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

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Message-ID: <20140116125727.GI30257@mudshark.cambridge.arm.com>
Date:	Thu, 16 Jan 2014 12:57:27 +0000
From:	Will Deacon <will.deacon@....com>
To:	Jean Pihet <jean.pihet@...aro.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Arnaldo <acme@...stprotocols.net>,
	"patches@...aro.org" <patches@...aro.org>,
	Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] ARM64: perf: support dwarf unwinding in compat mode

On Thu, Jan 16, 2014 at 12:26:53PM +0000, Jean Pihet wrote:
> On 16 January 2014 12:56, Will Deacon <will.deacon@....com> 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.

Will
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