Re: [PATCH] ARM64: perf: support dwarf unwinding in compat mode

From: Will Deacon
Date: Thu Jan 16 2014 - 07:58:15 EST

On Thu, Jan 16, 2014 at 12:26:53PM +0000, Jean Pihet wrote:
> On 16 January 2014 12:56, Will Deacon <will.deacon@xxxxxxx> wrote:
> > In your previous series, compat backtracing is actually split out into a
> > separate function (compat_user_backtrace), so it would be more consistent to
> > have a compat_user_stack_pointer macro, rather than add this check here.
> Do you mean this change instead?

I don't think so...

> diff --git a/kernel/events/internal.h b/kernel/events/internal.h
> index 569b2187..9b88d2e 100644
> --- a/kernel/events/internal.h
> +++ b/kernel/events/internal.h
> @@ -185,7 +185,8 @@ static inline bool arch_perf_have_user_stack_dump(void)
> return true;
> }
> -#define perf_user_stack_pointer(regs) user_stack_pointer(regs)
> +#define perf_user_stack_pointer(regs) \
> + (!compat_user_mode(regs)) ? ((regs)->sp) : ((regs)->compat_sp)

This doesn't belong in core code; compat_user_mode and the fields of regs
are arm64-specific. So I suppose you need to rework your original patch to
call compat_user_stack_pointer (which we already define in compat.h for
arm64) if compat_user_mode(regs)).

The problem there is the inconsistency with respect to the regs argument:

user_stack_pointer(regs) // Returns user stack pointer for regs
current_user_stack_pointer() // Returns current user stack pointer
compat_user_stack_pointer() // Doesn't take a regs argument!

On top of that, x86 treats those last two functions differently when current
is a compat task.

So the simplest thing would be to make compat_user_stack_pointer expand to
user_stack_pointer(current_pt_regs()) on arm64 and merge that in with your
original patch fixing user_stack_pointer.

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