Re: [PATCH v2 1/2] sched: move stack_canary field at the top of task_struct

[Christophe LEROY]

Le 19/09/2018 Ã 13:58, Peter Zijlstra a ÃcritÂ:
> 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@xxxxxx>
> > ---
> >   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