[PATCH v2] ARM: unwinder: Fix handling of 'fp' in thumb2 mode

[Dave Martin]

On Fri, May 02, 2014 at 04:37:36PM +0100, Nikolay Borisov wrote:
> The thread_save_fp macro has been defined so that it always reads the fp member
> of the cpu_context_save struct. However, in the case of THUMB2 the fp is saved
> not in the fp (r11) member but rather in r7.
> 
> This patch changes the way the macro is defined such that FP is read from the
> correct place depending on whether we are a THUMB2 kernel or not. This enables
> the backtrace in sitaution such as "echo t > /proc/sysrq-trigger" or the
> function in which a process sleeping when "ps -Al" is invoked.
> 
> Another place where similar change is requried is when working with pt_reg.
> regs->ARM_fp is defined such that r11 is always referenced as the frame pointer,
> so the definition of ARM_fp needs to change depending on the type of kernel we
> are running.

(Keep your S-o-b line here, but move the "change log" stuff after the
--- tearoff line so it doesn't form part of the commit message.)

> Changes since v1:
> * Added changes for the pt_reg structure.
> 
> Signed-off-by: Nikolay Borisov <Nikolay.Borisov at arm.com>
> 
> handle the case when we've got regs present
> ---
>  arch/arm/include/asm/thread_info.h | 6 ++++++
>  arch/arm/include/uapi/asm/ptrace.h | 4 ++++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/arch/arm/include/asm/thread_info.h b/arch/arm/include/asm/thread_info.h
> index f989d7c..e4e4208 100644
> --- a/arch/arm/include/asm/thread_info.h
> +++ b/arch/arm/include/asm/thread_info.h
> @@ -114,8 +114,14 @@ static inline struct thread_info *current_thread_info(void)
>  	((unsigned long)(task_thread_info(tsk)->cpu_context.pc))
>  #define thread_saved_sp(tsk)	\
>  	((unsigned long)(task_thread_info(tsk)->cpu_context.sp))
> +
> +#ifndef CONFIG_THUMB2_KERNEL
>  #define thread_saved_fp(tsk)	\
>  	((unsigned long)(task_thread_info(tsk)->cpu_context.fp))
> +#else
> +#define thread_saved_fp(tsk)	\
> +	((unsigned long)(task_thread_info(tsk)->cpu_context.r7))
> +#endif
>  
>  extern void crunch_task_disable(struct thread_info *);
>  extern void crunch_task_copy(struct thread_info *, void *);
> diff --git a/arch/arm/include/uapi/asm/ptrace.h b/arch/arm/include/uapi/asm/ptrace.h
> index 5af0ed1..92e1bae 100644
> --- a/arch/arm/include/uapi/asm/ptrace.h
> +++ b/arch/arm/include/uapi/asm/ptrace.h
> @@ -131,7 +131,11 @@ struct pt_regs {
>  #define ARM_lr		uregs[14]
>  #define ARM_sp		uregs[13]
>  #define ARM_ip		uregs[12]
> +#ifndef CONFIG_THUMB2_KERNEL
>  #define ARM_fp		uregs[11]
> +#else
> +#define ARM_fp		uregs[7]
> +#endif

I'm not sure about this.  There seem to be other bits of code that
assume the ARM_fp name maps to the fixed register defined by the PCS,
i.e. always r11 and not the "frame pointer".  For example, see
arch/arm/kernel/kgdb.c: I think we'll end up passing the wrong regs to
gdb in this case.

It may be better to work around the usage of these #defines locally in
the unwinder code instead.

Cheers
---Dave