[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:

quoted

From: Nikolay Borisov <redacted>

Currently all the code which deals with backtrace support assumes

that R11

quoted

is the frame-pointer. While this is the case for ARM mode and is

explicitly

quoted

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

quoted

and GCC uses R7 as FP and given that R7's usage is so intertwined

within GCC's

quoted

mechanics it is unlikely to change, so fixing backtrace in THUMB2

mode seems

quoted

in order.

This patch series rectifies the problem by first fixing the
thread_save_fp macro to reference the correct register. Furthermore,

there

quoted

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

Regards,
Nikolay


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