linux-kernel - Re: [PATCH v13 13/14] unwind_user/x86: Enable frame pointer unwinding on x86

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Message-ID: <20250711094321.3c64757c@pumpkin>
Date: Fri, 11 Jul 2025 09:43:21 +0100
From: David Laight <david.laight.linux@...il.com>
To: Steven Rostedt <rostedt@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 bpf@...r.kernel.org, x86@...nel.org, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Josh Poimboeuf <jpoimboe@...nel.org>, 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>, Jens Remus <jremus@...ux.ibm.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: [PATCH v13 13/14] unwind_user/x86: Enable frame pointer
 unwinding on x86

On Mon, 07 Jul 2025 21:22:52 -0400
Steven Rostedt <rostedt@...nel.org> wrote:

> From: Josh Poimboeuf <jpoimboe@...nel.org>
> 
> Use ARCH_INIT_USER_FP_FRAME to describe how frame pointers are unwound
> on x86, and enable CONFIG_HAVE_UNWIND_USER_FP accordingly so the
> unwind_user interfaces can be used.

How is that going to work?
Pretty much all x86 userspace is compiled with bp as a general
purpose register not a frame pointer.

	David

> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
>  arch/x86/Kconfig                   |  1 +
>  arch/x86/include/asm/unwind_user.h | 11 +++++++++++
>  2 files changed, 12 insertions(+)
>  create mode 100644 arch/x86/include/asm/unwind_user.h
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 71019b3b54ea..5862433c81e1 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -302,6 +302,7 @@ config X86
>  	select HAVE_SYSCALL_TRACEPOINTS
>  	select HAVE_UACCESS_VALIDATION		if HAVE_OBJTOOL
>  	select HAVE_UNSTABLE_SCHED_CLOCK
> +	select HAVE_UNWIND_USER_FP		if X86_64
>  	select HAVE_USER_RETURN_NOTIFIER
>  	select HAVE_GENERIC_VDSO
>  	select VDSO_GETRANDOM			if X86_64
> diff --git a/arch/x86/include/asm/unwind_user.h b/arch/x86/include/asm/unwind_user.h
> new file mode 100644
> index 000000000000..8597857bf896
> --- /dev/null
> +++ b/arch/x86/include/asm/unwind_user.h
> @@ -0,0 +1,11 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_X86_UNWIND_USER_H
> +#define _ASM_X86_UNWIND_USER_H
> +
> +#define ARCH_INIT_USER_FP_FRAME							\
> +	.cfa_off	= (s32)sizeof(long) *  2,				\
> +	.ra_off		= (s32)sizeof(long) * -1,				\
> +	.fp_off		= (s32)sizeof(long) * -2,				\
> +	.use_fp		= true,
> +
> +#endif /* _ASM_X86_UNWIND_USER_H */