linux-kernel - Re: [PATCH v2 1/2] unwind_user/x86: Enable frame pointer unwinding on x86

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Message-ID: <20250923105130.GG3419281@noisy.programming.kicks-ass.net>
Date: Tue, 23 Sep 2025 12:51:30 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Steven Rostedt <rostedt@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	x86@...nel.org, Masami Hiramatsu <mhiramat@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>
Subject: Re: [PATCH v2 1/2] unwind_user/x86: Enable frame pointer unwinding
 on x86

On Wed, Aug 27, 2025 at 03:36:45PM -0400, Steven Rostedt 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.
> 
> 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 58d890fe2100..8f94c58d4de8 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -299,6 +299,7 @@ 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 */

Moo, and now you have me look at unwind/user.c:

        /* Make sure that the address is word aligned */
        shift = sizeof(long) == 4 ? 2 : 3;
        if (cfa & ((1 << shift) - 1))
                return -EINVAL;

Isn't that just:

	if (cfa & (sizeof(long) - 1))

?

Let me go add a patch to clean that up...