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

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Date:   Wed, 19 Sep 2018 14:25:00 +0200
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Peter Zijlstra <peterz@...radead.org>
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



Le 19/09/2018 à 13:58, Peter Zijlstra a écrit :
> 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?

I have not been able to find a way to define the compilation flags AFTER 
building asm-offsets.h, see https://patchwork.ozlabs.org/patch/971521/

If you have a suggestion, it is welcomed.

> 
>> 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.
> 

Yes, it doesn't make much difference, don't any arch expect state at 
offset 0 ?

Christophe