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Date:   Wed, 18 Jul 2018 14:46:34 +0100
From:   Al Viro <viro@ZenIV.linux.org.uk>
To:     Miklos Szeredi <mszeredi@redhat.com>
Cc:     Stephen Rothwell <sfr@canb.auug.org.au>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        David Howells <dhowells@redhat.com>
Subject: Re: vfs / overlayfs conflict resolution for linux-next
Message-ID: <20180718134634.GT30522@ZenIV.linux.org.uk>
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On Wed, Jul 18, 2018 at 01:43:40PM +0100, Al Viro wrote:

> It *is* broken.  For now leave override_creds() as in your variant, but
> we really want to deal with that crap eventually.
> 
> > Okay, so ->open() is a file op, and file ops should use file->f_cred,
> > but how are we going to enforce this?
> 
> I'd say we cut down on the use of current_cred() when deep in call chain...

Actually, how about this:

#define call_with_creds(__cred, expr) ({		\
	__typeof__(expr) ____res;			\
	const struct cred *____old = current->cred;	\
	const struct cred *____new = (__cred);		\
	rcu_assign_pointer(current->cred, ____new);	\
	____res = expr;					\
	BUG_ON(current->cred != ____new);		\
	rcu_assign_pointer(current->cred, ____old);	\
	____res;					\
})

and replacing
                error = open(inode, f);
with
		error = call_with_cred(f->f_cred, open(inode, f));

possibly with similar at other methods callsites?  Unlike override_creds()
and revert_creds() it's cheap - no validation of creds, no cacheline
ping-pong...

Folks?