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 C creates/removes another symlink in the stack directory, so the result of the step 3 is outdated.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 A creates/removes devlink according to the outdated result in the step 3.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;

         }