Re: [PATCH v1] gdb: align siginfo_t with the Linux kernel definition

[Thiago Jung Bauermann <thiago.bauermann@linaro.org>]

Matthieu Longo <matthieu.longo@arm.com> writes:

> On 18/06/2026 04:02, Thiago Jung Bauermann wrote:
>> Matthieu Longo <matthieu.longo@arm.com> writes:
>> 
>>> On 17/06/2026 07:07, Thiago Jung Bauermann wrote:
>>>> compat_siginfo_t is used when the inferior is an AArch32 process while
>>>> GDB is AArch64. This will cause the 64-bit kernel to use the syscall
>>>> compatibility layer — and thus the 32-bit definition of siginfo_t —
>>>> which GDB's compat_siginfo_t aims to represent.
>>>
>>> I imagined that compat_siginfo_t was also used in the case when the inferior is an
>>> AArch64
>>> process, while GDB is AArch32.
>>> Hence ...
>> 
>> Ah, ok. This isn't supported, at least in the Arm port. I think GDB in
>> general doesn't support 32-bit GDB debugging 64-bit inferior, but I
>> could be wrong. It's certainly not well tested though. :)
>> 
>>>>> However, I am confused by the definition of compat_siginfo_t in gdb/nat/aarch64-linux.h
>>>>> for instance. I see that addresses like _sigfaul._addr are converted to 'unsigned int'
>>>>> (that should be equivalent to uint32_t I guess). Why is this working ?
>>>>>
>>>>> Please, could you provide explanations on the above, and guidances on the changes
>>>>> required
>>>>> in struct compat_siginfo_t ?
>>>> I'm not sure I understand why it wouldn't work. In the example of
>>>> _sigfault._addr, it is an unsigned int (equivalent to uint32_t as you
>>>> point out) so it will match the width of the void * used for the
>>>> corresponding void * field in 32-bit userspace. This is converted to
>>>> siginfo_t's 64-bit void * in aarch64_siginfo_from_compat_siginfo with:
>>>>     to->si_addr = (void *) (intptr_t) from->cpt_si_addr;
>>>> Do you see a problem in this process?
>>>
>>> my surprise when I say the definition of _sigfault._addr as a uint32_t. This would not
>>> have worked in my understanding.
>>
>> Yes, indeed in that case it wouldn't work.
>
> If the feature that requires new fields in siginfo_t, is only available AArch64 only,
> there should not be any reason to change aarch64_siginfo_from_compat_siginfo and
> aarch64_compat_siginfo_from_siginfo. Is this correct ?

Yes, that is correct. compact_siginfo_t only needs to reflect the
siginfo_t that the Linux kernel passes to AArch32 processes.

> If so, then the current patch is complete and ready to be reviewed.

Ok, I will start reviewing it today.

-- 
Thiago
(he/him)