[PATCH 2/2] aarch64: Add workaround for GDB bug handling string literals

[Adhemerval Zanella Netto]

On 01/07/26 05:39, Yury Khrustalev wrote:
> On Tue, Jun 30, 2026 at 01:15:04PM -0300, Adhemerval Zanella Netto wrote:
>>
>> On 30/06/26 12:29, Yury Khrustalev wrote:
>>> On Tue, Jun 30, 2026 at 12:17:37PM -0300, Adhemerval Zanella Netto wrote:
>>>>
>>>> On 30/06/26 11:15, Wilco Dijkstra wrote:
>>>>> Hi,
>>>>>
>>>>>> I don't like it either, but I don't see a better alternative to keep some
>>>>>> compatibility with old gdb.
>>>>>>
>>>>>> But I also think this approach is fragile: it require some assumptions on how
>>>>>> gdb implements this and adds some magic number.� I think a better alternative
>>>>>> is invert the assumption: the only exactly legitimate caller of an IFUNC
>>>>>> resolver is the the dynamic loader's relocation machinery
>>>>>> (elf_ifunc_invoke / _dl_fixup):
>>>>>
>>>>> On AArch64 we can just check for !(arg0 & IFUNC_ARG_HWCAP) since we always
>>>>> invoke ifuncs with that bit set from GLIBC (obviously the value is way too large to
>>>>> ever be a valid allocation). You could also check arg0 == GLRO (dl_hwcap) for other
>>>>> targets.
>>>
>>> Yes, we should do this, I'll make a patch.
>>
>> I think if we want to add this workaround, doing in a platform neutral is better
>> than adding arch-specific knobs.
> 
> You probably missed second part of my reply. The idea is not to provide
> a work-around for the problem in GDB. The idea is to fail early with a
> clear error message when resolver is called incorrectly.
> 
> The patch series at the start of this thread *is* "arch-specific knobs".

I got that and my suggestion is doing exactly what you suggested [1],
but in the generic way (enforce all ifunc alls originate inside ld.so, 
so __builtin_return_address(0) lies within _dl_rtld_map's [l_map_start, l
_map_end).

This will allow other ABIs to enable malloc ifunc without the need to come
up with arch-specific heuristics.

And I think we should *decouple* the 1. stop the corruption goal from 2. 
make GDB's string-literal path work.

[1] https://sourceware.org/pipermail/libc-alpha/2026-July/178497.html

> 
> Also, we're talking about code under sysdeps/aarch64 so I don't see
> anything wrong with it being arch-specific.
> 
> As machine maintainer for AArch64 I'm against the Florian's patch and I
> think the right way ahead is to implement Wilco's suggestion.
> 
> Thanks,
> Yury
>