linux-kernel - Re: [PATCH v7 2/6] x86/asm: Avoid emitting DWARF CFI for non-VDSO

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Message-ID: <bd2fdf83-e460-46ad-8617-16319bc55ca5@zytor.com>
Date: Tue, 3 Feb 2026 16:43:44 -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@...hat.com>,
        "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 v7 2/6] x86/asm: Avoid emitting DWARF CFI for non-VDSO

On 2026-02-03 09:19, Jens Remus wrote:
> From: Josh Poimboeuf <jpoimboe@...nel.org>
> 
> It was decided years ago that .cfi_* annotations aren't maintainable in
> the kernel.  They were replaced by objtool unwind hints.  For the kernel
> proper, ensure the CFI_* macros don't do anything.
> 
> On the other hand the VDSO library *does* use them, so user space can
> unwind through it.
> 
> Make sure these macros only work for VDSO.  They aren't actually being
> used outside of VDSO anyway, so there's no functional change.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> Signed-off-by: Jens Remus <jremus@...ux.ibm.com>
> ---
>  arch/x86/include/asm/dwarf2.h | 51 ++++++++++++++++++++++++-----------
>  1 file changed, 35 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/x86/include/asm/dwarf2.h b/arch/x86/include/asm/dwarf2.h
> index 09c9684d3ad6..7cc30500c095 100644
> --- a/arch/x86/include/asm/dwarf2.h
> +++ b/arch/x86/include/asm/dwarf2.h
> @@ -6,6 +6,15 @@
>  #warning "asm/dwarf2.h should be only included in pure assembly files"
>  #endif
>  
> +#ifdef BUILD_VDSO
> +
> +	/*
> +	 * For the vDSO, emit both runtime unwind information and debug
> +	 * symbols for the .dbg file.
> +	 */
> +
> +	.cfi_sections .eh_frame, .debug_frame
> +
>  #define CFI_STARTPROC		.cfi_startproc
>  #define CFI_ENDPROC		.cfi_endproc
>  #define CFI_DEF_CFA		.cfi_def_cfa
> @@ -22,21 +31,31 @@
>  #define CFI_ESCAPE		.cfi_escape
>  #define CFI_SIGNAL_FRAME	.cfi_signal_frame
>  
> -#ifndef BUILD_VDSO
> -	/*
> -	 * Emit CFI data in .debug_frame sections, not .eh_frame sections.
> -	 * The latter we currently just discard since we don't do DWARF
> -	 * unwinding at runtime.  So only the offline DWARF information is
> -	 * useful to anyone.  Note we should not use this directive if we
> -	 * ever decide to enable DWARF unwinding at runtime.
> -	 */
> -	.cfi_sections .debug_frame
> -#else
> -	 /*
> -	  * For the vDSO, emit both runtime unwind information and debug
> -	  * symbols for the .dbg file.
> -	  */
> -	.cfi_sections .eh_frame, .debug_frame
> -#endif
> +#else /* !BUILD_VDSO */
> +
> +/*
> + * On x86, these macros aren't used outside VDSO.  As well they shouldn't be:
> + * they're fragile and very difficult to maintain.
> + */
> +
> +.macro nocfi args:vararg
> +.endm
> +
> +#define CFI_STARTPROC		nocfi
> +#define CFI_ENDPROC		nocfi
> +#define CFI_DEF_CFA		nocfi
> +#define CFI_DEF_CFA_REGISTER	nocfi
> +#define CFI_DEF_CFA_OFFSET	nocfi
> +#define CFI_ADJUST_CFA_OFFSET	nocfi
> +#define CFI_OFFSET		nocfi
> +#define CFI_REL_OFFSET		nocfi
> +#define CFI_REGISTER		nocfi
> +#define CFI_RESTORE		nocfi
> +#define CFI_REMEMBER_STATE	nocfi
> +#define CFI_RESTORE_STATE	nocfi
> +#define CFI_UNDEFINED		nocfi
> +#define CFI_ESCAPE		nocfi
> +
> +#endif /* !BUILD_VDSO */
>  
>  #endif /* _ASM_X86_DWARF2_H */

I guess patch 3 justifies this.

Acked-by: H. Peter Anvin (Intel) <hpa@...or.com>