[PATCH v2] fs: always return zero on success from replace_fd()

["Thomas Weißschuh" <thomas.weissschuh@linutronix.de>]

replace_fd() returns the number of the new file descriptor through the
return value of do_dup2(). However its callers never care about the
specific number. In fact the caller in receive_fd_replace() treats any
non-zero return value as an error and therefore never calls
__receive_sock() for most file descriptors, which is a bug.

To fix the bug in receive_fd_replace() and to avoid the same issue
happening in future callers, signal success through a plain zero.

Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/lkml/20250801220215.GS222315@ZenIV/
Fixes: 173817151b15 ("fs: Expand __receive_fd() to accept existing fd")
Fixes: 42eb0d54c08a ("fs: split receive_fd_replace from __receive_fd")
Cc: stable@vger.kernel.org
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
Changes in v2:
- Move the fix to replace_fd() (Al)
- Link to v1: https://lore.kernel.org/r/20250801-fix-receive_fd_replace-v1-1-d46d600c74d6@linutronix.de
---
Untested, it stuck out while reading the code.
---
 fs/file.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/file.c b/fs/file.c
index 6d2275c3be9c6967d16c75d1b6521f9b58980926..f8a271265913951d755a5db559938d589219c4f2 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -1330,7 +1330,10 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
 	err = expand_files(files, fd);
 	if (unlikely(err < 0))
 		goto out_unlock;
-	return do_dup2(files, file, fd, flags);
+	err = do_dup2(files, file, fd, flags);
+	if (err < 0)
+		goto out_unlock;
+	err = 0;
 
 out_unlock:
 	spin_unlock(&files->file_lock);

---
base-commit: d2eedaa3909be9102d648a4a0a50ccf64f96c54f
change-id: 20250801-fix-receive_fd_replace-7fdd5ce6532d

Best regards,
-- 
Thomas Weißschuh <thomas.weissschuh@linutronix.de>