loop-util: use new LOOP_CONFIGURE ioctl

LOOP_CONFIGURE allows us to configure a loopback device in one ioctl
instead of two, which is not just faster but also removes the race that
udev might start probing the device before we adjusted things properly.

Unfortunately LOOP_CONFIGURE is broken in regards to LO_FLAGS_PARTSCAN
as of kernel 5.8.0. This patch contains a work-around for that, to
fallback to old behaviour if partition scanning is requested but does
not work. Sucks a bit.

Proposed upstream fix for that issue:

https://lkml.org/lkml/2020/8/6/97

diff --git a/src/basic/missing_loop.h b/src/basic/missing_loop.h
new file mode 100644 (file)
index 0000000..5326777
--- /dev/null
+++ b/src/basic/missing_loop.h
@@ -0,0 +1,15 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+#pragma once
+
+#include <linux/loop.h>
+
+#ifndef LOOP_CONFIGURE
+struct loop_config {
+        __u32 fd;
+        __u32 block_size;
+        struct loop_info64 info;
+        __u64 __reserved[8];
+};
+
+#define LOOP_CONFIGURE 0x4C0A
+#endif

diff --git a/src/shared/loop-util.c b/src/shared/loop-util.c
index 4a593b05f3c90073b6fd87e8ad60d66d040b7562..8db2fed66ff3ee4336e0a68678000e1f099998ad 100644 (file)
--- a/src/shared/loop-util.c
+++ b/src/shared/loop-util.c
@@ -14,20 +14,68 @@
 #include <unistd.h>
 
 #include "alloc-util.h"
+#include "blockdev-util.h"
 #include "errno-util.h"
 #include "fd-util.h"
 #include "fileio.h"
 #include "loop-util.h"
+#include "missing_loop.h"
 #include "parse-util.h"
 #include "stat-util.h"
 #include "stdio-util.h"
 #include "string-util.h"
 
 static void cleanup_clear_loop_close(int *fd) {
-        if (*fd >= 0) {
-                (void) ioctl(*fd, LOOP_CLR_FD);
-                (void) safe_close(*fd);
+        if (*fd < 0)
+                return;
+
+        (void) ioctl(*fd, LOOP_CLR_FD);
+        (void) safe_close(*fd);
+}
+
+static int loop_configure(int fd, const struct loop_config *c) {
+        int r;
+
+        assert(fd >= 0);
+        assert(c);
+
+        if (ioctl(fd, LOOP_CONFIGURE, c) < 0) {
+                /* Do fallback only if LOOP_CONFIGURE is not supported, propagate all other errors. Note that
+                 * the kernel is weird: non-existing ioctls currently return EINVAL rather than ENOTTY on
+                 * loopback block devices. They should fix that in the kernel, but in the meantime we accept
+                 * both here. */
+                if (!ERRNO_IS_NOT_SUPPORTED(errno) && errno != EINVAL)
+                        return -errno;
+        } else {
+                if (!FLAGS_SET(c->info.lo_flags, LO_FLAGS_PARTSCAN))
+                        return 0;
+
+                /* Kernel 5.8 vanilla doesn't properly propagate the partition scanning flag into the
+                 * block device. Let's hence verify if things work correctly here before returning. */
+
+                r = blockdev_partscan_enabled(fd);
+                if (r < 0)
+                        goto fail;
+                if (r > 0)
+                        return 0; /* All is good. */
+
+                /* Otherwise, undo the attachment and use the old APIs */
+                (void) ioctl(fd, LOOP_CLR_FD);
+        }
+
+        if (ioctl(fd, LOOP_SET_FD, c->fd) < 0)
+                return -errno;
+
+        if (ioctl(fd, LOOP_SET_STATUS64, &c->info) < 0) {
+                r = -errno;
+                goto fail;
         }
+
+        return 0;
+
+fail:
+        (void) ioctl(fd, LOOP_CLR_FD);
+        return r;
 }
 
 int loop_device_make(
@@ -39,7 +87,7 @@ int loop_device_make(
                 LoopDevice **ret) {
 
         _cleanup_free_ char *loopdev = NULL;
-        struct loop_info64 info;
+        struct loop_config config;
         LoopDevice *d = NULL;
         struct stat st;
         int nr = -1, r;
@@ -52,14 +100,14 @@ int loop_device_make(
                 return -errno;
 
         if (S_ISBLK(st.st_mode)) {
-                if (ioctl(fd, LOOP_GET_STATUS64, &info) >= 0) {
+                if (ioctl(fd, LOOP_GET_STATUS64, &config.info) >= 0) {
                         /* Oh! This is a loopback device? That's interesting! */
 
 #if HAVE_VALGRIND_MEMCHECK_H
                         /* Valgrind currently doesn't know LOOP_GET_STATUS64. Remove this once it does */
-                        VALGRIND_MAKE_MEM_DEFINED(&info, sizeof(info));
+                        VALGRIND_MAKE_MEM_DEFINED(&config.info, sizeof(config.info));
 #endif
-                        nr = info.lo_number;
+                        nr = config.info.lo_number;
 
                         if (asprintf(&loopdev, "/dev/loop%i", nr) < 0)
                                 return -ENOMEM;
@@ -100,6 +148,16 @@ int loop_device_make(
         if (control < 0)
                 return -errno;
 
+        config = (struct loop_config) {
+                .fd = fd,
+                .info = {
+                        /* Use the specified flags, but configure the read-only flag from the open flags, and force autoclear */
+                        .lo_flags = (loop_flags & ~LO_FLAGS_READ_ONLY) | ((loop_flags & O_ACCMODE) == O_RDONLY ? LO_FLAGS_READ_ONLY : 0) | LO_FLAGS_AUTOCLEAR,
+                        .lo_offset = offset,
+                        .lo_sizelimit = size == UINT64_MAX ? 0 : size,
+                },
+        };
+
         /* Loop around LOOP_CTL_GET_FREE, since at the moment we attempt to open the returned device it might
          * be gone already, taken by somebody else racing against us. */
         for (unsigned n_attempts = 0;;) {
@@ -119,12 +177,13 @@ int loop_device_make(
                         if (errno != ENOENT)
                                 return -errno;
                 } else {
-                        if (ioctl(loop, LOOP_SET_FD, fd) >= 0) {
+                        r = loop_configure(loop, &config);
+                        if (r >= 0) {
                                 loop_with_fd = TAKE_FD(loop);
                                 break;
                         }
-                        if (errno != EBUSY)
-                                return -errno;
+                        if (r != -EBUSY)
+                                return r;
                 }
 
                 if (++n_attempts >= 64) /* Give up eventually */
@@ -133,16 +192,6 @@ int loop_device_make(
                 loopdev = mfree(loopdev);
         }
 
-        info = (struct loop_info64) {
-                /* Use the specified flags, but configure the read-only flag from the open flags, and force autoclear */
-                .lo_flags = (loop_flags & ~LO_FLAGS_READ_ONLY) | ((loop_flags & O_ACCMODE) == O_RDONLY ? LO_FLAGS_READ_ONLY : 0) | LO_FLAGS_AUTOCLEAR,
-                .lo_offset = offset,
-                .lo_sizelimit = size == UINT64_MAX ? 0 : size,
-        };
-
-        if (ioctl(loop_with_fd, LOOP_SET_STATUS64, &info) < 0)
-                return -errno;
-
         d = new(LoopDevice, 1);
         if (!d)
                 return -ENOMEM;