fuse: fix clearing suid, sgid for chown()

commit c01638f5 upstream.

Basically, the pjdfstests set the ownership of a file to 06555, and then
chowns it (as root) to a new uid/gid. Prior to commit a09f99ed ("fuse:
fix killing s[ug]id in setattr"), fuse would send down a setattr with both
the uid/gid change and a new mode.  Now, it just sends down the uid/gid
change.

Technically this is NOTABUG, since POSIX doesn't _require_ that we clear
these bits for a privileged process, but Linux (wisely) has done that and I
think we don't want to change that behavior here.

This is caused by the use of should_remove_suid(), which will always return
0 when the process has CAP_FSETID.

In fact we really don't need to be calling should_remove_suid() at all,
since we've already been indicated that we should remove the suid, we just
don't want to use a (very) stale mode for that.

This patch should fix the above as well as simplify the logic.
Reported-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: a09f99ed ("fuse: fix killing s[ug]id in setattr")
Reviewed-by: Jeff Layton <jlayton@redhat.com>
[bwh: Backported to 3.16: adjust context, indentation]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/fuse/dir.c

@@ -1829,8 +1829,6 @@ static int fuse_setattr(struct dentry *entry, struct iattr *attr)
 		return -EACCES;
 
 	if (attr->ia_valid & (ATTR_KILL_SUID | ATTR_KILL_SGID)) {
-		int kill;
-
 		attr->ia_valid &= ~(ATTR_KILL_SUID | ATTR_KILL_SGID |
 				    ATTR_MODE);
 		/*
@@ -1842,12 +1840,11 @@ static int fuse_setattr(struct dentry *entry, struct iattr *attr)
 			return ret;
 
 		attr->ia_mode = inode->i_mode;
-		kill = should_remove_suid(entry);
-		if (kill & ATTR_KILL_SUID) {
+		if (inode->i_mode & S_ISUID) {
 			attr->ia_valid |= ATTR_MODE;
 			attr->ia_mode &= ~S_ISUID;
 		}
-		if (kill & ATTR_KILL_SGID) {
+		if ((inode->i_mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
 			attr->ia_valid |= ATTR_MODE;
 			attr->ia_mode &= ~S_ISGID;
 		}