[PATCH] ovl: Handle ENOSYS when fileattr support is missing in lower/upper fs

["Christian Kohlschütter" <christian@kohlschutter.com>]

overlayfs may fail to complete updates when a filesystem lacks
fileattr/xattr syscall support and responds with an ENOSYS error code,
resulting in an unexpected "Function not implemented" error.

This bug may occur with FUSE filesystems, such as davfs2.

Steps to reproduce:

  # install davfs2, e.g., apk add davfs2
  mkdir /test mkdir /test/lower /test/upper /test/work /test/mnt
  yes '' | mount -t davfs -o ro http://some-web-dav-server/path \
    /test/lower
  mount -t overlay -o upperdir=/test/upper,lowerdir=/test/lower \
    -o workdir=/test/work overlay /test/mnt

  # when "some-file" exists in the lowerdir, this fails with "Function
  # not implemented", with dmesg showing "overlayfs: failed to retrieve
  # lower fileattr (/some-file, err=-38)"
  touch /test/mnt/some-file

This bug is related to a regression in v5.15 that was partially fixed in
v5.16.

This patch also adds checks for ENOSYS in case the upper file system
does not support file attributes. That change is related to a partial
fix in v5.17.

Reported-by: Christian Kohlschütter <christian@kohlschutter.com>
Fixes: 5b0a414d06c ("ovl: fix filattr copy-up failure")
Fixes: 24d7f48c723 ("ovl: don't fail copy up if no fileattr support on upper")
Cc: <stable@vger.kernel.org> # v5.15
Signed-off-by: Christian Kohlschütter <christian@kohlschutter.com>
---
 fs/overlayfs/copy_up.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
index 714ec569d25b..0ad88573e77a 100644
--- a/fs/overlayfs/copy_up.c
+++ b/fs/overlayfs/copy_up.c
@@ -142,7 +142,7 @@ static int ovl_copy_fileattr(struct inode *inode, struct path *old,
 	err = ovl_real_fileattr_get(old, &oldfa);
 	if (err) {
 		/* Ntfs-3g returns -EINVAL for "no fileattr support" */
-		if (err == -ENOTTY || err == -EINVAL)
+		if (err == -ENOTTY || err == -EINVAL || err == -ENOSYS)
 			return 0;
 		pr_warn("failed to retrieve lower fileattr (%pd2, err=%i)\n",
 			old->dentry, err);
@@ -173,7 +173,7 @@ static int ovl_copy_fileattr(struct inode *inode, struct path *old,
 		 * Returning an error if upper doesn't support fileattr will
 		 * result in a regression, so revert to the old behavior.
 		 */
-		if (err == -ENOTTY || err == -EINVAL) {
+		if (err == -ENOTTY || err == -EINVAL || err == -ENOSYS) {
 			pr_warn_once("copying fileattr: no support on upper\n");
 			return 0;
 		}
-- 
2.36.1