linux-kernel - Re: PATCH? process_32.c:__switch_to() calls __math_state_restore() before updating current

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Date:	Sat, 6 Feb 2010 13:06:40 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: PATCH? process_32.c:__switch_to() calls __math_state_restore()
	before updating current_task

On 02/05, Oleg Nesterov wrote:
>
> In short: why restore_i387_xstate() does init_fpu() when !used_math(),
> can't (or shouldn't) it merely do set_used_math() ?
>
> 	restore_i387_xstate:
> 		
> 		if (!used_math()) {
> 			err = init_fpu(tsk);
> 			if (err)
> 				return err;
> 		}
>
> note that buf != NULL. This means that used_math() was true when
> get_sigframe() was called, otherwise buf == sigcontext->fpstate
> would be NULL, right?
>
> So, the task must have the valid ->thread.xstate,

This is probably correct. But a malicious user can set sc->fpsate
and fool the kernel, so we must assume buf != NULL implies the
valid ->xstate.

Oleg.

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