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libdm: create symlinks to real /dev nodes when using alternative dev dir

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When DM_DEV_DIR is set to an alternative directory (e.g., for test
environments) and fallback node creation is triggered, check if the
real /dev node already exists. If it does, create a symlink to it
instead of creating a duplicate device node with mknod().

This ensures that operations on nodes in the alternative directory
actually operate on the real /dev nodes, which properly triggers udev
events. Previously, duplicate nodes were created which didn't generate
udev events when accessed, causing test failures for udev-dependent
functionality.

Additionally, enhance symlink handling in device node creation:
- Detect when a valid symlink to the correct device already exists
  (using lstat to check if the path is a symlink after stat confirms
  it points to the right device)
- Clean up dangling symlinks before creating new nodes (when stat fails
  with ENOENT but lstat shows a symlink exists, indicating the target
  device no longer exists)
- Add comprehensive comments explaining stat() vs lstat() behavior and
  the symlink creation strategy

Benefits:
- wipefs and other operations on fake dev nodes now trigger udev events
- Test environment behaves more like production with udev integration
- Eliminates duplicate device nodes that can cause confusion
- Properly detects and reuses existing symlinks to avoid errors
- Cleans up stale symlinks from previous test runs
- lvmdbusd udev monitoring works correctly in test environment

Co-Authored-By: Claude <noreply@anthropic.com>
This commit is contained in:
Zdenek Kabelac
2025-10-27 13:25:07 +01:00
parent 0a16d082e0
commit 3942b356e5
2 changed files with 134 additions and 10 deletions
Download Patch File Download Diff File Expand all files Collapse all files

72
device_mapper/libdm-common.c
View File

@@ -1,6 +1,6 @@
/*
* Copyright (C) 2001-2004 Sistina Software, Inc. All rights reserved.
* Copyright (C) 2004-2012 Red Hat, Inc. All rights reserved.
* Copyright (C) 2004-2025 Red Hat, Inc. All rights reserved.
*
* This file is part of the device-mapper userspace tools.
*
@@ -1044,12 +1044,19 @@ static int _add_dev_node(const char *dev_name, uint32_t major, uint32_t minor,
{
char path[PATH_MAX];
struct stat info;
struct stat linfo;
dev_t dev = MKDEV(major, minor);
mode_t old_mask;
if (!_build_dev_path(path, sizeof(path), dev_name))
return_0;
/*
* Check if the device node already exists.
* Note: stat() follows symlinks, so this checks the target device,
* not the symlink itself. This works correctly for both real nodes
* and symlinks pointing to the right device.
*/
if (stat(path, &info) >= 0) {
if (!S_ISBLK(info.st_mode)) {
log_error("A non-block device file at '%s' "
@@ -1058,16 +1065,71 @@ static int _add_dev_node(const char *dev_name, uint32_t major, uint32_t minor,
}
/* If right inode already exists we don't touch uid etc. */
if (info.st_rdev == dev)
if (info.st_rdev == dev) {
/*
* Correct device exists (either as real node or symlink).
* Use lstat() to distinguish between them for logging.
*/
if (lstat(path, &linfo) >= 0 && S_ISLNK(linfo.st_mode))
log_debug_activation("Symlink %s to correct device already exists", path);
return 1;
}
if (unlink(path) && (errno != ENOENT)) {
log_sys_error("unlink", path);
return 0;
}
} else if (_warn_if_op_needed(warn_if_udev_failed))
log_warn("%s not set up by udev: Falling back to direct "
"node creation.", path);
} else {
/*
* stat() failed. Check for dangling symlinks (where lstat succeeds
* but stat fails with ENOENT because the symlink target doesn't exist).
* Remove the dangling symlink before attempting to create a new node.
*/
if (errno == ENOENT && lstat(path, &linfo) >= 0 && S_ISLNK(linfo.st_mode)) {
log_debug_activation("Removing dangling symlink %s", path);
if (unlink(path) && (errno != ENOENT)) {
log_sys_error("unlink", path);
return 0;
}
}
if (_warn_if_op_needed(warn_if_udev_failed))
log_warn("%s not set up by udev: Falling back to direct "
"node creation.", path);
}
/*
* Test environment optimization: If using alternative dev dir (e.g., /tmp/LVMTEST/dev)
* and the real /dev node already exists, create a symlink instead of a duplicate node.
* This ensures operations trigger udev events which only monitors /dev.
*/
if (strcmp(_dm_dir, DEV_DIR) != 0) {
char real_path[PATH_MAX];
struct stat real_stat;
/* Build path to real /dev node (kernel always creates /dev/dm-N) */
if (dm_snprintf(real_path, sizeof(real_path), DEV_DIR "dm-%u", minor) >= 0) {
/* Check if real node exists with matching dev */
if (stat(real_path, &real_stat) >= 0 &&
S_ISBLK(real_stat.st_mode) &&
real_stat.st_rdev == dev) {
/*
* Real /dev/dm-N exists. Create symlink from alternative location.
* This allows operations to work through the symlink and trigger
* udev events on the real device.
*/
log_debug_activation("Creating symlink %s -> %s", path, real_path);
(void) dm_prepare_selinux_context(path, S_IFLNK);
if (symlink(real_path, path) < 0) {
log_sys_error("symlink", path);
(void) dm_prepare_selinux_context(NULL, 0);
return 0;
}
(void) dm_prepare_selinux_context(NULL, 0);
log_debug_activation("Created symlink %s -> %s", path, real_path);
return 1;
}
}
}
(void) dm_prepare_selinux_context(path, S_IFBLK);
old_mask = umask(0);

72
libdm/libdm-common.c
View File

@@ -1,6 +1,6 @@
/*
* Copyright (C) 2001-2004 Sistina Software, Inc. All rights reserved.
* Copyright (C) 2004-2012 Red Hat, Inc. All rights reserved.
* Copyright (C) 2004-2025 Red Hat, Inc. All rights reserved.
*
* This file is part of the device-mapper userspace tools.
*
@@ -1042,12 +1042,19 @@ static int _add_dev_node(const char *dev_name, uint32_t major, uint32_t minor,
{
char path[PATH_MAX];
struct stat info;
struct stat linfo;
dev_t dev = MKDEV(major, minor);
mode_t old_mask;
if (!_build_dev_path(path, sizeof(path), dev_name))
return_0;
/*
* Check if the device node already exists.
* Note: stat() follows symlinks, so this checks the target device,
* not the symlink itself. This works correctly for both real nodes
* and symlinks pointing to the right device.
*/
if (stat(path, &info) >= 0) {
if (!S_ISBLK(info.st_mode)) {
log_error("A non-block device file at '%s' "
@@ -1056,16 +1063,71 @@ static int _add_dev_node(const char *dev_name, uint32_t major, uint32_t minor,
}
/* If right inode already exists we don't touch uid etc. */
if (info.st_rdev == dev)
if (info.st_rdev == dev) {
/*
* Correct device exists (either as real node or symlink).
* Use lstat() to distinguish between them for logging.
*/
if (lstat(path, &linfo) >= 0 && S_ISLNK(linfo.st_mode))
log_debug_activation("Symlink %s to correct device already exists", path);
return 1;
}
if (unlink(path) && (errno != ENOENT)) {
log_sys_error("unlink", path);
return 0;
}
} else if (_warn_if_op_needed(warn_if_udev_failed))
log_warn("%s not set up by udev: Falling back to direct "
"node creation.", path);
} else {
/*
* stat() failed. Check for dangling symlinks (where lstat succeeds
* but stat fails with ENOENT because the symlink target doesn't exist).
* Remove the dangling symlink before attempting to create a new node.
*/
if (errno == ENOENT && lstat(path, &linfo) >= 0 && S_ISLNK(linfo.st_mode)) {
log_debug_activation("Removing dangling symlink %s", path);
if (unlink(path) && (errno != ENOENT)) {
log_sys_error("unlink", path);
return 0;
}
}
if (_warn_if_op_needed(warn_if_udev_failed))
log_warn("%s not set up by udev: Falling back to direct "
"node creation.", path);
}
/*
* Test environment optimization: If using alternative dev dir (e.g., /tmp/LVMTEST/dev)
* and the real /dev node already exists, create a symlink instead of a duplicate node.
* This ensures operations trigger udev events which only monitors /dev.
*/
if (strcmp(_dm_dir, DEV_DIR) != 0) {
char real_path[PATH_MAX];
struct stat real_stat;
/* Build path to real /dev node (kernel always creates /dev/dm-N) */
if (dm_snprintf(real_path, sizeof(real_path), DEV_DIR "dm-%u", minor) >= 0) {
/* Check if real node exists with matching dev */
if (stat(real_path, &real_stat) >= 0 &&
S_ISBLK(real_stat.st_mode) &&
real_stat.st_rdev == dev) {
/*
* Real /dev/dm-N exists. Create symlink from alternative location.
* This allows operations to work through the symlink and trigger
* udev events on the real device.
*/
log_debug_activation("Creating symlink %s -> %s", path, real_path);
(void) dm_prepare_selinux_context(path, S_IFLNK);
if (symlink(real_path, path) < 0) {
log_sys_error("symlink", path);
(void) dm_prepare_selinux_context(NULL, 0);
return 0;
}
(void) dm_prepare_selinux_context(NULL, 0);
log_debug_activation("Created symlink %s -> %s", path, real_path);
return 1;
}
}
}
(void) dm_prepare_selinux_context(path, S_IFBLK);
old_mask = umask(0);