[PATCH] fs: add bounds checking when updating ctime

[Jeff Layton <jlayton@kernel.org>]

During some discussion around another patch, Linus pointed out that
we don't want to skip updating the ctime unless the current coarse
time is in a reasonable range.

When updating the ctime on a multigrain filesystem, only keep the
current ctime if the coarse time is less than 2 jiffies earlier.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
While we're still discussing whether to keep this series in, here's the
fix for the issue that Linus identified yesterday, basically that a
large clock jump backward could result in the m/ctime not being updated
properly.
---
 fs/inode.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index 54237f4242ff..f3d68e4b8df7 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2571,6 +2571,13 @@ struct timespec64 current_time(struct inode *inode)
 }
 EXPORT_SYMBOL(current_time);
 
+/*
+ * Coarse timer ticks happen (roughly) every jiffy. If we see a coarse time
+ * more than 2 jiffies earlier than the current ctime, then we need to
+ * update it. This is the max delta allowed (in ns).
+ */
+#define COARSE_TIME_MAX_DELTA (2 / HZ * NSEC_PER_SEC)
+
 /**
  * inode_set_ctime_current - set the ctime to current_time
  * @inode: inode
@@ -2599,8 +2606,19 @@ struct timespec64 inode_set_ctime_current(struct inode *inode)
 		 * existing ctime. Just keep the existing value if so.
 		 */
 		ctime.tv_sec = inode->__i_ctime.tv_sec;
-		if (timespec64_compare(&ctime, &now) > 0)
-			return ctime;
+		if (timespec64_compare(&ctime, &now) > 0) {
+			struct timespec64	limit = now;
+
+			/*
+			 * If the current coarse-grained clock is earlier than
+			 * it should be, then that's an indication that there
+			 * may have been a backward clock jump, and that the
+			 * update should not be skipped.
+			 */
+			timespec64_add_ns(&limit, COARSE_TIME_MAX_DELTA);
+			if (timespec64_compare(&ctime, &limit) < 0)
+				return ctime;
+		}
 
 		/*
 		 * Ctime updates are usually protected by the inode_lock, but

---
base-commit: 0b4cbb6924ecf459c12b2b5ff4370ae29a276fee
change-id: 20230919-ctime-14ad041893fb

Best regards,
-- 
Jeff Layton <jlayton@kernel.org>