linux-kernel - Re: [RFC PATCH v1 06/16] unwind_user: Enable archs that define CFA = SP

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Message-ID: <qoiocmdhuuaox5v5ig2ui67qbuxkvzl4z3ft4gdp7p3c4b4zfq@trjthmmculkf>
Date: Wed, 16 Jul 2025 14:32:21 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Jens Remus <jremus@...ux.ibm.com>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
	bpf@...r.kernel.org, x86@...nel.org, Steven Rostedt <rostedt@...nel.org>, 
	Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>, 
	Ilya Leoshkevich <iii@...ux.ibm.com>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Andrii Nakryiko <andrii@...nel.org>, 
	Indu Bhagat <indu.bhagat@...cle.com>, "Jose E. Marchesi" <jemarch@....org>, 
	Beau Belgrave <beaub@...ux.microsoft.com>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Jens Axboe <axboe@...nel.dk>, 
	Florian Weimer <fweimer@...hat.com>, Sam James <sam@...too.org>
Subject: Re: [RFC PATCH v1 06/16] unwind_user: Enable archs that define CFA =
 SP_callsite + offset

On Thu, Jul 10, 2025 at 06:35:12PM +0200, Jens Remus wrote:
> Most architectures define their CFA as the value of the stack pointer
> (SP) at the call site in the previous frame, as suggested by the DWARF
> standard:
> 
>   CFA = <SP at call site>
> 
> Enable unwinding of user space for architectures, such as s390, which
> define their CFA as the value of the SP at the call site in the previous
> frame with an offset:
> 
>   CFA = <SP at call site> + offset

This is a bit confusing, as the comment and code define it as

    SP = CFA + offset

Should the commit log be updated to match that?

> +++ b/arch/x86/include/asm/unwind_user.h
> @@ -8,6 +8,7 @@
>  	.cfa_off	= (s32)sizeof(long) *  2,				\
>  	.ra_off		= (s32)sizeof(long) * -1,				\
>  	.fp_off		= (s32)sizeof(long) * -2,				\
> +	.sp_val_off	= (s32)0,						\

IIUC, this is similar to ra_off and fp_off in that its an offset from
the CFA.  Can we call it "sp_off"?

-- 
Josh