[PATCH v2] gas: sframe: Represent .cfi_undefined RA as SFrame FDE without FREs

[Jens Remus]

Hello Indu!

On 22.07.2025 16:31, Jens Remus wrote:
> On 22.07.2025 05:00, Indu Bhagat via Binutils wrote:
>> On 7/21/25 9:23 AM, Jens Remus wrote:
>>> In DWARF CFI an "undefined" register rule for the return address (RA)
>>> register indicates that there is no return address and the stack trace
>>> is complete.  Represent DW_CFA_undefined as SFrame FDE without any FREs,
>>> so that a stack tracer implementation can use this as indication that
>>> an outermost frame has been reached and the trace is complete.
>>>
>>> This representation is backwards compatible, as existing stack tracers
>>> should already deal with the case, that an SFrame FDE has a FRE count of
>>> zero and stop the trace.
> 
>> While representing the construct "DW_CFA_undefined RA" with an SFrame
>> FDE with 0 FREs is OK, I think using this as an unambiguous marker for
>> outermost frame may be not be
>>
>> I think of case when, say, we may need to carry some function-specific
>> information in the SFrame FDE, but cannot represent the stack trace
>> informtion in SFrame format (for various reasons).  In such a case, we
>> may want an SFrame FDE with 0 FREs (with markers on FDE as necessary).
>> Yes that implcitly indicates that one cannot stack trace beyond this
>> function (as there is no information for stacktracing), but is that
>> necessarily the outermost frame? May be not.
>>
>> Which is why I think for a stack tracer, using "SFrame FDE with 0 FREs"
>> _together_ with a marker like FP is zero on AMD64 and FP/LR are zero on
>> AArch64 is still reasonable.  If a similar check is not possible for
>> s390x as you point out (although I have a question on that, posted on
>> the thread on libc-alpha), an option is to use the spare bit in FDE
>> func info to unambiguously mark outermost frame along with an SFrame
>> FDE with 0 FREs.
> 
> As replied in the other thread on libc-aplha there is no such indication
> on s390x (s390 64-bit).  I'll post a V3 of this patch, which introduces
> a SFrame FDE infor word flag "ra_undefined".

An attempt to build Glibc with my Binutils patch applied fails due to 
one or more relocation(s) out or range during link.  I guess the .sframe
merge logic chokes on a FDE without any FREs.

Please let me know if you gave any guess, where I should start.  I will
continue tomorrow.

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

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