FAILED: patch "[PATCH] splice: avoid stale f_pos writeback on error" failed to apply to 5.15-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 79b496ebe382866e3a05090b7dc9ae95e85e65b2
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107061356-failed-5.15-79b496ebe382@kernel.org' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 79b496ebe382866e3a05090b7dc9ae95e85e65b2 Mon Sep 17 00:00:00 2001
From: Daehyeon Ko <4ncienth@gmail.com>
Date: Sun, 18 Oct 2026 14:17:08 +0900
Subject: [PATCH] splice: avoid stale f_pos writeback on error

do_splice() snapshots an implicit file position before calling the splice
helper. Since splice does not take f_pos_lock, another operation on the
same open file description can update f_pos while the helper is blocked.
If the helper then fails, the unconditional writeback restores the stale
snapshot.

For a directory without ->splice_read, a file-to-pipe splice fails with
-EINVAL. A concurrent directory mutation and getdents() call can advance
f_pos and refresh the filesystem's revalidation cookie before that failure
restores only the old position. The next ext2 readdir can then trust an
offset inside a directory entry and copy data from the adjacent page to
userspace.

Only update an implicit file position when splice returns a non-negative
result, matching read and write. Keep explicit offset updates unchanged.

Fixes: 7995bd287134 ("splice: don't pass the address of ->f_pos to methods")
Cc: stable@vger.kernel.org
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
Link: https://patch.msgid.link/20261018051708.2422394-1-4ncienth@gmail.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/splice.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/fs/splice.c b/fs/splice.c
index bc243ab8dc431..f4bce35890671 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -1377,10 +1377,10 @@ ssize_t do_splice(struct file *in, loff_t *off_in, struct file *out,
 		ret = do_splice_from(ipipe, out, &offset, len, flags);
 		file_end_write(out);
 
-		if (!off_out)
-			out->f_pos = offset;
-		else
+		if (off_out)
 			*off_out = offset;
+		else if (ret >= 0)
+			out->f_pos = offset;
 	} else if (opipe) {
 		if (off_out)
 			return -ESPIPE;
@@ -1401,10 +1401,10 @@ ssize_t do_splice(struct file *in, loff_t *off_in, struct file *out,
 
 		ret = splice_file_to_pipe(in, opipe, &offset, len, flags);
 
-		if (!off_in)
-			in->f_pos = offset;
-		else
+		if (off_in)
 			*off_in = offset;
+		else if (ret >= 0)
+			in->f_pos = offset;
 	} else {
 		ret = -EINVAL;
 	}
-- 
2.53.0