[PATCH v2 0/7] Fix backtrace support in THUMB2 mode

Nikolay Borisov Nikolay.Borisov at arm.com
Mon Jun 2 01:34:58 PDT 2014

[Adding Dave Martin as CC]

> -----Original Message-----
> From: Arnd Bergmann [mailto:arnd at arndb.de]
> Sent: 31 May 2014 19:32
> To: linux-arm-kernel at lists.infradead.org
> Cc: Nikolay Borisov; rric at kernel.org; Will Deacon; u.kleine-
> koenig at pengutronix.de
> Subject: Re: [PATCH v2 0/7] Fix backtrace support in THUMB2 mode
> On Friday 30 May 2014 17:59:44 Nikolay Borisov wrote:
> > From: Nikolay Borisov <Nikolay.Borisov at arm.com>
> >
> > Currently all the code which deals with backtrace support assumes
> that R11
> > is the frame-pointer. While this is the case for ARM mode and is
> explicitly
> > documented in the AAPCS, this is not the case for THUMB2 mode.
> >
> > There is no official document requiring that R11 has to be the frame
> pointer
> > and GCC uses R7 as FP and given that R7's usage is so intertwined
> within GCC's
> > mechanics it is unlikely to change, so fixing backtrace in THUMB2
> mode seems
> > in order.
> >
> > This patch series rectifies the problem by first fixing the
> > thread_save_fp macro to reference the correct register. Furthermore,
> there
> > a lot of repetetive sequences of code such as :
> >
> > stackframe.fp = pt_regs->ARM_fp
> > stackframe.lr = pt_regs->ARM_lr
> >
> > so introducing a function arm_get_current_stack_frame which both
> > hides this repetition and also utilizes teh frame_pointer(regs) macro
> > to reference the correct register depending on the mode.
> >
> > Finally, change all the call sites so that they utilize the new
> routine.
> Sorry for the stupid question, but does your series relate to the
> #warning "TODO: return_address should use unwind tables" that we see
> all the time in arch/arm/kernel/return_address.c?

My changes have nothing to do with the use of unwind table whatsoever,
I guess this warning is a result of a completely different issue. Probably
Dave Martin could shed more light on this.

> If I remember the story correctly, the problem is that so far we could
> never use the frame pointer in thumb2 mode, and nobody has implemented
> a version of that function using the arm unwinder.
> Are we now able to use the frame pointer after all?

What my changes implement, boils down to referring the correct register
Which acts as a frame pointer depending on whether we are running a THUMB2
Kernel or not.

>       Arnd


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