linux-kernel - [PATCH 2/2] fs: don't update the atime if existing atime is newer than "now"

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Message-Id: <20230907-ctime-fixes-v1-2-3b74c970d934@kernel.org>
Date:   Thu, 07 Sep 2023 12:33:48 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>
Cc:     linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jeff Layton <jlayton@...nel.org>,
        kernel test robot <oliver.sang@...el.com>
Subject: [PATCH 2/2] fs: don't update the atime if existing atime is newer
 than "now"

It's possible for the atime to be updated with a fine-grained timestamp
and then later get an update that uses a coarse-grained timestamp which
makes the atime appear to go backward.

Fix this by only updating the atime if "now" is later than the current
value.

Reported-by: kernel test robot <oliver.sang@...el.com>
Closes: https://lore.kernel.org/oe-lkp/202309071017.a64aca5e-oliver.sang@intel.com
Signed-off-by: Jeff Layton <jlayton@...nel.org>
---
 fs/inode.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index 54237f4242ff..cf4726b7f4b5 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -1905,7 +1905,7 @@ int inode_update_timestamps(struct inode *inode, int flags)
 	}
 
 	if (flags & S_ATIME) {
-		if (!timespec64_equal(&now, &inode->i_atime)) {
+		if (timespec64_compare(&inode->i_atime, &now) < 0) {
 			inode->i_atime = now;
 			updated |= S_ATIME;
 		}
@@ -1991,7 +1991,7 @@ bool atime_needs_update(const struct path *path, struct inode *inode)
 	if (!relatime_need_update(mnt, inode, now))
 		return false;
 
-	if (timespec64_equal(&inode->i_atime, &now))
+	if (timespec64_compare(&inode->i_atime, &now) >= 0)
 		return false;
 
 	return true;

-- 
2.41.0