linux-kernel - Re: [PATCH] signals: check_kill_permission: don't check creds if same_thread

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Date:	Tue, 18 May 2010 16:08:40 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrew Tridgell <tridge@...ba.org>,
	Eric Paris <eparis@...isplace.org>,
	Jakub Jelinek <jakub@...hat.com>,
	James Morris <jmorris@...ei.org>,
	Roland McGrath <roland@...hat.com>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] signals: check_kill_permission: don't check creds if
	same_thread_group()

On 05/18, David Howells wrote:
>
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> > Yes, but both current_cred() and same_thread_group(current, t) call
> > get_current(), and gcc doesn't cache the result because we call
> > audit_signal_info() in between.
>
> Sorry, yes.  I was reading get_current() as current_cred() for some reason.
>
> However, you are _still_ calling get_current() twice...  So that bit of your
> changelog isn't really correct.

If I read kernel/signal.s correctly - no.

Well, yes, get_current() is still called twice inside check_kill_permission().
But this is because we have audit_signal_info()->audit_dummy_context() which
uses current too.

But "cred = current_cred()" and same_thread_group(current, t) read
gs:current_task only once, so this change really helps (although the
optimization is very minor, of course).

> In fact, get_current() should be __attribute_const__ since it can't change
> whilst you're looking at it, except within switch_to(), probably in a piece of
> assembly code, so gcc should be free to cache it as long as it likes.

Agreed! I thought about this many times.

Oleg.

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