From 16a6007cc8881ef19cc97de676d3b2b36b2def82 Mon Sep 17 00:00:00 2001
From: Yu Watanabe <watanabe.yu+github@gmail.com>
Date: Wed, 1 Sep 2021 12:57:40 +0900
Subject: [PATCH] udev-node: always update timestamp of stack directory
Please see the comments in the code.
(cherry picked from commit 6df797f75fa08bb1a9e657001229bd47903e6174)
Related: #1977994
---
src/udev/udev-node.c | 90 ++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 87 insertions(+), 3 deletions(-)
diff --git a/src/udev/udev-node.c b/src/udev/udev-node.c
index 5d6aae0bd4..0de848da19 100644
--- a/src/udev/udev-node.c
+++ b/src/udev/udev-node.c
@@ -32,6 +32,7 @@
#define CREATE_LINK_MAX_RETRIES 128
#define LINK_UPDATE_MAX_RETRIES 128
#define CREATE_STACK_LINK_MAX_RETRIES 128
+#define UPDATE_TIMESTAMP_MAX_RETRIES 128
#define UDEV_NODE_HASH_KEY SD_ID128_MAKE(b9,6a,f1,ce,40,31,44,1a,9e,19,ec,8b,ae,f3,e3,2f)
static int create_symlink(const char *target, const char *slink) {
@@ -285,9 +286,60 @@ toolong:
return size - 1;
}
+static int update_timestamp(sd_device *dev, const char *path, struct stat *prev) {
+ assert(path);
+ assert(prev);
+
+ /* Even if a symlink in the stack directory is created/removed, the mtime of the directory may
+ * not be changed. Why? Let's consider the following situation. For simplicity, let's assume
+ * there exist three udev workers (A, B, and C) and all of them calls link_update() for the
+ * same devlink simultaneously.
+ *
+ * 1. B creates/removes a symlink in the stack directory.
+ * 2. A calls the first stat() in the loop of link_update().
+ * 3. A calls link_find_prioritized().
+ * 4. C creates/removes another symlink in the stack directory, so the result of the step 3 is outdated.
+ * 5. B and C finish link_update().
+ * 6. A creates/removes devlink according to the outdated result in the step 3.
+ * 7. A calls the second stat() in the loop of link_update().
+ *
+ * If these 7 steps are processed in this order within a short time period that kernel's timer
+ * does not increase, then even if the contents in the stack directory is changed, the results
+ * of two stat() called by A shows the same timestamp, and A cannot detect the change.
+ *
+ * By calling this function after creating/removing symlinks in the stack directory, the
+ * timestamp of the stack directory is always increased at least in the above step 5, so A can
+ * detect the update. */
+
+ if ((prev->st_mode & S_IFMT) == 0)
+ return 0; /* Does not exist, or previous stat() failed. */
+
+ for (unsigned i = 0; i < UPDATE_TIMESTAMP_MAX_RETRIES; i++) {
+ struct stat st;
+
+ if (stat(path, &st) < 0)
+ return -errno;
+
+ if (!stat_inode_unmodified(prev, &st))
+ return 0;
+
+ log_device_debug(dev,
+ "%s is modified, but its timestamp is not changed, "
+ "updating timestamp after 10ms.",
+ path);
+
+ (void) usleep(10 * USEC_PER_MSEC);
+ if (utimensat(AT_FDCWD, path, NULL, 0) < 0)
+ return -errno;
+ }
+
+ return -ELOOP;
+}
+
static int update_stack_directory(sd_device *dev, const char *dirname, bool add) {
_cleanup_free_ char *filename = NULL, *data = NULL, *buf = NULL;
const char *devname, *id;
+ struct stat st = {};
int priority, r;
assert(dev);
@@ -302,10 +354,31 @@ static int update_stack_directory(sd_device *dev, const char *dirname, bool add)
return log_oom_debug();
if (!add) {
- if (unlink(filename) < 0 && errno != ENOENT)
- log_device_debug_errno(dev, errno, "Failed to remove %s, ignoring: %m", filename);
+ bool unlink_failed = false;
+
+ if (stat(dirname, &st) < 0) {
+ if (errno == ENOENT)
+ return 0; /* The stack directory is already removed. That's OK. */
+ log_device_debug_errno(dev, errno, "Failed to stat %s, ignoring: %m", dirname);
+ }
+
+ if (unlink(filename) < 0) {
+ unlink_failed = true;
+ if (errno != ENOENT)
+ log_device_debug_errno(dev, errno, "Failed to remove %s, ignoring: %m", filename);
+ }
+
+ if (rmdir(dirname) >= 0 || errno == ENOENT)
+ return 0;
+
+ if (unlink_failed)
+ return 0; /* If we failed to remove the symlink, there is almost nothing we can do. */
+
+ /* The symlink was removed. Check if the timestamp of directory is changed. */
+ r = update_timestamp(dev, dirname, &st);
+ if (r < 0 && r != -ENOENT)
+ return log_device_debug_errno(dev, r, "Failed to update timestamp of %s: %m", dirname);
- (void) rmdir(dirname);
return 0;
}
@@ -335,12 +408,23 @@ static int update_stack_directory(sd_device *dev, const char *dirname, bool add)
if (r < 0)
return log_device_debug_errno(dev, r, "Failed to create directory %s: %m", dirname);
+ if (stat(dirname, &st) < 0) {
+ if (errno == ENOENT)
+ continue;
+ return log_device_debug_errno(dev, errno, "Failed to stat %s: %m", dirname);
+ }
+
if (symlink(data, filename) < 0) {
if (errno == ENOENT)
continue;
return log_device_debug_errno(dev, errno, "Failed to create symbolic link %s: %m", filename);
}
+ /* The symlink was created. Check if the timestamp of directory is changed. */
+ r = update_timestamp(dev, dirname, &st);
+ if (r < 0)
+ return log_device_debug_errno(dev, r, "Failed to update timestamp of %s: %m", dirname);
+
return 0;
}