NFS: if ATTR_KILL_S*ID bits are set, then skip mode change

If the ATTR_KILL_S*ID bits are set then any mode change is only for clearing
the setuid/setgid bits.  For NFS, skip the mode change and let the server
handle it.
Signed-off-by: NJeff Layton <jlayton@redhat.com>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
Cc: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/nfs/inode.c

@@ -357,6 +357,10 @@ nfs_setattr(struct dentry *dentry, struct iattr *attr)
 
 	nfs_inc_stats(inode, NFSIOS_VFSSETATTR);
 
+	/* skip mode change if it's just for clearing setuid/setgid */
+	if (attr->ia_valid & (ATTR_KILL_SUID | ATTR_KILL_SGID))
+		attr->ia_valid &= ~ATTR_MODE;
+
 	if (attr->ia_valid & ATTR_SIZE) {
 		if (!S_ISREG(inode->i_mode) || attr->ia_size == i_size_read(inode))
 			attr->ia_valid &= ~ATTR_SIZE;