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

[Jens Remus <jremus@linux.ibm.com>]

On 2/4/2026 1:43 AM, H. Peter Anvin wrote:
> On 2026-02-03 09:19, Jens Remus wrote:
>> From: Josh Poimboeuf <jpoimboe@kernel.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@kernel.org>
>> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
>> Signed-off-by: Jens Remus <jremus@linux.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@zytor.com>
> 

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

IBM Deutschland Research & Development GmbH; Vorsitzender des Aufsichtsrats: Wolfgang Wendt; Geschäftsführung: David Faller; Sitz der Gesellschaft: Ehningen; Registergericht: Amtsgericht Stuttgart, HRB 243294
IBM Data Privacy Statement: https://www.ibm.com/privacy/