linux-kernel - Re: [PATCH v8 6/6] x86/vdso: Enable sframe generation in VDSO

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Message-ID: <4304d18a-f647-4709-9f29-43d9995cc24e@zytor.com>
Date: Fri, 6 Feb 2026 15:08:17 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Jens Remus <jremus@...ux.ibm.com>, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org,
        x86@...nel.org, linux-mm@...ck.org,
        Steven Rostedt <rostedt@...nel.org>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>,
        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>,
        Arnaldo Carvalho de Melo <acme@...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>,
        Florian Weimer <fweimer@...hat.com>, Kees Cook <kees@...nel.org>,
        "Carlos O'Donell" <codonell@...hat.com>, Sam James <sam@...too.org>,
        Dylan Hatch <dylanbhatch@...gle.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...nel.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
        Michal Hocko
 <mhocko@...e.com>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        "Steven Rostedt (Google)" <rostedt@...dmis.org>
Subject: Re: [PATCH v8 6/6] x86/vdso: Enable sframe generation in VDSO

On 2026-02-06 11:36, Jens Remus wrote:
> From: Josh Poimboeuf <jpoimboe@...nel.org>
> 
> Enable sframe generation in the VDSO library so kernel and user space
> can unwind through it.
> 
> SFrame isn't supported for x32 or x86-32.  Discard .sframe sections for
> those VDSOs.
> 
> [ Jens Remus: Add support for SFrame V3.  Prevent GNU_SFRAME program
> table entry to empty .sframe section. ]
> 

This will not break the x86-32 build if the assembler encounters .sframe?

> Notes (jremus):
>     Changes in v8:
>     - Discard .sframe for x32 and x86-32 VDSOs. (Josh/Indu)
>       Note that the use of KEEP_SFRAME enables to define it for x86-64
>       VDSO only.  Unlike CONFIG_AS_SFRAME, which may also be defined
>       for x32 and x86-32 VDSO.  In x32 VDSO it would result in superfluous
>       .sframe (copied from the x86-64 build - could be removed in X32
>       build step).  In x86-32 VDSO it would cause a bogus GNU_SFRAME
>       program table entry.

For x32, this would be a "valid" sframe, right, even if the tools currently
don't know how to consume it (and potentially never will)? If so, is there
really any reason to explicitly remove it?

>  	/*
>  	 * Text is well-separated from actual data: there's plenty of
>  	 * stuff that isn't used at runtime in between.
> @@ -80,6 +87,10 @@ SECTIONS
>  		*(.discard)
>  		*(.discard.*)
>  		*(__bug_table)
> +#ifndef KEEP_SFRAME
> +		*(.sframe)
> +		*(.sframe.*)
> +#endif

This #ifndef is actually not necessary: if we have already "consumed" the
.sframe* sections they will not be encountered here.

I would prefer to have KEEP_SFRAME always defined (as true or false, and using
#if) instead of using #ifdef. I believe that also means you can do:

#define KEEP_SFRAME IS_ENABLED(CONFIG_AS_SFRAME)

... instead of #ifdef.

	-hpa