linux-kernel - Re: [PATCH v2 1/2] sched: move stack_canary field at the top of task

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Message-ID: <20180919115836.GE24124@hirez.programming.kicks-ass.net>
Date:   Wed, 19 Sep 2018 13:58:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Ingo Molnar <mingo@...hat.com>, segher@...nel.crashing.org,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 1/2] sched: move stack_canary field at the top of
 task_struct

On Wed, Sep 19, 2018 at 11:14:43AM +0000, Christophe Leroy wrote:
> In order to allow the use of non global stack protector canary,
> the stack canary needs to be located at a know offset defined
> in Makefile via -mstack-protector-guard-offset.
> 
> On powerpc/32, register r2 points to current task_struct at
> all time, the stack_canary located inside task_struct can be
> used directly if it is located in a known place.
> 
> In order to allow that, this patch moves the stack_canary field
> out of the randomized area of task_struct.

And you cannot use something like asm-offsets to extract this?

> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> ---
>  include/linux/sched.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 977cb57d7bc9..1d977b8a4bac 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -601,6 +601,10 @@ struct task_struct {
>  	/* -1 unrunnable, 0 runnable, >0 stopped: */
>  	volatile long			state;
>  
> +#ifdef CONFIG_STACKPROTECTOR
> +	/* Canary value for the -fstack-protector GCC feature: */
> +	unsigned long			stack_canary;
> +#endif
>  	/*
>  	 * This begins the randomizable portion of task_struct. Only
>  	 * scheduling-critical items should be added above here.

Might as well put it before state, right after the task_info thing.