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

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Message-ID: <832897ac-f1d4-4f91-8034-f6b2d9e86fa4@linux.ibm.com>
Date: Fri, 6 Feb 2026 15:01:19 +0100
From: Jens Remus <jremus@...ux.ibm.com>
To: "H. Peter Anvin" <hpa@...or.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 2/4/2026 1:43 AM, H. Peter Anvin wrote:
> 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

I missed that this is new on tip:x86/entry and thus also needs to be
added below in the !BUILD_VDSO case.

>>  
>> -#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

#define CFI_SIGNAL_FRAME		nocfi

>> +
>> +#endif /* !BUILD_VDSO */
>>  
>>  #endif /* _ASM_X86_DWARF2_H */
> 
> I guess patch 3 justifies this.
> 
> Acked-by: H. Peter Anvin (Intel) <hpa@...or.com>
> 

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
jremus@...ibm.com / jremus@...ux.ibm.com

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