Merge pull request #9193 from keszybz/coverity

[thirdparty/systemd.git] / src / basic / copy.c

/* SPDX-License-Identifier: LGPL-2.1+ */
/***
  This file is part of systemd.

  Copyright 2014 Lennart Poettering
***/

#include <dirent.h>
#include <errno.h>
#include <fcntl.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/sendfile.h>
#include <sys/stat.h>
#include <sys/xattr.h>
#include <time.h>
#include <unistd.h>

#include "alloc-util.h"
#include "btrfs-util.h"
#include "chattr-util.h"
#include "copy.h"
#include "dirent-util.h"
#include "fd-util.h"
#include "fileio.h"
#include "fs-util.h"
#include "io-util.h"
#include "macro.h"
#include "missing.h"
#include "mount-util.h"
#include "string-util.h"
#include "strv.h"
#include "time-util.h"
#include "umask-util.h"
#include "user-util.h"
#include "xattr-util.h"

#define COPY_BUFFER_SIZE (16U*1024U)

/* A safety net for descending recursively into file system trees to copy. On Linux PATH_MAX is 4096, which means the
 * deepest valid path one can build is around 2048, which we hence use as a safety net here, to not spin endlessly in
 * case of bind mount cycles and suchlike. */
#define COPY_DEPTH_MAX 2048U

static ssize_t try_copy_file_range(
                int fd_in, loff_t *off_in,
                int fd_out, loff_t *off_out,
                size_t len,
                unsigned int flags) {

        static int have = -1;
        ssize_t r;

        if (have == 0)
                return -ENOSYS;

        r = copy_file_range(fd_in, off_in, fd_out, off_out, len, flags);
        if (have < 0)
                have = r >= 0 || errno != ENOSYS;
        if (r < 0)
                return -errno;

        return r;
}

enum {
        FD_IS_NO_PIPE,
        FD_IS_BLOCKING_PIPE,
        FD_IS_NONBLOCKING_PIPE,
};

static int fd_is_nonblock_pipe(int fd) {
        struct stat st;
        int flags;

        /* Checks whether the specified file descriptor refers to a pipe, and if so if O_NONBLOCK is set. */

        if (fstat(fd, &st) < 0)
                return -errno;

        if (!S_ISFIFO(st.st_mode))
                return FD_IS_NO_PIPE;

        flags = fcntl(fd, F_GETFL);
        if (flags < 0)
                return -errno;

        return FLAGS_SET(flags, O_NONBLOCK) ? FD_IS_NONBLOCKING_PIPE : FD_IS_BLOCKING_PIPE;
}

int copy_bytes_full(
                int fdf, int fdt,
                uint64_t max_bytes,
                CopyFlags copy_flags,
                void **ret_remains,
                size_t *ret_remains_size) {

        bool try_cfr = true, try_sendfile = true, try_splice = true;
        int r, nonblock_pipe = -1;
        size_t m = SSIZE_MAX; /* that is the maximum that sendfile and c_f_r accept */

        assert(fdf >= 0);
        assert(fdt >= 0);

        /* Tries to copy bytes from the file descriptor 'fdf' to 'fdt' in the smartest possible way. Copies a maximum
         * of 'max_bytes', which may be specified as UINT64_MAX, in which no maximum is applied. Returns negative on
         * error, zero if EOF is hit before the bytes limit is hit and positive otherwise. If the copy fails for some
         * reason but we read but didn't yet write some data an ret_remains/ret_remains_size is not NULL, then it will
         * be initialized with an allocated buffer containing this "remaining" data. Note that these two parameters are
         * initialized with a valid buffer only on failure and only if there's actually data already read. Otherwise
         * these parameters if non-NULL are set to NULL. */

        if (ret_remains)
                *ret_remains = NULL;
        if (ret_remains_size)
                *ret_remains_size = 0;

        /* Try btrfs reflinks first. This only works on regular, seekable files, hence let's check the file offsets of
         * source and destination first. */
        if ((copy_flags & COPY_REFLINK)) {
                off_t foffset;

                foffset = lseek(fdf, 0, SEEK_CUR);
                if (foffset >= 0) {
                        off_t toffset;

                        toffset = lseek(fdt, 0, SEEK_CUR);
                        if (toffset >= 0) {

                                if (foffset == 0 && toffset == 0 && max_bytes == UINT64_MAX)
                                        r = btrfs_reflink(fdf, fdt); /* full file reflink */
                                else
                                        r = btrfs_clone_range(fdf, foffset, fdt, toffset, max_bytes == UINT64_MAX ? 0 : max_bytes); /* partial reflink */
                                if (r >= 0) {
                                        off_t t;

                                        /* This worked, yay! Now — to be fully correct — let's adjust the file pointers */
                                        if (max_bytes == UINT64_MAX) {

                                                /* We cloned to the end of the source file, let's position the read
                                                 * pointer there, and query it at the same time. */
                                                t = lseek(fdf, 0, SEEK_END);
                                                if (t < 0)
                                                        return -errno;
                                                if (t < foffset)
                                                        return -ESPIPE;

                                                /* Let's adjust the destination file write pointer by the same number
                                                 * of bytes. */
                                                t = lseek(fdt, toffset + (t - foffset), SEEK_SET);
                                                if (t < 0)
                                                        return -errno;

                                                return 0; /* we copied the whole thing, hence hit EOF, return 0 */
                                        } else {
                                                t = lseek(fdf, foffset + max_bytes, SEEK_SET);
                                                if (t < 0)
                                                        return -errno;

                                                t = lseek(fdt, toffset + max_bytes, SEEK_SET);
                                                if (t < 0)
                                                        return -errno;

                                                return 1; /* we copied only some number of bytes, which worked, but this means we didn't hit EOF, return 1 */
                                        }
                                }

                                log_debug_errno(r, "Reflinking didn't work, falling back to non-reflink copying: %m");
                        }
                }
        }

        for (;;) {
                ssize_t n;

                if (max_bytes <= 0)
                        return 1; /* return > 0 if we hit the max_bytes limit */

                if (max_bytes != UINT64_MAX && m > max_bytes)
                        m = max_bytes;

                /* First try copy_file_range(), unless we already tried */
                if (try_cfr) {
                        n = try_copy_file_range(fdf, NULL, fdt, NULL, m, 0u);
                        if (n < 0) {
                                if (!IN_SET(n, -EINVAL, -ENOSYS, -EXDEV, -EBADF))
                                        return n;

                                try_cfr = false;
                                /* use fallback below */
                        } else if (n == 0) /* EOF */
                                break;
                        else
                                /* Success! */
                                goto next;
                }

                /* First try sendfile(), unless we already tried */
                if (try_sendfile) {
                        n = sendfile(fdt, fdf, NULL, m);
                        if (n < 0) {
                                if (!IN_SET(errno, EINVAL, ENOSYS))
                                        return -errno;

                                try_sendfile = false;
                                /* use fallback below */
                        } else if (n == 0) /* EOF */
                                break;
                        else
                                /* Success! */
                                goto next;
                }

                /* Then try splice, unless we already tried. */
                if (try_splice) {

                        /* splice()'s asynchronous I/O support is a bit weird. When it encounters a pipe file
                         * descriptor, then it will ignore its O_NONBLOCK flag and instead only honour the
                         * SPLICE_F_NONBLOCK flag specified in its flag parameter. Let's hide this behaviour here, and
                         * check if either of the specified fds are a pipe, and if so, let's pass the flag
                         * automatically, depending on O_NONBLOCK being set.
                         *
                         * Here's a twist though: when we use it to move data between two pipes of which one has
                         * O_NONBLOCK set and the other has not, then we have no individual control over O_NONBLOCK
                         * behaviour. Hence in that case we can't use splice() and still guarantee systematic
                         * O_NONBLOCK behaviour, hence don't. */

                        if (nonblock_pipe < 0) {
                                int a, b;

                                /* Check if either of these fds is a pipe, and if so non-blocking or not */
                                a = fd_is_nonblock_pipe(fdf);
                                if (a < 0)
                                        return a;

                                b = fd_is_nonblock_pipe(fdt);
                                if (b < 0)
                                        return b;

                                if ((a == FD_IS_NO_PIPE && b == FD_IS_NO_PIPE) ||
                                    (a == FD_IS_BLOCKING_PIPE && b == FD_IS_NONBLOCKING_PIPE) ||
                                    (a == FD_IS_NONBLOCKING_PIPE && b == FD_IS_BLOCKING_PIPE))

                                        /* splice() only works if one of the fds is a pipe. If neither is, let's skip
                                         * this step right-away. As mentioned above, if one of the two fds refers to a
                                         * blocking pipe and the other to a non-blocking pipe, we can't use splice()
                                         * either, hence don't try either. This hence means we can only use splice() if
                                         * either only one of the two fds is a pipe, or if both are pipes with the same
                                         * nonblocking flag setting. */

                                        try_splice = false;
                                else
                                        nonblock_pipe = a == FD_IS_NONBLOCKING_PIPE || b == FD_IS_NONBLOCKING_PIPE;
                        }
                }

                if (try_splice) {
                        n = splice(fdf, NULL, fdt, NULL, m, nonblock_pipe ? SPLICE_F_NONBLOCK : 0);
                        if (n < 0) {
                                if (!IN_SET(errno, EINVAL, ENOSYS))
                                        return -errno;

                                try_splice = false;
                                /* use fallback below */
                        } else if (n == 0) /* EOF */
                                break;
                        else
                                /* Success! */
                                goto next;
                }

                /* As a fallback just copy bits by hand */
                {
                        uint8_t buf[MIN(m, COPY_BUFFER_SIZE)], *p = buf;
                        ssize_t z;

                        n = read(fdf, buf, sizeof buf);
                        if (n < 0)
                                return -errno;
                        if (n == 0) /* EOF */
                                break;

                        z = (size_t) n;
                        do {
                                ssize_t k;

                                k = write(fdt, p, z);
                                if (k < 0) {
                                        r = -errno;

                                        if (ret_remains) {
                                                void *copy;

                                                copy = memdup(p, z);
                                                if (!copy)
                                                        return -ENOMEM;

                                                *ret_remains = copy;
                                        }

                                        if (ret_remains_size)
                                                *ret_remains_size = z;

                                        return r;
                                }

                                assert(k <= z);
                                z -= k;
                                p += k;
                        } while (z > 0);
                }

        next:
                if (max_bytes != (uint64_t) -1) {
                        assert(max_bytes >= (uint64_t) n);
                        max_bytes -= n;
                }
                /* sendfile accepts at most SSIZE_MAX-offset bytes to copy,
                 * so reduce our maximum by the amount we already copied,
                 * but don't go below our copy buffer size, unless we are
                 * close the limit of bytes we are allowed to copy. */
                m = MAX(MIN(COPY_BUFFER_SIZE, max_bytes), m - n);
        }

        return 0; /* return 0 if we hit EOF earlier than the size limit */
}

static int fd_copy_symlink(
                int df,
                const char *from,
                const struct stat *st,
                int dt,
                const char *to,
                uid_t override_uid,
                gid_t override_gid,
                CopyFlags copy_flags) {

        _cleanup_free_ char *target = NULL;
        int r;

        assert(from);
        assert(st);
        assert(to);

        r = readlinkat_malloc(df, from, &target);
        if (r < 0)
                return r;

        if (symlinkat(target, dt, to) < 0)
                return -errno;

        if (fchownat(dt, to,
                     uid_is_valid(override_uid) ? override_uid : st->st_uid,
                     gid_is_valid(override_gid) ? override_gid : st->st_gid,
                     AT_SYMLINK_NOFOLLOW) < 0)
                return -errno;

        return 0;
}

static int fd_copy_regular(
                int df,
                const char *from,
                const struct stat *st,
                int dt,
                const char *to,
                uid_t override_uid,
                gid_t override_gid,
                CopyFlags copy_flags) {

        _cleanup_close_ int fdf = -1, fdt = -1;
        struct timespec ts[2];
        int r, q;

        assert(from);
        assert(st);
        assert(to);

        fdf = openat(df, from, O_RDONLY|O_CLOEXEC|O_NOCTTY|O_NOFOLLOW);
        if (fdf < 0)
                return -errno;

        fdt = openat(dt, to, O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC|O_NOCTTY|O_NOFOLLOW, st->st_mode & 07777);
        if (fdt < 0)
                return -errno;

        r = copy_bytes(fdf, fdt, (uint64_t) -1, copy_flags);
        if (r < 0) {
                (void) unlinkat(dt, to, 0);
                return r;
        }

        if (fchown(fdt,
                   uid_is_valid(override_uid) ? override_uid : st->st_uid,
                   gid_is_valid(override_gid) ? override_gid : st->st_gid) < 0)
                r = -errno;

        if (fchmod(fdt, st->st_mode & 07777) < 0)
                r = -errno;

        ts[0] = st->st_atim;
        ts[1] = st->st_mtim;
        (void) futimens(fdt, ts);
        (void) copy_xattr(fdf, fdt);

        q = close(fdt);
        fdt = -1;

        if (q < 0) {
                r = -errno;
                (void) unlinkat(dt, to, 0);
        }

        return r;
}

static int fd_copy_fifo(
                int df,
                const char *from,
                const struct stat *st,
                int dt,
                const char *to,
                uid_t override_uid,
                gid_t override_gid,
                CopyFlags copy_flags) {
        int r;

        assert(from);
        assert(st);
        assert(to);

        r = mkfifoat(dt, to, st->st_mode & 07777);
        if (r < 0)
                return -errno;

        if (fchownat(dt, to,
                     uid_is_valid(override_uid) ? override_uid : st->st_uid,
                     gid_is_valid(override_gid) ? override_gid : st->st_gid,
                     AT_SYMLINK_NOFOLLOW) < 0)
                r = -errno;

        if (fchmodat(dt, to, st->st_mode & 07777, 0) < 0)
                r = -errno;

        return r;
}

static int fd_copy_node(
                int df,
                const char *from,
                const struct stat *st,
                int dt,
                const char *to,
                uid_t override_uid,
                gid_t override_gid,
                CopyFlags copy_flags) {
        int r;

        assert(from);
        assert(st);
        assert(to);

        r = mknodat(dt, to, st->st_mode, st->st_rdev);
        if (r < 0)
                return -errno;

        if (fchownat(dt, to,
                     uid_is_valid(override_uid) ? override_uid : st->st_uid,
                     gid_is_valid(override_gid) ? override_gid : st->st_gid,
                     AT_SYMLINK_NOFOLLOW) < 0)
                r = -errno;

        if (fchmodat(dt, to, st->st_mode & 07777, 0) < 0)
                r = -errno;

        return r;
}

static int fd_copy_directory(
                int df,
                const char *from,
                const struct stat *st,
                int dt,
                const char *to,
                dev_t original_device,
                unsigned depth_left,
                uid_t override_uid,
                gid_t override_gid,
                CopyFlags copy_flags) {

        _cleanup_close_ int fdf = -1, fdt = -1;
        _cleanup_closedir_ DIR *d = NULL;
        struct dirent *de;
        bool created;
        int r;

        assert(st);
        assert(to);

        if (depth_left == 0)
                return -ENAMETOOLONG;

        if (from)
                fdf = openat(df, from, O_RDONLY|O_DIRECTORY|O_CLOEXEC|O_NOCTTY|O_NOFOLLOW);
        else
                fdf = fcntl(df, F_DUPFD_CLOEXEC, 3);
        if (fdf < 0)
                return -errno;

        d = fdopendir(fdf);
        if (!d)
                return -errno;
        fdf = -1;

        r = mkdirat(dt, to, st->st_mode & 07777);
        if (r >= 0)
                created = true;
        else if (errno == EEXIST && (copy_flags & COPY_MERGE))
                created = false;
        else
                return -errno;

        fdt = openat(dt, to, O_RDONLY|O_DIRECTORY|O_CLOEXEC|O_NOCTTY|O_NOFOLLOW);
        if (fdt < 0)
                return -errno;

        r = 0;

        FOREACH_DIRENT_ALL(de, d, return -errno) {
                struct stat buf;
                int q;

                if (dot_or_dot_dot(de->d_name))
                        continue;

                if (fstatat(dirfd(d), de->d_name, &buf, AT_SYMLINK_NOFOLLOW) < 0) {
                        r = -errno;
                        continue;
                }

                if (S_ISDIR(buf.st_mode)) {
                        /*
                         * Don't descend into directories on other file systems, if this is requested. We do a simple
                         * .st_dev check here, which basically comes for free. Note that we do this check only on
                         * directories, not other kind of file system objects, for two reason:
                         *
                         * • The kernel's overlayfs pseudo file system that overlays multiple real file systems
                         *   propagates the .st_dev field of the file system a file originates from all the way up
                         *   through the stack to stat(). It doesn't do that for directories however. This means that
                         *   comparing .st_dev on non-directories suggests that they all are mount points. To avoid
                         *   confusion we hence avoid relying on this check for regular files.
                         *
                         * • The main reason we do this check at all is to protect ourselves from bind mount cycles,
                         *   where we really want to avoid descending down in all eternity. However the .st_dev check
                         *   is usually not sufficient for this protection anyway, as bind mount cycles from the same
                         *   file system onto itself can't be detected that way. (Note we also do a recursion depth
                         *   check, which is probably the better protection in this regard, which is why
                         *   COPY_SAME_MOUNT is optional).
                         */

                        if (FLAGS_SET(copy_flags, COPY_SAME_MOUNT)) {
                                if (buf.st_dev != original_device)
                                        continue;

                                r = fd_is_mount_point(dirfd(d), de->d_name, 0);
                                if (r < 0)
                                        return r;
                                if (r > 0)
                                        continue;
                        }

                        q = fd_copy_directory(dirfd(d), de->d_name, &buf, fdt, de->d_name, original_device, depth_left-1, override_uid, override_gid, copy_flags);
                } else if (S_ISREG(buf.st_mode))
                        q = fd_copy_regular(dirfd(d), de->d_name, &buf, fdt, de->d_name, override_uid, override_gid, copy_flags);
                else if (S_ISLNK(buf.st_mode))
                        q = fd_copy_symlink(dirfd(d), de->d_name, &buf, fdt, de->d_name, override_uid, override_gid, copy_flags);
                else if (S_ISFIFO(buf.st_mode))
                        q = fd_copy_fifo(dirfd(d), de->d_name, &buf, fdt, de->d_name, override_uid, override_gid, copy_flags);
                else if (S_ISBLK(buf.st_mode) || S_ISCHR(buf.st_mode) || S_ISSOCK(buf.st_mode))
                        q = fd_copy_node(dirfd(d), de->d_name, &buf, fdt, de->d_name, override_uid, override_gid, copy_flags);
                else
                        q = -EOPNOTSUPP;

                if (q == -EEXIST && (copy_flags & COPY_MERGE))
                        q = 0;

                if (q < 0)
                        r = q;
        }

        if (created) {
                struct timespec ut[2] = {
                        st->st_atim,
                        st->st_mtim
                };

                if (fchown(fdt,
                           uid_is_valid(override_uid) ? override_uid : st->st_uid,
                           gid_is_valid(override_gid) ? override_gid : st->st_gid) < 0)
                        r = -errno;

                if (fchmod(fdt, st->st_mode & 07777) < 0)
                        r = -errno;

                (void) copy_xattr(dirfd(d), fdt);
                (void) futimens(fdt, ut);
        }

        return r;
}

int copy_tree_at(int fdf, const char *from, int fdt, const char *to, uid_t override_uid, gid_t override_gid, CopyFlags copy_flags) {
        struct stat st;

        assert(from);
        assert(to);

        if (fstatat(fdf, from, &st, AT_SYMLINK_NOFOLLOW) < 0)
                return -errno;

        if (S_ISREG(st.st_mode))
                return fd_copy_regular(fdf, from, &st, fdt, to, override_uid, override_gid, copy_flags);
        else if (S_ISDIR(st.st_mode))
                return fd_copy_directory(fdf, from, &st, fdt, to, st.st_dev, COPY_DEPTH_MAX, override_uid, override_gid, copy_flags);
        else if (S_ISLNK(st.st_mode))
                return fd_copy_symlink(fdf, from, &st, fdt, to, override_uid, override_gid, copy_flags);
        else if (S_ISFIFO(st.st_mode))
                return fd_copy_fifo(fdf, from, &st, fdt, to, override_uid, override_gid, copy_flags);
        else if (S_ISBLK(st.st_mode) || S_ISCHR(st.st_mode) || S_ISSOCK(st.st_mode))
                return fd_copy_node(fdf, from, &st, fdt, to, override_uid, override_gid, copy_flags);
        else
                return -EOPNOTSUPP;
}

int copy_tree(const char *from, const char *to, uid_t override_uid, gid_t override_gid, CopyFlags copy_flags) {
        return copy_tree_at(AT_FDCWD, from, AT_FDCWD, to, override_uid, override_gid, copy_flags);
}

int copy_directory_fd(int dirfd, const char *to, CopyFlags copy_flags) {
        struct stat st;

        assert(dirfd >= 0);
        assert(to);

        if (fstat(dirfd, &st) < 0)
                return -errno;

        if (!S_ISDIR(st.st_mode))
                return -ENOTDIR;

        return fd_copy_directory(dirfd, NULL, &st, AT_FDCWD, to, st.st_dev, COPY_DEPTH_MAX, UID_INVALID, GID_INVALID, copy_flags);
}

int copy_directory(const char *from, const char *to, CopyFlags copy_flags) {
        struct stat st;

        assert(from);
        assert(to);

        if (lstat(from, &st) < 0)
                return -errno;

        if (!S_ISDIR(st.st_mode))
                return -ENOTDIR;

        return fd_copy_directory(AT_FDCWD, from, &st, AT_FDCWD, to, st.st_dev, COPY_DEPTH_MAX, UID_INVALID, GID_INVALID, copy_flags);
}

int copy_file_fd(const char *from, int fdt, CopyFlags copy_flags) {
        _cleanup_close_ int fdf = -1;
        int r;

        assert(from);
        assert(fdt >= 0);

        fdf = open(from, O_RDONLY|O_CLOEXEC|O_NOCTTY);
        if (fdf < 0)
                return -errno;

        r = copy_bytes(fdf, fdt, (uint64_t) -1, copy_flags);

        (void) copy_times(fdf, fdt);
        (void) copy_xattr(fdf, fdt);

        return r;
}

int copy_file(const char *from, const char *to, int flags, mode_t mode, unsigned chattr_flags, CopyFlags copy_flags) {
        int fdt = -1, r;

        assert(from);
        assert(to);

        RUN_WITH_UMASK(0000) {
                fdt = open(to, flags|O_WRONLY|O_CREAT|O_CLOEXEC|O_NOCTTY, mode);
                if (fdt < 0)
                        return -errno;
        }

        if (chattr_flags != 0)
                (void) chattr_fd(fdt, chattr_flags, (unsigned) -1);

        r = copy_file_fd(from, fdt, copy_flags);
        if (r < 0) {
                close(fdt);
                (void) unlink(to);
                return r;
        }

        if (close(fdt) < 0) {
                unlink_noerrno(to);
                return -errno;
        }

        return 0;
}

int copy_file_atomic(const char *from, const char *to, mode_t mode, unsigned chattr_flags, CopyFlags copy_flags) {
        _cleanup_free_ char *t = NULL;
        int r;

        assert(from);
        assert(to);

        r = tempfn_random(to, NULL, &t);
        if (r < 0)
                return r;

        r = copy_file(from, t, O_NOFOLLOW|O_EXCL, mode, chattr_flags, copy_flags);
        if (r < 0)
                return r;

        if (copy_flags & COPY_REPLACE) {
                r = renameat(AT_FDCWD, t, AT_FDCWD, to);
                if (r < 0)
                        r = -errno;
        } else
                r = rename_noreplace(AT_FDCWD, t, AT_FDCWD, to);
        if (r < 0) {
                (void) unlink(t);
                return r;
        }

        return 0;
}

int copy_times(int fdf, int fdt) {
        struct timespec ut[2];
        struct stat st;
        usec_t crtime = 0;

        assert(fdf >= 0);
        assert(fdt >= 0);

        if (fstat(fdf, &st) < 0)
                return -errno;

        ut[0] = st.st_atim;
        ut[1] = st.st_mtim;

        if (futimens(fdt, ut) < 0)
                return -errno;

        if (fd_getcrtime(fdf, &crtime) >= 0)
                (void) fd_setcrtime(fdt, crtime);

        return 0;
}

int copy_xattr(int fdf, int fdt) {
        _cleanup_free_ char *bufa = NULL, *bufb = NULL;
        size_t sza = 100, szb = 100;
        ssize_t n;
        int ret = 0;
        const char *p;

        for (;;) {
                bufa = malloc(sza);
                if (!bufa)
                        return -ENOMEM;

                n = flistxattr(fdf, bufa, sza);
                if (n == 0)
                        return 0;
                if (n > 0)
                        break;
                if (errno != ERANGE)
                        return -errno;

                sza *= 2;

                bufa = mfree(bufa);
        }

        p = bufa;
        while (n > 0) {
                size_t l;

                l = strlen(p);
                assert(l < (size_t) n);

                if (startswith(p, "user.")) {
                        ssize_t m;

                        if (!bufb) {
                                bufb = malloc(szb);
                                if (!bufb)
                                        return -ENOMEM;
                        }

                        m = fgetxattr(fdf, p, bufb, szb);
                        if (m < 0) {
                                if (errno == ERANGE) {
                                        szb *= 2;
                                        bufb = mfree(bufb);
                                        continue;
                                }

                                return -errno;
                        }

                        if (fsetxattr(fdt, p, bufb, m, 0) < 0)
                                ret = -errno;
                }

                p += l + 1;
                n -= l + 1;
        }

        return ret;
}