[PATCH 4.19 097/425] security: commoncap: fix -Wstringop-overread warning

[Greg Kroah-Hartman]

From: Arnd Bergmann <arnd@xxxxxxxx>

commit 82e5d8cc768b0c7b03c551a9ab1f8f3f68d5f83f upstream.

gcc-11 introdces a harmless warning for cap_inode_getsecurity:

security/commoncap.c: In function ‘cap_inode_getsecurity’:
security/commoncap.c:440:33: error: ‘memcpy’ reading 16 bytes from a region of size 0 [-Werror=stringop-overread]
  440 |                                 memcpy(&nscap->data, &cap->data, sizeof(__le32) * 2 * VFS_CAP_U32);
      |                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The problem here is that tmpbuf is initialized to NULL, so gcc assumes
it is not accessible unless it gets set by vfs_getxattr_alloc().  This is
a legitimate warning as far as I can tell, but the code is correct since
it correctly handles the error when that function fails.

Add a separate NULL check to tell gcc about it as well.

Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
Acked-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
Signed-off-by: James Morris <jamorris@xxxxxxxxxxxxxxxxxxx>
Cc: Andrey Zhizhikin <andrey.z@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 security/commoncap.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -397,7 +397,7 @@ int cap_inode_getsecurity(struct inode *
 				 &tmpbuf, size, GFP_NOFS);
 	dput(dentry);
 
-	if (ret < 0)
+	if (ret < 0 || !tmpbuf)
 		return ret;
 
 	fs_ns = inode->i_sb->s_user_ns;