[PATCH v2 1/3] fs: stamp the current time for a stale delegated ctime update

[Jeff Layton <jlayton@kernel.org>]

inode_set_ctime_deleg() drops an update that does not advance the ctime.
That is correct when a client reports a timestamp that it advanced on its
own. It is wrong when the client moves the timestamp backwards on purpose,
as utimensat() does.

Stamp the current time in that case. The ctime still never moves backwards.

Fixes: 3952f1cbcbc4 ("nfsd: fix SETATTR updates for delegated timestamps")
Assisted-by: LLM
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/inode.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index ba7da39be4a3..8391814a4016 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2959,11 +2959,17 @@ EXPORT_SYMBOL(inode_set_ctime_current);
  * inode attributes, including the mtime. When updating the mtime, update
  * the ctime to a value at least equal to that.
  *
- * This can race with concurrent updates to the inode, in which
- * case the update is skipped.
+ * The ctime never moves backwards. An @update that does not advance the ctime
+ * records the current time instead, so that the delegated change is still
+ * visible in the ctime.
+ *
+ * This can still race with a concurrent update to the inode. That stamp takes
+ * precedence, and is at least as recent as the one it displaces.
  *
  * Note that this works even when multigrain timestamps are not enabled,
  * so it is used in either case.
+ *
+ * Returns the resulting ctime.
  */
 struct timespec64 inode_set_ctime_deleg(struct inode *inode, struct timespec64 update)
 {
@@ -2975,9 +2981,12 @@ struct timespec64 inode_set_ctime_deleg(struct inode *inode, struct timespec64 u
 	cur_ts.tv_nsec = cur & ~I_CTIME_QUERIED;
 	cur_ts.tv_sec = inode_get_ctime_sec(inode);
 
-	/* If the update is older than the existing value, skip it. */
+	/*
+	 * The update does not advance the ctime. Stamp the current time, so
+	 * that the delegated change is still visible in the ctime.
+	 */
 	if (timespec64_compare(&update, &cur_ts) <= 0)
-		return cur_ts;
+		return inode_set_ctime_current(inode);
 
 	ktime_get_coarse_real_ts64_mg(&now);
 

-- 
2.55.0