Merge tag 'leds-next-6.6' of git://git.kernel.org/pub/scm/linux/kernel/git/lee/leds

[thirdparty/linux.git] / lib / iov_iter.c

// SPDX-License-Identifier: GPL-2.0-only
#include <crypto/hash.h>
#include <linux/export.h>
#include <linux/bvec.h>
#include <linux/fault-inject-usercopy.h>
#include <linux/uio.h>
#include <linux/pagemap.h>
#include <linux/highmem.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <linux/splice.h>
#include <linux/compat.h>
#include <net/checksum.h>
#include <linux/scatterlist.h>
#include <linux/instrumented.h>

/* covers ubuf and kbuf alike */
#define iterate_buf(i, n, base, len, off, __p, STEP) {          \
        size_t __maybe_unused off = 0;                          \
        len = n;                                                \
        base = __p + i->iov_offset;                             \
        len -= (STEP);                                          \
        i->iov_offset += len;                                   \
        n = len;                                                \
}

/* covers iovec and kvec alike */
#define iterate_iovec(i, n, base, len, off, __p, STEP) {        \
        size_t off = 0;                                         \
        size_t skip = i->iov_offset;                            \
        do {                                                    \
                len = min(n, __p->iov_len - skip);              \
                if (likely(len)) {                              \
                        base = __p->iov_base + skip;            \
                        len -= (STEP);                          \
                        off += len;                             \
                        skip += len;                            \
                        n -= len;                               \
                        if (skip < __p->iov_len)                \
                                break;                          \
                }                                               \
                __p++;                                          \
                skip = 0;                                       \
        } while (n);                                            \
        i->iov_offset = skip;                                   \
        n = off;                                                \
}

#define iterate_bvec(i, n, base, len, off, p, STEP) {           \
        size_t off = 0;                                         \
        unsigned skip = i->iov_offset;                          \
        while (n) {                                             \
                unsigned offset = p->bv_offset + skip;          \
                unsigned left;                                  \
                void *kaddr = kmap_local_page(p->bv_page +      \
                                        offset / PAGE_SIZE);    \
                base = kaddr + offset % PAGE_SIZE;              \
                len = min(min(n, (size_t)(p->bv_len - skip)),   \
                     (size_t)(PAGE_SIZE - offset % PAGE_SIZE)); \
                left = (STEP);                                  \
                kunmap_local(kaddr);                            \
                len -= left;                                    \
                off += len;                                     \
                skip += len;                                    \
                if (skip == p->bv_len) {                        \
                        skip = 0;                               \
                        p++;                                    \
                }                                               \
                n -= len;                                       \
                if (left)                                       \
                        break;                                  \
        }                                                       \
        i->iov_offset = skip;                                   \
        n = off;                                                \
}

#define iterate_xarray(i, n, base, len, __off, STEP) {          \
        __label__ __out;                                        \
        size_t __off = 0;                                       \
        struct folio *folio;                                    \
        loff_t start = i->xarray_start + i->iov_offset;         \
        pgoff_t index = start / PAGE_SIZE;                      \
        XA_STATE(xas, i->xarray, index);                        \
                                                                \
        len = PAGE_SIZE - offset_in_page(start);                \
        rcu_read_lock();                                        \
        xas_for_each(&xas, folio, ULONG_MAX) {                  \
                unsigned left;                                  \
                size_t offset;                                  \
                if (xas_retry(&xas, folio))                     \
                        continue;                               \
                if (WARN_ON(xa_is_value(folio)))                \
                        break;                                  \
                if (WARN_ON(folio_test_hugetlb(folio)))         \
                        break;                                  \
                offset = offset_in_folio(folio, start + __off); \
                while (offset < folio_size(folio)) {            \
                        base = kmap_local_folio(folio, offset); \
                        len = min(n, len);                      \
                        left = (STEP);                          \
                        kunmap_local(base);                     \
                        len -= left;                            \
                        __off += len;                           \
                        n -= len;                               \
                        if (left || n == 0)                     \
                                goto __out;                     \
                        offset += len;                          \
                        len = PAGE_SIZE;                        \
                }                                               \
        }                                                       \
__out:                                                          \
        rcu_read_unlock();                                      \
        i->iov_offset += __off;                                 \
        n = __off;                                              \
}

#define __iterate_and_advance(i, n, base, len, off, I, K) {     \
        if (unlikely(i->count < n))                             \
                n = i->count;                                   \
        if (likely(n)) {                                        \
                if (likely(iter_is_ubuf(i))) {                  \
                        void __user *base;                      \
                        size_t len;                             \
                        iterate_buf(i, n, base, len, off,       \
                                                i->ubuf, (I))   \
                } else if (likely(iter_is_iovec(i))) {          \
                        const struct iovec *iov = iter_iov(i);  \
                        void __user *base;                      \
                        size_t len;                             \
                        iterate_iovec(i, n, base, len, off,     \
                                                iov, (I))       \
                        i->nr_segs -= iov - iter_iov(i);        \
                        i->__iov = iov;                         \
                } else if (iov_iter_is_bvec(i)) {               \
                        const struct bio_vec *bvec = i->bvec;   \
                        void *base;                             \
                        size_t len;                             \
                        iterate_bvec(i, n, base, len, off,      \
                                                bvec, (K))      \
                        i->nr_segs -= bvec - i->bvec;           \
                        i->bvec = bvec;                         \
                } else if (iov_iter_is_kvec(i)) {               \
                        const struct kvec *kvec = i->kvec;      \
                        void *base;                             \
                        size_t len;                             \
                        iterate_iovec(i, n, base, len, off,     \
                                                kvec, (K))      \
                        i->nr_segs -= kvec - i->kvec;           \
                        i->kvec = kvec;                         \
                } else if (iov_iter_is_xarray(i)) {             \
                        void *base;                             \
                        size_t len;                             \
                        iterate_xarray(i, n, base, len, off,    \
                                                        (K))    \
                }                                               \
                i->count -= n;                                  \
        }                                                       \
}
#define iterate_and_advance(i, n, base, len, off, I, K) \
        __iterate_and_advance(i, n, base, len, off, I, ((void)(K),0))

static int copyout(void __user *to, const void *from, size_t n)
{
        if (should_fail_usercopy())
                return n;
        if (access_ok(to, n)) {
                instrument_copy_to_user(to, from, n);
                n = raw_copy_to_user(to, from, n);
        }
        return n;
}

static int copyout_nofault(void __user *to, const void *from, size_t n)
{
        long res;

        if (should_fail_usercopy())
                return n;

        res = copy_to_user_nofault(to, from, n);

        return res < 0 ? n : res;
}

static int copyin(void *to, const void __user *from, size_t n)
{
        size_t res = n;

        if (should_fail_usercopy())
                return n;
        if (access_ok(from, n)) {
                instrument_copy_from_user_before(to, from, n);
                res = raw_copy_from_user(to, from, n);
                instrument_copy_from_user_after(to, from, n, res);
        }
        return res;
}

/*
 * fault_in_iov_iter_readable - fault in iov iterator for reading
 * @i: iterator
 * @size: maximum length
 *
 * Fault in one or more iovecs of the given iov_iter, to a maximum length of
 * @size.  For each iovec, fault in each page that constitutes the iovec.
 *
 * Returns the number of bytes not faulted in (like copy_to_user() and
 * copy_from_user()).
 *
 * Always returns 0 for non-userspace iterators.
 */
size_t fault_in_iov_iter_readable(const struct iov_iter *i, size_t size)
{
        if (iter_is_ubuf(i)) {
                size_t n = min(size, iov_iter_count(i));
                n -= fault_in_readable(i->ubuf + i->iov_offset, n);
                return size - n;
        } else if (iter_is_iovec(i)) {
                size_t count = min(size, iov_iter_count(i));
                const struct iovec *p;
                size_t skip;

                size -= count;
                for (p = iter_iov(i), skip = i->iov_offset; count; p++, skip = 0) {
                        size_t len = min(count, p->iov_len - skip);
                        size_t ret;

                        if (unlikely(!len))
                                continue;
                        ret = fault_in_readable(p->iov_base + skip, len);
                        count -= len - ret;
                        if (ret)
                                break;
                }
                return count + size;
        }
        return 0;
}
EXPORT_SYMBOL(fault_in_iov_iter_readable);

/*
 * fault_in_iov_iter_writeable - fault in iov iterator for writing
 * @i: iterator
 * @size: maximum length
 *
 * Faults in the iterator using get_user_pages(), i.e., without triggering
 * hardware page faults.  This is primarily useful when we already know that
 * some or all of the pages in @i aren't in memory.
 *
 * Returns the number of bytes not faulted in, like copy_to_user() and
 * copy_from_user().
 *
 * Always returns 0 for non-user-space iterators.
 */
size_t fault_in_iov_iter_writeable(const struct iov_iter *i, size_t size)
{
        if (iter_is_ubuf(i)) {
                size_t n = min(size, iov_iter_count(i));
                n -= fault_in_safe_writeable(i->ubuf + i->iov_offset, n);
                return size - n;
        } else if (iter_is_iovec(i)) {
                size_t count = min(size, iov_iter_count(i));
                const struct iovec *p;
                size_t skip;

                size -= count;
                for (p = iter_iov(i), skip = i->iov_offset; count; p++, skip = 0) {
                        size_t len = min(count, p->iov_len - skip);
                        size_t ret;

                        if (unlikely(!len))
                                continue;
                        ret = fault_in_safe_writeable(p->iov_base + skip, len);
                        count -= len - ret;
                        if (ret)
                                break;
                }
                return count + size;
        }
        return 0;
}
EXPORT_SYMBOL(fault_in_iov_iter_writeable);

void iov_iter_init(struct iov_iter *i, unsigned int direction,
                        const struct iovec *iov, unsigned long nr_segs,
                        size_t count)
{
        WARN_ON(direction & ~(READ | WRITE));
        *i = (struct iov_iter) {
                .iter_type = ITER_IOVEC,
                .copy_mc = false,
                .nofault = false,
                .user_backed = true,
                .data_source = direction,
                .__iov = iov,
                .nr_segs = nr_segs,
                .iov_offset = 0,
                .count = count
        };
}
EXPORT_SYMBOL(iov_iter_init);

static __wsum csum_and_memcpy(void *to, const void *from, size_t len,
                              __wsum sum, size_t off)
{
        __wsum next = csum_partial_copy_nocheck(from, to, len);
        return csum_block_add(sum, next, off);
}

size_t _copy_to_iter(const void *addr, size_t bytes, struct iov_iter *i)
{
        if (WARN_ON_ONCE(i->data_source))
                return 0;
        if (user_backed_iter(i))
                might_fault();
        iterate_and_advance(i, bytes, base, len, off,
                copyout(base, addr + off, len),
                memcpy(base, addr + off, len)
        )

        return bytes;
}
EXPORT_SYMBOL(_copy_to_iter);

#ifdef CONFIG_ARCH_HAS_COPY_MC
static int copyout_mc(void __user *to, const void *from, size_t n)
{
        if (access_ok(to, n)) {
                instrument_copy_to_user(to, from, n);
                n = copy_mc_to_user((__force void *) to, from, n);
        }
        return n;
}

/**
 * _copy_mc_to_iter - copy to iter with source memory error exception handling
 * @addr: source kernel address
 * @bytes: total transfer length
 * @i: destination iterator
 *
 * The pmem driver deploys this for the dax operation
 * (dax_copy_to_iter()) for dax reads (bypass page-cache and the
 * block-layer). Upon #MC read(2) aborts and returns EIO or the bytes
 * successfully copied.
 *
 * The main differences between this and typical _copy_to_iter().
 *
 * * Typical tail/residue handling after a fault retries the copy
 *   byte-by-byte until the fault happens again. Re-triggering machine
 *   checks is potentially fatal so the implementation uses source
 *   alignment and poison alignment assumptions to avoid re-triggering
 *   hardware exceptions.
 *
 * * ITER_KVEC and ITER_BVEC can return short copies.  Compare to
 *   copy_to_iter() where only ITER_IOVEC attempts might return a short copy.
 *
 * Return: number of bytes copied (may be %0)
 */
size_t _copy_mc_to_iter(const void *addr, size_t bytes, struct iov_iter *i)
{
        if (WARN_ON_ONCE(i->data_source))
                return 0;
        if (user_backed_iter(i))
                might_fault();
        __iterate_and_advance(i, bytes, base, len, off,
                copyout_mc(base, addr + off, len),
                copy_mc_to_kernel(base, addr + off, len)
        )

        return bytes;
}
EXPORT_SYMBOL_GPL(_copy_mc_to_iter);
#endif /* CONFIG_ARCH_HAS_COPY_MC */

static void *memcpy_from_iter(struct iov_iter *i, void *to, const void *from,
                                 size_t size)
{
        if (iov_iter_is_copy_mc(i))
                return (void *)copy_mc_to_kernel(to, from, size);
        return memcpy(to, from, size);
}

size_t _copy_from_iter(void *addr, size_t bytes, struct iov_iter *i)
{
        if (WARN_ON_ONCE(!i->data_source))
                return 0;

        if (user_backed_iter(i))
                might_fault();
        iterate_and_advance(i, bytes, base, len, off,
                copyin(addr + off, base, len),
                memcpy_from_iter(i, addr + off, base, len)
        )

        return bytes;
}
EXPORT_SYMBOL(_copy_from_iter);

size_t _copy_from_iter_nocache(void *addr, size_t bytes, struct iov_iter *i)
{
        if (WARN_ON_ONCE(!i->data_source))
                return 0;

        iterate_and_advance(i, bytes, base, len, off,
                __copy_from_user_inatomic_nocache(addr + off, base, len),
                memcpy(addr + off, base, len)
        )

        return bytes;
}
EXPORT_SYMBOL(_copy_from_iter_nocache);

#ifdef CONFIG_ARCH_HAS_UACCESS_FLUSHCACHE
/**
 * _copy_from_iter_flushcache - write destination through cpu cache
 * @addr: destination kernel address
 * @bytes: total transfer length
 * @i: source iterator
 *
 * The pmem driver arranges for filesystem-dax to use this facility via
 * dax_copy_from_iter() for ensuring that writes to persistent memory
 * are flushed through the CPU cache. It is differentiated from
 * _copy_from_iter_nocache() in that guarantees all data is flushed for
 * all iterator types. The _copy_from_iter_nocache() only attempts to
 * bypass the cache for the ITER_IOVEC case, and on some archs may use
 * instructions that strand dirty-data in the cache.
 *
 * Return: number of bytes copied (may be %0)
 */
size_t _copy_from_iter_flushcache(void *addr, size_t bytes, struct iov_iter *i)
{
        if (WARN_ON_ONCE(!i->data_source))
                return 0;

        iterate_and_advance(i, bytes, base, len, off,
                __copy_from_user_flushcache(addr + off, base, len),
                memcpy_flushcache(addr + off, base, len)
        )

        return bytes;
}
EXPORT_SYMBOL_GPL(_copy_from_iter_flushcache);
#endif

static inline bool page_copy_sane(struct page *page, size_t offset, size_t n)
{
        struct page *head;
        size_t v = n + offset;

        /*
         * The general case needs to access the page order in order
         * to compute the page size.
         * However, we mostly deal with order-0 pages and thus can
         * avoid a possible cache line miss for requests that fit all
         * page orders.
         */
        if (n <= v && v <= PAGE_SIZE)
                return true;

        head = compound_head(page);
        v += (page - head) << PAGE_SHIFT;

        if (WARN_ON(n > v || v > page_size(head)))
                return false;
        return true;
}

size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
                         struct iov_iter *i)
{
        size_t res = 0;
        if (!page_copy_sane(page, offset, bytes))
                return 0;
        if (WARN_ON_ONCE(i->data_source))
                return 0;
        page += offset / PAGE_SIZE; // first subpage
        offset %= PAGE_SIZE;
        while (1) {
                void *kaddr = kmap_local_page(page);
                size_t n = min(bytes, (size_t)PAGE_SIZE - offset);
                n = _copy_to_iter(kaddr + offset, n, i);
                kunmap_local(kaddr);
                res += n;
                bytes -= n;
                if (!bytes || !n)
                        break;
                offset += n;
                if (offset == PAGE_SIZE) {
                        page++;
                        offset = 0;
                }
        }
        return res;
}
EXPORT_SYMBOL(copy_page_to_iter);

size_t copy_page_to_iter_nofault(struct page *page, unsigned offset, size_t bytes,
                                 struct iov_iter *i)
{
        size_t res = 0;

        if (!page_copy_sane(page, offset, bytes))
                return 0;
        if (WARN_ON_ONCE(i->data_source))
                return 0;
        page += offset / PAGE_SIZE; // first subpage
        offset %= PAGE_SIZE;
        while (1) {
                void *kaddr = kmap_local_page(page);
                size_t n = min(bytes, (size_t)PAGE_SIZE - offset);

                iterate_and_advance(i, n, base, len, off,
                        copyout_nofault(base, kaddr + offset + off, len),
                        memcpy(base, kaddr + offset + off, len)
                )
                kunmap_local(kaddr);
                res += n;
                bytes -= n;
                if (!bytes || !n)
                        break;
                offset += n;
                if (offset == PAGE_SIZE) {
                        page++;
                        offset = 0;
                }
        }
        return res;
}
EXPORT_SYMBOL(copy_page_to_iter_nofault);

size_t copy_page_from_iter(struct page *page, size_t offset, size_t bytes,
                         struct iov_iter *i)
{
        size_t res = 0;
        if (!page_copy_sane(page, offset, bytes))
                return 0;
        page += offset / PAGE_SIZE; // first subpage
        offset %= PAGE_SIZE;
        while (1) {
                void *kaddr = kmap_local_page(page);
                size_t n = min(bytes, (size_t)PAGE_SIZE - offset);
                n = _copy_from_iter(kaddr + offset, n, i);
                kunmap_local(kaddr);
                res += n;
                bytes -= n;
                if (!bytes || !n)
                        break;
                offset += n;
                if (offset == PAGE_SIZE) {
                        page++;
                        offset = 0;
                }
        }
        return res;
}
EXPORT_SYMBOL(copy_page_from_iter);

size_t iov_iter_zero(size_t bytes, struct iov_iter *i)
{
        iterate_and_advance(i, bytes, base, len, count,
                clear_user(base, len),
                memset(base, 0, len)
        )

        return bytes;
}
EXPORT_SYMBOL(iov_iter_zero);

size_t copy_page_from_iter_atomic(struct page *page, size_t offset,
                size_t bytes, struct iov_iter *i)
{
        size_t n, copied = 0;

        if (!page_copy_sane(page, offset, bytes))
                return 0;
        if (WARN_ON_ONCE(!i->data_source))
                return 0;

        do {
                char *p;

                n = bytes - copied;
                if (PageHighMem(page)) {
                        page += offset / PAGE_SIZE;
                        offset %= PAGE_SIZE;
                        n = min_t(size_t, n, PAGE_SIZE - offset);
                }

                p = kmap_atomic(page) + offset;
                iterate_and_advance(i, n, base, len, off,
                        copyin(p + off, base, len),
                        memcpy_from_iter(i, p + off, base, len)
                )
                kunmap_atomic(p);
                copied += n;
                offset += n;
        } while (PageHighMem(page) && copied != bytes && n > 0);

        return copied;
}
EXPORT_SYMBOL(copy_page_from_iter_atomic);

static void iov_iter_bvec_advance(struct iov_iter *i, size_t size)
{
        const struct bio_vec *bvec, *end;

        if (!i->count)
                return;
        i->count -= size;

        size += i->iov_offset;

        for (bvec = i->bvec, end = bvec + i->nr_segs; bvec < end; bvec++) {
                if (likely(size < bvec->bv_len))
                        break;
                size -= bvec->bv_len;
        }
        i->iov_offset = size;
        i->nr_segs -= bvec - i->bvec;
        i->bvec = bvec;
}

static void iov_iter_iovec_advance(struct iov_iter *i, size_t size)
{
        const struct iovec *iov, *end;

        if (!i->count)
                return;
        i->count -= size;

        size += i->iov_offset; // from beginning of current segment
        for (iov = iter_iov(i), end = iov + i->nr_segs; iov < end; iov++) {
                if (likely(size < iov->iov_len))
                        break;
                size -= iov->iov_len;
        }
        i->iov_offset = size;
        i->nr_segs -= iov - iter_iov(i);
        i->__iov = iov;
}

void iov_iter_advance(struct iov_iter *i, size_t size)
{
        if (unlikely(i->count < size))
                size = i->count;
        if (likely(iter_is_ubuf(i)) || unlikely(iov_iter_is_xarray(i))) {
                i->iov_offset += size;
                i->count -= size;
        } else if (likely(iter_is_iovec(i) || iov_iter_is_kvec(i))) {
                /* iovec and kvec have identical layouts */
                iov_iter_iovec_advance(i, size);
        } else if (iov_iter_is_bvec(i)) {
                iov_iter_bvec_advance(i, size);
        } else if (iov_iter_is_discard(i)) {
                i->count -= size;
        }
}
EXPORT_SYMBOL(iov_iter_advance);

void iov_iter_revert(struct iov_iter *i, size_t unroll)
{
        if (!unroll)
                return;
        if (WARN_ON(unroll > MAX_RW_COUNT))
                return;
        i->count += unroll;
        if (unlikely(iov_iter_is_discard(i)))
                return;
        if (unroll <= i->iov_offset) {
                i->iov_offset -= unroll;
                return;
        }
        unroll -= i->iov_offset;
        if (iov_iter_is_xarray(i) || iter_is_ubuf(i)) {
                BUG(); /* We should never go beyond the start of the specified
                        * range since we might then be straying into pages that
                        * aren't pinned.
                        */
        } else if (iov_iter_is_bvec(i)) {
                const struct bio_vec *bvec = i->bvec;
                while (1) {
                        size_t n = (--bvec)->bv_len;
                        i->nr_segs++;
                        if (unroll <= n) {
                                i->bvec = bvec;
                                i->iov_offset = n - unroll;
                                return;
                        }
                        unroll -= n;
                }
        } else { /* same logics for iovec and kvec */
                const struct iovec *iov = iter_iov(i);
                while (1) {
                        size_t n = (--iov)->iov_len;
                        i->nr_segs++;
                        if (unroll <= n) {
                                i->__iov = iov;
                                i->iov_offset = n - unroll;
                                return;
                        }
                        unroll -= n;
                }
        }
}
EXPORT_SYMBOL(iov_iter_revert);

/*
 * Return the count of just the current iov_iter segment.
 */
size_t iov_iter_single_seg_count(const struct iov_iter *i)
{
        if (i->nr_segs > 1) {
                if (likely(iter_is_iovec(i) || iov_iter_is_kvec(i)))
                        return min(i->count, iter_iov(i)->iov_len - i->iov_offset);
                if (iov_iter_is_bvec(i))
                        return min(i->count, i->bvec->bv_len - i->iov_offset);
        }
        return i->count;
}
EXPORT_SYMBOL(iov_iter_single_seg_count);

void iov_iter_kvec(struct iov_iter *i, unsigned int direction,
                        const struct kvec *kvec, unsigned long nr_segs,
                        size_t count)
{
        WARN_ON(direction & ~(READ | WRITE));
        *i = (struct iov_iter){
                .iter_type = ITER_KVEC,
                .copy_mc = false,
                .data_source = direction,
                .kvec = kvec,
                .nr_segs = nr_segs,
                .iov_offset = 0,
                .count = count
        };
}
EXPORT_SYMBOL(iov_iter_kvec);

void iov_iter_bvec(struct iov_iter *i, unsigned int direction,
                        const struct bio_vec *bvec, unsigned long nr_segs,
                        size_t count)
{
        WARN_ON(direction & ~(READ | WRITE));
        *i = (struct iov_iter){
                .iter_type = ITER_BVEC,
                .copy_mc = false,
                .data_source = direction,
                .bvec = bvec,
                .nr_segs = nr_segs,
                .iov_offset = 0,
                .count = count
        };
}
EXPORT_SYMBOL(iov_iter_bvec);

/**
 * iov_iter_xarray - Initialise an I/O iterator to use the pages in an xarray
 * @i: The iterator to initialise.
 * @direction: The direction of the transfer.
 * @xarray: The xarray to access.
 * @start: The start file position.
 * @count: The size of the I/O buffer in bytes.
 *
 * Set up an I/O iterator to either draw data out of the pages attached to an
 * inode or to inject data into those pages.  The pages *must* be prevented
 * from evaporation, either by taking a ref on them or locking them by the
 * caller.
 */
void iov_iter_xarray(struct iov_iter *i, unsigned int direction,
                     struct xarray *xarray, loff_t start, size_t count)
{
        BUG_ON(direction & ~1);
        *i = (struct iov_iter) {
                .iter_type = ITER_XARRAY,
                .copy_mc = false,
                .data_source = direction,
                .xarray = xarray,
                .xarray_start = start,
                .count = count,
                .iov_offset = 0
        };
}
EXPORT_SYMBOL(iov_iter_xarray);

/**
 * iov_iter_discard - Initialise an I/O iterator that discards data
 * @i: The iterator to initialise.
 * @direction: The direction of the transfer.
 * @count: The size of the I/O buffer in bytes.
 *
 * Set up an I/O iterator that just discards everything that's written to it.
 * It's only available as a READ iterator.
 */
void iov_iter_discard(struct iov_iter *i, unsigned int direction, size_t count)
{
        BUG_ON(direction != READ);
        *i = (struct iov_iter){
                .iter_type = ITER_DISCARD,
                .copy_mc = false,
                .data_source = false,
                .count = count,
                .iov_offset = 0
        };
}
EXPORT_SYMBOL(iov_iter_discard);

static bool iov_iter_aligned_iovec(const struct iov_iter *i, unsigned addr_mask,
                                   unsigned len_mask)
{
        size_t size = i->count;
        size_t skip = i->iov_offset;
        unsigned k;

        for (k = 0; k < i->nr_segs; k++, skip = 0) {
                const struct iovec *iov = iter_iov(i) + k;
                size_t len = iov->iov_len - skip;

                if (len > size)
                        len = size;
                if (len & len_mask)
                        return false;
                if ((unsigned long)(iov->iov_base + skip) & addr_mask)
                        return false;

                size -= len;
                if (!size)
                        break;
        }
        return true;
}

static bool iov_iter_aligned_bvec(const struct iov_iter *i, unsigned addr_mask,
                                  unsigned len_mask)
{
        size_t size = i->count;
        unsigned skip = i->iov_offset;
        unsigned k;

        for (k = 0; k < i->nr_segs; k++, skip = 0) {
                size_t len = i->bvec[k].bv_len - skip;

                if (len > size)
                        len = size;
                if (len & len_mask)
                        return false;
                if ((unsigned long)(i->bvec[k].bv_offset + skip) & addr_mask)
                        return false;

                size -= len;
                if (!size)
                        break;
        }
        return true;
}

/**
 * iov_iter_is_aligned() - Check if the addresses and lengths of each segments
 *      are aligned to the parameters.
 *
 * @i: &struct iov_iter to restore
 * @addr_mask: bit mask to check against the iov element's addresses
 * @len_mask: bit mask to check against the iov element's lengths
 *
 * Return: false if any addresses or lengths intersect with the provided masks
 */
bool iov_iter_is_aligned(const struct iov_iter *i, unsigned addr_mask,
                         unsigned len_mask)
{
        if (likely(iter_is_ubuf(i))) {
                if (i->count & len_mask)
                        return false;
                if ((unsigned long)(i->ubuf + i->iov_offset) & addr_mask)
                        return false;
                return true;
        }

        if (likely(iter_is_iovec(i) || iov_iter_is_kvec(i)))
                return iov_iter_aligned_iovec(i, addr_mask, len_mask);

        if (iov_iter_is_bvec(i))
                return iov_iter_aligned_bvec(i, addr_mask, len_mask);

        if (iov_iter_is_xarray(i)) {
                if (i->count & len_mask)
                        return false;
                if ((i->xarray_start + i->iov_offset) & addr_mask)
                        return false;
        }

        return true;
}
EXPORT_SYMBOL_GPL(iov_iter_is_aligned);

static unsigned long iov_iter_alignment_iovec(const struct iov_iter *i)
{
        unsigned long res = 0;
        size_t size = i->count;
        size_t skip = i->iov_offset;
        unsigned k;

        for (k = 0; k < i->nr_segs; k++, skip = 0) {
                const struct iovec *iov = iter_iov(i) + k;
                size_t len = iov->iov_len - skip;
                if (len) {
                        res |= (unsigned long)iov->iov_base + skip;
                        if (len > size)
                                len = size;
                        res |= len;
                        size -= len;
                        if (!size)
                                break;
                }
        }
        return res;
}

static unsigned long iov_iter_alignment_bvec(const struct iov_iter *i)
{
        unsigned res = 0;
        size_t size = i->count;
        unsigned skip = i->iov_offset;
        unsigned k;

        for (k = 0; k < i->nr_segs; k++, skip = 0) {
                size_t len = i->bvec[k].bv_len - skip;
                res |= (unsigned long)i->bvec[k].bv_offset + skip;
                if (len > size)
                        len = size;
                res |= len;
                size -= len;
                if (!size)
                        break;
        }
        return res;
}

unsigned long iov_iter_alignment(const struct iov_iter *i)
{
        if (likely(iter_is_ubuf(i))) {
                size_t size = i->count;
                if (size)
                        return ((unsigned long)i->ubuf + i->iov_offset) | size;
                return 0;
        }

        /* iovec and kvec have identical layouts */
        if (likely(iter_is_iovec(i) || iov_iter_is_kvec(i)))
                return iov_iter_alignment_iovec(i);

        if (iov_iter_is_bvec(i))
                return iov_iter_alignment_bvec(i);

        if (iov_iter_is_xarray(i))
                return (i->xarray_start + i->iov_offset) | i->count;

        return 0;
}
EXPORT_SYMBOL(iov_iter_alignment);

unsigned long iov_iter_gap_alignment(const struct iov_iter *i)
{
        unsigned long res = 0;
        unsigned long v = 0;
        size_t size = i->count;
        unsigned k;

        if (iter_is_ubuf(i))
                return 0;

        if (WARN_ON(!iter_is_iovec(i)))
                return ~0U;

        for (k = 0; k < i->nr_segs; k++) {
                const struct iovec *iov = iter_iov(i) + k;
                if (iov->iov_len) {
                        unsigned long base = (unsigned long)iov->iov_base;
                        if (v) // if not the first one
                                res |= base | v; // this start | previous end
                        v = base + iov->iov_len;
                        if (size <= iov->iov_len)
                                break;
                        size -= iov->iov_len;
                }
        }
        return res;
}
EXPORT_SYMBOL(iov_iter_gap_alignment);

static int want_pages_array(struct page ***res, size_t size,
                            size_t start, unsigned int maxpages)
{
        unsigned int count = DIV_ROUND_UP(size + start, PAGE_SIZE);

        if (count > maxpages)
                count = maxpages;
        WARN_ON(!count);        // caller should've prevented that
        if (!*res) {
                *res = kvmalloc_array(count, sizeof(struct page *), GFP_KERNEL);
                if (!*res)
                        return 0;
        }
        return count;
}

static ssize_t iter_xarray_populate_pages(struct page **pages, struct xarray *xa,
                                          pgoff_t index, unsigned int nr_pages)
{
        XA_STATE(xas, xa, index);
        struct page *page;
        unsigned int ret = 0;

        rcu_read_lock();
        for (page = xas_load(&xas); page; page = xas_next(&xas)) {
                if (xas_retry(&xas, page))
                        continue;

                /* Has the page moved or been split? */
                if (unlikely(page != xas_reload(&xas))) {
                        xas_reset(&xas);
                        continue;
                }

                pages[ret] = find_subpage(page, xas.xa_index);
                get_page(pages[ret]);
                if (++ret == nr_pages)
                        break;
        }
        rcu_read_unlock();
        return ret;
}

static ssize_t iter_xarray_get_pages(struct iov_iter *i,
                                     struct page ***pages, size_t maxsize,
                                     unsigned maxpages, size_t *_start_offset)
{
        unsigned nr, offset, count;
        pgoff_t index;
        loff_t pos;

        pos = i->xarray_start + i->iov_offset;
        index = pos >> PAGE_SHIFT;
        offset = pos & ~PAGE_MASK;
        *_start_offset = offset;

        count = want_pages_array(pages, maxsize, offset, maxpages);
        if (!count)
                return -ENOMEM;
        nr = iter_xarray_populate_pages(*pages, i->xarray, index, count);
        if (nr == 0)
                return 0;

        maxsize = min_t(size_t, nr * PAGE_SIZE - offset, maxsize);
        i->iov_offset += maxsize;
        i->count -= maxsize;
        return maxsize;
}

/* must be done on non-empty ITER_UBUF or ITER_IOVEC one */
static unsigned long first_iovec_segment(const struct iov_iter *i, size_t *size)
{
        size_t skip;
        long k;

        if (iter_is_ubuf(i))
                return (unsigned long)i->ubuf + i->iov_offset;

        for (k = 0, skip = i->iov_offset; k < i->nr_segs; k++, skip = 0) {
                const struct iovec *iov = iter_iov(i) + k;
                size_t len = iov->iov_len - skip;

                if (unlikely(!len))
                        continue;
                if (*size > len)
                        *size = len;
                return (unsigned long)iov->iov_base + skip;
        }
        BUG(); // if it had been empty, we wouldn't get called
}

/* must be done on non-empty ITER_BVEC one */
static struct page *first_bvec_segment(const struct iov_iter *i,
                                       size_t *size, size_t *start)
{
        struct page *page;
        size_t skip = i->iov_offset, len;

        len = i->bvec->bv_len - skip;
        if (*size > len)
                *size = len;
        skip += i->bvec->bv_offset;
        page = i->bvec->bv_page + skip / PAGE_SIZE;
        *start = skip % PAGE_SIZE;
        return page;
}

static ssize_t __iov_iter_get_pages_alloc(struct iov_iter *i,
                   struct page ***pages, size_t maxsize,
                   unsigned int maxpages, size_t *start)
{
        unsigned int n, gup_flags = 0;

        if (maxsize > i->count)
                maxsize = i->count;
        if (!maxsize)
                return 0;
        if (maxsize > MAX_RW_COUNT)
                maxsize = MAX_RW_COUNT;

        if (likely(user_backed_iter(i))) {
                unsigned long addr;
                int res;

                if (iov_iter_rw(i) != WRITE)
                        gup_flags |= FOLL_WRITE;
                if (i->nofault)
                        gup_flags |= FOLL_NOFAULT;

                addr = first_iovec_segment(i, &maxsize);
                *start = addr % PAGE_SIZE;
                addr &= PAGE_MASK;
                n = want_pages_array(pages, maxsize, *start, maxpages);
                if (!n)
                        return -ENOMEM;
                res = get_user_pages_fast(addr, n, gup_flags, *pages);
                if (unlikely(res <= 0))
                        return res;
                maxsize = min_t(size_t, maxsize, res * PAGE_SIZE - *start);
                iov_iter_advance(i, maxsize);
                return maxsize;
        }
        if (iov_iter_is_bvec(i)) {
                struct page **p;
                struct page *page;

                page = first_bvec_segment(i, &maxsize, start);
                n = want_pages_array(pages, maxsize, *start, maxpages);
                if (!n)
                        return -ENOMEM;
                p = *pages;
                for (int k = 0; k < n; k++)
                        get_page(p[k] = page + k);
                maxsize = min_t(size_t, maxsize, n * PAGE_SIZE - *start);
                i->count -= maxsize;
                i->iov_offset += maxsize;
                if (i->iov_offset == i->bvec->bv_len) {
                        i->iov_offset = 0;
                        i->bvec++;
                        i->nr_segs--;
                }
                return maxsize;
        }
        if (iov_iter_is_xarray(i))
                return iter_xarray_get_pages(i, pages, maxsize, maxpages, start);
        return -EFAULT;
}

ssize_t iov_iter_get_pages2(struct iov_iter *i, struct page **pages,
                size_t maxsize, unsigned maxpages, size_t *start)
{
        if (!maxpages)
                return 0;
        BUG_ON(!pages);

        return __iov_iter_get_pages_alloc(i, &pages, maxsize, maxpages, start);
}
EXPORT_SYMBOL(iov_iter_get_pages2);

ssize_t iov_iter_get_pages_alloc2(struct iov_iter *i,
                struct page ***pages, size_t maxsize, size_t *start)
{
        ssize_t len;

        *pages = NULL;

        len = __iov_iter_get_pages_alloc(i, pages, maxsize, ~0U, start);
        if (len <= 0) {
                kvfree(*pages);
                *pages = NULL;
        }
        return len;
}
EXPORT_SYMBOL(iov_iter_get_pages_alloc2);

size_t csum_and_copy_from_iter(void *addr, size_t bytes, __wsum *csum,
                               struct iov_iter *i)
{
        __wsum sum, next;
        sum = *csum;
        if (WARN_ON_ONCE(!i->data_source))
                return 0;

        iterate_and_advance(i, bytes, base, len, off, ({
                next = csum_and_copy_from_user(base, addr + off, len);
                sum = csum_block_add(sum, next, off);
                next ? 0 : len;
        }), ({
                sum = csum_and_memcpy(addr + off, base, len, sum, off);
        })
        )
        *csum = sum;
        return bytes;
}
EXPORT_SYMBOL(csum_and_copy_from_iter);

size_t csum_and_copy_to_iter(const void *addr, size_t bytes, void *_csstate,
                             struct iov_iter *i)
{
        struct csum_state *csstate = _csstate;
        __wsum sum, next;

        if (WARN_ON_ONCE(i->data_source))
                return 0;
        if (unlikely(iov_iter_is_discard(i))) {
                // can't use csum_memcpy() for that one - data is not copied
                csstate->csum = csum_block_add(csstate->csum,
                                               csum_partial(addr, bytes, 0),
                                               csstate->off);
                csstate->off += bytes;
                return bytes;
        }

        sum = csum_shift(csstate->csum, csstate->off);
        iterate_and_advance(i, bytes, base, len, off, ({
                next = csum_and_copy_to_user(addr + off, base, len);
                sum = csum_block_add(sum, next, off);
                next ? 0 : len;
        }), ({
                sum = csum_and_memcpy(base, addr + off, len, sum, off);
        })
        )
        csstate->csum = csum_shift(sum, csstate->off);
        csstate->off += bytes;
        return bytes;
}
EXPORT_SYMBOL(csum_and_copy_to_iter);

size_t hash_and_copy_to_iter(const void *addr, size_t bytes, void *hashp,
                struct iov_iter *i)
{
#ifdef CONFIG_CRYPTO_HASH
        struct ahash_request *hash = hashp;
        struct scatterlist sg;
        size_t copied;

        copied = copy_to_iter(addr, bytes, i);
        sg_init_one(&sg, addr, copied);
        ahash_request_set_crypt(hash, &sg, NULL, copied);
        crypto_ahash_update(hash);
        return copied;
#else
        return 0;
#endif
}
EXPORT_SYMBOL(hash_and_copy_to_iter);

static int iov_npages(const struct iov_iter *i, int maxpages)
{
        size_t skip = i->iov_offset, size = i->count;
        const struct iovec *p;
        int npages = 0;

        for (p = iter_iov(i); size; skip = 0, p++) {
                unsigned offs = offset_in_page(p->iov_base + skip);
                size_t len = min(p->iov_len - skip, size);

                if (len) {
                        size -= len;
                        npages += DIV_ROUND_UP(offs + len, PAGE_SIZE);
                        if (unlikely(npages > maxpages))
                                return maxpages;
                }
        }
        return npages;
}

static int bvec_npages(const struct iov_iter *i, int maxpages)
{
        size_t skip = i->iov_offset, size = i->count;
        const struct bio_vec *p;
        int npages = 0;

        for (p = i->bvec; size; skip = 0, p++) {
                unsigned offs = (p->bv_offset + skip) % PAGE_SIZE;
                size_t len = min(p->bv_len - skip, size);

                size -= len;
                npages += DIV_ROUND_UP(offs + len, PAGE_SIZE);
                if (unlikely(npages > maxpages))
                        return maxpages;
        }
        return npages;
}

int iov_iter_npages(const struct iov_iter *i, int maxpages)
{
        if (unlikely(!i->count))
                return 0;
        if (likely(iter_is_ubuf(i))) {
                unsigned offs = offset_in_page(i->ubuf + i->iov_offset);
                int npages = DIV_ROUND_UP(offs + i->count, PAGE_SIZE);
                return min(npages, maxpages);
        }
        /* iovec and kvec have identical layouts */
        if (likely(iter_is_iovec(i) || iov_iter_is_kvec(i)))
                return iov_npages(i, maxpages);
        if (iov_iter_is_bvec(i))
                return bvec_npages(i, maxpages);
        if (iov_iter_is_xarray(i)) {
                unsigned offset = (i->xarray_start + i->iov_offset) % PAGE_SIZE;
                int npages = DIV_ROUND_UP(offset + i->count, PAGE_SIZE);
                return min(npages, maxpages);
        }
        return 0;
}
EXPORT_SYMBOL(iov_iter_npages);

const void *dup_iter(struct iov_iter *new, struct iov_iter *old, gfp_t flags)
{
        *new = *old;
        if (iov_iter_is_bvec(new))
                return new->bvec = kmemdup(new->bvec,
                                    new->nr_segs * sizeof(struct bio_vec),
                                    flags);
        else if (iov_iter_is_kvec(new) || iter_is_iovec(new))
                /* iovec and kvec have identical layout */
                return new->__iov = kmemdup(new->__iov,
                                   new->nr_segs * sizeof(struct iovec),
                                   flags);
        return NULL;
}
EXPORT_SYMBOL(dup_iter);

static __noclone int copy_compat_iovec_from_user(struct iovec *iov,
                const struct iovec __user *uvec, unsigned long nr_segs)
{
        const struct compat_iovec __user *uiov =
                (const struct compat_iovec __user *)uvec;
        int ret = -EFAULT, i;

        if (!user_access_begin(uiov, nr_segs * sizeof(*uiov)))
                return -EFAULT;

        for (i = 0; i < nr_segs; i++) {
                compat_uptr_t buf;
                compat_ssize_t len;

                unsafe_get_user(len, &uiov[i].iov_len, uaccess_end);
                unsafe_get_user(buf, &uiov[i].iov_base, uaccess_end);

                /* check for compat_size_t not fitting in compat_ssize_t .. */
                if (len < 0) {
                        ret = -EINVAL;
                        goto uaccess_end;
                }
                iov[i].iov_base = compat_ptr(buf);
                iov[i].iov_len = len;
        }

        ret = 0;
uaccess_end:
        user_access_end();
        return ret;
}

static __noclone int copy_iovec_from_user(struct iovec *iov,
                const struct iovec __user *uiov, unsigned long nr_segs)
{
        int ret = -EFAULT;

        if (!user_access_begin(uiov, nr_segs * sizeof(*uiov)))
                return -EFAULT;

        do {
                void __user *buf;
                ssize_t len;

                unsafe_get_user(len, &uiov->iov_len, uaccess_end);
                unsafe_get_user(buf, &uiov->iov_base, uaccess_end);

                /* check for size_t not fitting in ssize_t .. */
                if (unlikely(len < 0)) {
                        ret = -EINVAL;
                        goto uaccess_end;
                }
                iov->iov_base = buf;
                iov->iov_len = len;

                uiov++; iov++;
        } while (--nr_segs);

        ret = 0;
uaccess_end:
        user_access_end();
        return ret;
}

struct iovec *iovec_from_user(const struct iovec __user *uvec,
                unsigned long nr_segs, unsigned long fast_segs,
                struct iovec *fast_iov, bool compat)
{
        struct iovec *iov = fast_iov;
        int ret;

        /*
         * SuS says "The readv() function *may* fail if the iovcnt argument was
         * less than or equal to 0, or greater than {IOV_MAX}.  Linux has
         * traditionally returned zero for zero segments, so...
         */
        if (nr_segs == 0)
                return iov;
        if (nr_segs > UIO_MAXIOV)
                return ERR_PTR(-EINVAL);
        if (nr_segs > fast_segs) {
                iov = kmalloc_array(nr_segs, sizeof(struct iovec), GFP_KERNEL);
                if (!iov)
                        return ERR_PTR(-ENOMEM);
        }

        if (unlikely(compat))
                ret = copy_compat_iovec_from_user(iov, uvec, nr_segs);
        else
                ret = copy_iovec_from_user(iov, uvec, nr_segs);
        if (ret) {
                if (iov != fast_iov)
                        kfree(iov);
                return ERR_PTR(ret);
        }

        return iov;
}

/*
 * Single segment iovec supplied by the user, import it as ITER_UBUF.
 */
static ssize_t __import_iovec_ubuf(int type, const struct iovec __user *uvec,
                                   struct iovec **iovp, struct iov_iter *i,
                                   bool compat)
{
        struct iovec *iov = *iovp;
        ssize_t ret;

        if (compat)
                ret = copy_compat_iovec_from_user(iov, uvec, 1);
        else
                ret = copy_iovec_from_user(iov, uvec, 1);
        if (unlikely(ret))
                return ret;

        ret = import_ubuf(type, iov->iov_base, iov->iov_len, i);
        if (unlikely(ret))
                return ret;
        *iovp = NULL;
        return i->count;
}

ssize_t __import_iovec(int type, const struct iovec __user *uvec,
                 unsigned nr_segs, unsigned fast_segs, struct iovec **iovp,
                 struct iov_iter *i, bool compat)
{
        ssize_t total_len = 0;
        unsigned long seg;
        struct iovec *iov;

        if (nr_segs == 1)
                return __import_iovec_ubuf(type, uvec, iovp, i, compat);

        iov = iovec_from_user(uvec, nr_segs, fast_segs, *iovp, compat);
        if (IS_ERR(iov)) {
                *iovp = NULL;
                return PTR_ERR(iov);
        }

        /*
         * According to the Single Unix Specification we should return EINVAL if
         * an element length is < 0 when cast to ssize_t or if the total length
         * would overflow the ssize_t return value of the system call.
         *
         * Linux caps all read/write calls to MAX_RW_COUNT, and avoids the
         * overflow case.
         */
        for (seg = 0; seg < nr_segs; seg++) {
                ssize_t len = (ssize_t)iov[seg].iov_len;

                if (!access_ok(iov[seg].iov_base, len)) {
                        if (iov != *iovp)
                                kfree(iov);
                        *iovp = NULL;
                        return -EFAULT;
                }

                if (len > MAX_RW_COUNT - total_len) {
                        len = MAX_RW_COUNT - total_len;
                        iov[seg].iov_len = len;
                }
                total_len += len;
        }

        iov_iter_init(i, type, iov, nr_segs, total_len);
        if (iov == *iovp)
                *iovp = NULL;
        else
                *iovp = iov;
        return total_len;
}

/**
 * import_iovec() - Copy an array of &struct iovec from userspace
 *     into the kernel, check that it is valid, and initialize a new
 *     &struct iov_iter iterator to access it.
 *
 * @type: One of %READ or %WRITE.
 * @uvec: Pointer to the userspace array.
 * @nr_segs: Number of elements in userspace array.
 * @fast_segs: Number of elements in @iov.
 * @iovp: (input and output parameter) Pointer to pointer to (usually small
 *     on-stack) kernel array.
 * @i: Pointer to iterator that will be initialized on success.
 *
 * If the array pointed to by *@iov is large enough to hold all @nr_segs,
 * then this function places %NULL in *@iov on return. Otherwise, a new
 * array will be allocated and the result placed in *@iov. This means that
 * the caller may call kfree() on *@iov regardless of whether the small
 * on-stack array was used or not (and regardless of whether this function
 * returns an error or not).
 *
 * Return: Negative error code on error, bytes imported on success
 */
ssize_t import_iovec(int type, const struct iovec __user *uvec,
                 unsigned nr_segs, unsigned fast_segs,
                 struct iovec **iovp, struct iov_iter *i)
{
        return __import_iovec(type, uvec, nr_segs, fast_segs, iovp, i,
                              in_compat_syscall());
}
EXPORT_SYMBOL(import_iovec);

int import_single_range(int rw, void __user *buf, size_t len,
                 struct iovec *iov, struct iov_iter *i)
{
        if (len > MAX_RW_COUNT)
                len = MAX_RW_COUNT;
        if (unlikely(!access_ok(buf, len)))
                return -EFAULT;

        iov_iter_ubuf(i, rw, buf, len);
        return 0;
}
EXPORT_SYMBOL(import_single_range);

int import_ubuf(int rw, void __user *buf, size_t len, struct iov_iter *i)
{
        if (len > MAX_RW_COUNT)
                len = MAX_RW_COUNT;
        if (unlikely(!access_ok(buf, len)))
                return -EFAULT;

        iov_iter_ubuf(i, rw, buf, len);
        return 0;
}
EXPORT_SYMBOL_GPL(import_ubuf);

/**
 * iov_iter_restore() - Restore a &struct iov_iter to the same state as when
 *     iov_iter_save_state() was called.
 *
 * @i: &struct iov_iter to restore
 * @state: state to restore from
 *
 * Used after iov_iter_save_state() to bring restore @i, if operations may
 * have advanced it.
 *
 * Note: only works on ITER_IOVEC, ITER_BVEC, and ITER_KVEC
 */
void iov_iter_restore(struct iov_iter *i, struct iov_iter_state *state)
{
        if (WARN_ON_ONCE(!iov_iter_is_bvec(i) && !iter_is_iovec(i) &&
                         !iter_is_ubuf(i)) && !iov_iter_is_kvec(i))
                return;
        i->iov_offset = state->iov_offset;
        i->count = state->count;
        if (iter_is_ubuf(i))
                return;
        /*
         * For the *vec iters, nr_segs + iov is constant - if we increment
         * the vec, then we also decrement the nr_segs count. Hence we don't
         * need to track both of these, just one is enough and we can deduct
         * the other from that. ITER_KVEC and ITER_IOVEC are the same struct
         * size, so we can just increment the iov pointer as they are unionzed.
         * ITER_BVEC _may_ be the same size on some archs, but on others it is
         * not. Be safe and handle it separately.
         */
        BUILD_BUG_ON(sizeof(struct iovec) != sizeof(struct kvec));
        if (iov_iter_is_bvec(i))
                i->bvec -= state->nr_segs - i->nr_segs;
        else
                i->__iov -= state->nr_segs - i->nr_segs;
        i->nr_segs = state->nr_segs;
}

/*
 * Extract a list of contiguous pages from an ITER_XARRAY iterator.  This does not
 * get references on the pages, nor does it get a pin on them.
 */
static ssize_t iov_iter_extract_xarray_pages(struct iov_iter *i,
                                             struct page ***pages, size_t maxsize,
                                             unsigned int maxpages,
                                             iov_iter_extraction_t extraction_flags,
                                             size_t *offset0)
{
        struct page *page, **p;
        unsigned int nr = 0, offset;
        loff_t pos = i->xarray_start + i->iov_offset;
        pgoff_t index = pos >> PAGE_SHIFT;
        XA_STATE(xas, i->xarray, index);

        offset = pos & ~PAGE_MASK;
        *offset0 = offset;

        maxpages = want_pages_array(pages, maxsize, offset, maxpages);
        if (!maxpages)
                return -ENOMEM;
        p = *pages;

        rcu_read_lock();
        for (page = xas_load(&xas); page; page = xas_next(&xas)) {
                if (xas_retry(&xas, page))
                        continue;

                /* Has the page moved or been split? */
                if (unlikely(page != xas_reload(&xas))) {
                        xas_reset(&xas);
                        continue;
                }

                p[nr++] = find_subpage(page, xas.xa_index);
                if (nr == maxpages)
                        break;
        }
        rcu_read_unlock();

        maxsize = min_t(size_t, nr * PAGE_SIZE - offset, maxsize);
        iov_iter_advance(i, maxsize);
        return maxsize;
}

/*
 * Extract a list of contiguous pages from an ITER_BVEC iterator.  This does
 * not get references on the pages, nor does it get a pin on them.
 */
static ssize_t iov_iter_extract_bvec_pages(struct iov_iter *i,
                                           struct page ***pages, size_t maxsize,
                                           unsigned int maxpages,
                                           iov_iter_extraction_t extraction_flags,
                                           size_t *offset0)
{
        struct page **p, *page;
        size_t skip = i->iov_offset, offset;
        int k;

        for (;;) {
                if (i->nr_segs == 0)
                        return 0;
                maxsize = min(maxsize, i->bvec->bv_len - skip);
                if (maxsize)
                        break;
                i->iov_offset = 0;
                i->nr_segs--;
                i->bvec++;
                skip = 0;
        }

        skip += i->bvec->bv_offset;
        page = i->bvec->bv_page + skip / PAGE_SIZE;
        offset = skip % PAGE_SIZE;
        *offset0 = offset;

        maxpages = want_pages_array(pages, maxsize, offset, maxpages);
        if (!maxpages)
                return -ENOMEM;
        p = *pages;
        for (k = 0; k < maxpages; k++)
                p[k] = page + k;

        maxsize = min_t(size_t, maxsize, maxpages * PAGE_SIZE - offset);
        iov_iter_advance(i, maxsize);
        return maxsize;
}

/*
 * Extract a list of virtually contiguous pages from an ITER_KVEC iterator.
 * This does not get references on the pages, nor does it get a pin on them.
 */
static ssize_t iov_iter_extract_kvec_pages(struct iov_iter *i,
                                           struct page ***pages, size_t maxsize,
                                           unsigned int maxpages,
                                           iov_iter_extraction_t extraction_flags,
                                           size_t *offset0)
{
        struct page **p, *page;
        const void *kaddr;
        size_t skip = i->iov_offset, offset, len;
        int k;

        for (;;) {
                if (i->nr_segs == 0)
                        return 0;
                maxsize = min(maxsize, i->kvec->iov_len - skip);
                if (maxsize)
                        break;
                i->iov_offset = 0;
                i->nr_segs--;
                i->kvec++;
                skip = 0;
        }

        kaddr = i->kvec->iov_base + skip;
        offset = (unsigned long)kaddr & ~PAGE_MASK;
        *offset0 = offset;

        maxpages = want_pages_array(pages, maxsize, offset, maxpages);
        if (!maxpages)
                return -ENOMEM;
        p = *pages;

        kaddr -= offset;
        len = offset + maxsize;
        for (k = 0; k < maxpages; k++) {
                size_t seg = min_t(size_t, len, PAGE_SIZE);

                if (is_vmalloc_or_module_addr(kaddr))
                        page = vmalloc_to_page(kaddr);
                else
                        page = virt_to_page(kaddr);

                p[k] = page;
                len -= seg;
                kaddr += PAGE_SIZE;
        }

        maxsize = min_t(size_t, maxsize, maxpages * PAGE_SIZE - offset);
        iov_iter_advance(i, maxsize);
        return maxsize;
}

/*
 * Extract a list of contiguous pages from a user iterator and get a pin on
 * each of them.  This should only be used if the iterator is user-backed
 * (IOBUF/UBUF).
 *
 * It does not get refs on the pages, but the pages must be unpinned by the
 * caller once the transfer is complete.
 *
 * This is safe to be used where background IO/DMA *is* going to be modifying
 * the buffer; using a pin rather than a ref makes forces fork() to give the
 * child a copy of the page.
 */
static ssize_t iov_iter_extract_user_pages(struct iov_iter *i,
                                           struct page ***pages,
                                           size_t maxsize,
                                           unsigned int maxpages,
                                           iov_iter_extraction_t extraction_flags,
                                           size_t *offset0)
{
        unsigned long addr;
        unsigned int gup_flags = 0;
        size_t offset;
        int res;

        if (i->data_source == ITER_DEST)
                gup_flags |= FOLL_WRITE;
        if (extraction_flags & ITER_ALLOW_P2PDMA)
                gup_flags |= FOLL_PCI_P2PDMA;
        if (i->nofault)
                gup_flags |= FOLL_NOFAULT;

        addr = first_iovec_segment(i, &maxsize);
        *offset0 = offset = addr % PAGE_SIZE;
        addr &= PAGE_MASK;
        maxpages = want_pages_array(pages, maxsize, offset, maxpages);
        if (!maxpages)
                return -ENOMEM;
        res = pin_user_pages_fast(addr, maxpages, gup_flags, *pages);
        if (unlikely(res <= 0))
                return res;
        maxsize = min_t(size_t, maxsize, res * PAGE_SIZE - offset);
        iov_iter_advance(i, maxsize);
        return maxsize;
}

/**
 * iov_iter_extract_pages - Extract a list of contiguous pages from an iterator
 * @i: The iterator to extract from
 * @pages: Where to return the list of pages
 * @maxsize: The maximum amount of iterator to extract
 * @maxpages: The maximum size of the list of pages
 * @extraction_flags: Flags to qualify request
 * @offset0: Where to return the starting offset into (*@pages)[0]
 *
 * Extract a list of contiguous pages from the current point of the iterator,
 * advancing the iterator.  The maximum number of pages and the maximum amount
 * of page contents can be set.
 *
 * If *@pages is NULL, a page list will be allocated to the required size and
 * *@pages will be set to its base.  If *@pages is not NULL, it will be assumed
 * that the caller allocated a page list at least @maxpages in size and this
 * will be filled in.
 *
 * @extraction_flags can have ITER_ALLOW_P2PDMA set to request peer-to-peer DMA
 * be allowed on the pages extracted.
 *
 * The iov_iter_extract_will_pin() function can be used to query how cleanup
 * should be performed.
 *
 * Extra refs or pins on the pages may be obtained as follows:
 *
 *  (*) If the iterator is user-backed (ITER_IOVEC/ITER_UBUF), pins will be
 *      added to the pages, but refs will not be taken.
 *      iov_iter_extract_will_pin() will return true.
 *
 *  (*) If the iterator is ITER_KVEC, ITER_BVEC or ITER_XARRAY, the pages are
 *      merely listed; no extra refs or pins are obtained.
 *      iov_iter_extract_will_pin() will return 0.
 *
 * Note also:
 *
 *  (*) Use with ITER_DISCARD is not supported as that has no content.
 *
 * On success, the function sets *@pages to the new pagelist, if allocated, and
 * sets *offset0 to the offset into the first page.
 *
 * It may also return -ENOMEM and -EFAULT.
 */
ssize_t iov_iter_extract_pages(struct iov_iter *i,
                               struct page ***pages,
                               size_t maxsize,
                               unsigned int maxpages,
                               iov_iter_extraction_t extraction_flags,
                               size_t *offset0)
{
        maxsize = min_t(size_t, min_t(size_t, maxsize, i->count), MAX_RW_COUNT);
        if (!maxsize)
                return 0;

        if (likely(user_backed_iter(i)))
                return iov_iter_extract_user_pages(i, pages, maxsize,
                                                   maxpages, extraction_flags,
                                                   offset0);
        if (iov_iter_is_kvec(i))
                return iov_iter_extract_kvec_pages(i, pages, maxsize,
                                                   maxpages, extraction_flags,
                                                   offset0);
        if (iov_iter_is_bvec(i))
                return iov_iter_extract_bvec_pages(i, pages, maxsize,
                                                   maxpages, extraction_flags,
                                                   offset0);
        if (iov_iter_is_xarray(i))
                return iov_iter_extract_xarray_pages(i, pages, maxsize,
                                                     maxpages, extraction_flags,
                                                     offset0);
        return -EFAULT;
}
EXPORT_SYMBOL_GPL(iov_iter_extract_pages);