[PATCH v2 03/11] s390: Initial support to generate .sframe from CFI directives in assembler

[Jens Remus]

On 06.06.2025 19:38, Indu Bhagat via Binutils wrote:
> On 6/3/25 6:11 AM, Jens Remus wrote:
>>>> +@section s390x
>>>> +
>>>> +A stack tracer implementation must initialize the SP to the designated SP
>>>> +register value, the FP to the preferred FP register value, and the RA to the
>>>> +designated RA register value in the topmost stack frame of the callchain.  This
>>>> +is required, as either the SP or FP is used as CFA base register and as the FP
>>>> +and/or RA are not necessarily saved on the stack.  For RA this may only be the
>>>> +case in the topmost stack frame of the callchain.  For FP this may be the case
>>>> +in any stack frame.
>>>> +
>>>> +Irrespective of the ABI, the first stack offset is always used to locate the
>>>> +CFA, by interpreting it as: CFA = @code{BASE_REG} + offset1.
>>>> +The identification of the @code{BASE_REG} is done by using the
>>>> +@code{fre_cfa_base_reg_id} field in the SFrame FRE info word.
>>>> +
>>>> +The (64-bit) s390x ELF ABI does not mandate the precise location in a function
>>>> +where the return address (RA) and frame pointer (FP) are saved, if at all.
>>>> +Hence the need to track RA in the SFrame stack trace format.  As RA is being
>>>> +tracked in this ABI, the second stack offset is always used to locate the RA
>>>> +stack slot, by interpreting it as: RA = CFA + offset2.  RA remains unchanged,
>>>> +if the offset is not available.  The third stack offset is used to locate the
>>>> +FP stack slot, by interpreting it as: FP = CFA + offset3.  FP remains unchanged,
>>>> +if the offset is not available.
>>>> +It is recommended that a stack tracer implementation performs the required
>>>> +checks to ensure that RA remains unchanged only for the topmost stack frame
>>>> +in the callchain.
>>>> +
>>> I find the last statement difficult to follow. In light of the patches
>>> 04/11 and 05/11, which allow RA to be saved in say FPR, or RA to be
>>> not saved on stack respectively, I am not sure I fully grasp the scope
>>> of the statement "RA remains unchanged only for the topmost stack
>>> frame in the callchain".
>> Let me try to explain what I intended to convey.  Maybe you can help me
>> get the wording right.
>>
>> A stack tracer using SFrame first initializes:
>>
>>    SP = designated SP register content
>>    FP = designated (s390x: preferred) SP register content
>>    RA = designated RA register content, if arch uses RA tracking
>>
>> Then for each frame it uses the SFrame CFA, FP, and RA tracking info to
>> determine:
>>
>>    CFA = CFA_base_reg + CFA_offset
>>
>>    SP = CFA [+ SP_val_offset on s390x]
>>
>>    if FP_offset != 0:
>>     FP = restore FP from stack at CFA + FP_offset (or register on s390x)
>>
>>    if RA_offset == 0 && !topmost:
>>     error
>>    if RA_offset != 0:
>>     RA = restore RA from stack at CFA + RA_offset (or register on s390x)
>>
>> Whether RA may be unchanged (not saved on the stack nor in another
>> register) is only valid in the topmost frame.  For the FP that may
>> be the case in for any frame, as a frame is not forced to modify
>> the FP.  Saved in another register is also only valid in the topmost
>> frame, as the stack tracer only then has access to all register values.
> 
> (Sorry for the delay)
> 
> Thanks.  IMO, the following is clear(er):
> 
> It is recommended that a stack tracer implements a check to ensure
> that RA offset can have a value of SFRAME_FRE_RA_OFFSET_INVALID for
> topmost frame only.
> 
> And for better flow, I would recommend the above line be added before
> we start talking about the FP offset.  So, perhaps right after the
> statement : "RA remains unchanged, if the offset is not available or
> has a value of SFRAME_FRE_RA_OFFSET_INVALID"

I have reworded the paragraphs regarding RA tracking in my next version.

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

IBM

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