[PATCH 4.15 195/202] ovl: take mnt_want_write() for removing impure xattr

[Greg Kroah-Hartman]

4.15-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Amir Goldstein <amir73il@xxxxxxxxx>

commit a5a927a7c82e28ea76599dee4019c41e372c911f upstream.

The optimization in ovl_cache_get_impure() that tries to remove an
unneeded "impure" xattr needs to take mnt_want_write() on upper fs.

Fixes: 4edb83bb1041 ("ovl: constant d_ino for non-merge dirs")
Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/overlayfs/readdir.c |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -593,8 +593,15 @@ static struct ovl_dir_cache *ovl_cache_g
 		return ERR_PTR(res);
 	}
 	if (list_empty(&cache->entries)) {
-		/* Good oportunity to get rid of an unnecessary "impure" flag */
-		ovl_do_removexattr(ovl_dentry_upper(dentry), OVL_XATTR_IMPURE);
+		/*
+		 * A good opportunity to get rid of an unneeded "impure" flag.
+		 * Removing the "impure" xattr is best effort.
+		 */
+		if (!ovl_want_write(dentry)) {
+			ovl_do_removexattr(ovl_dentry_upper(dentry),
+					   OVL_XATTR_IMPURE);
+			ovl_drop_write(dentry);
+		}
 		ovl_clear_flag(OVL_IMPURE, d_inode(dentry));
 		kfree(cache);
 		return NULL;