Re: [RFC PATCH v1 06/16] unwind_user: Enable archs that define CFA = SP_callsite + offset

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

On 16.07.2025 23:32, Josh Poimboeuf wrote:
> On Thu, Jul 10, 2025 at 06:35:12PM +0200, Jens Remus wrote:
>> Most architectures define their CFA as the value of the stack pointer
>> (SP) at the call site in the previous frame, as suggested by the DWARF
>> standard:
>>
>>   CFA = <SP at call site>
>>
>> Enable unwinding of user space for architectures, such as s390, which
>> define their CFA as the value of the SP at the call site in the previous
>> frame with an offset:
>>
>>   CFA = <SP at call site> + offset
> 
> This is a bit confusing, as the comment and code define it as
> 
>     SP = CFA + offset
> 
> Should the commit log be updated to match that?

I agree that the commit message is confusing. Would it help if I replace
it with the following:

Most architectures define their CFA as the value of the stack pointer
(SP) at the call site in the previous frame, as suggested by the DWARF
standard.  Therefore the SP at call site can be unwound using an
implicitly assumed value offset from CFA rule with an offset of zero:

  .cfi_val_offset <SP>, 0

As a result the SP at call site computes as follows:

  SP = CFA

Enable unwinding of user space for architectures, such as s390, which
define their CFA as the value of the SP at the call site in the previous
frame with an offset.  Do so by enabling architectures to override the
default SP value offset from CFA of zero with an architecture-specific
one:

  .cfi_val_offset <SP>, offset
  
So that the SP at call site computes as follows:

  SP = CFA + offset

>> +++ b/arch/x86/include/asm/unwind_user.h
>> @@ -8,6 +8,7 @@
>>  	.cfa_off	= (s32)sizeof(long) *  2,				\
>>  	.ra_off		= (s32)sizeof(long) * -1,				\
>>  	.fp_off		= (s32)sizeof(long) * -2,				\
>> +	.sp_val_off	= (s32)0,						\
> 
> IIUC, this is similar to ra_off and fp_off in that its an offset from
> the CFA.  Can we call it "sp_off"?

My intent was to use the terminology from DWARF CFI (i.e. "offset(N)"
and "val_offset(N)") and the related assembler CFI directives:

  .cfi_offset register, offset:  Previous value of register is saved at
                                 offset from CFA.

  .cfi_val_offset register, offset:  Previous value of register is
                                     CFA + offset. 

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@de.ibm.com

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