Re: [PATCH v2 4/9] aarch64: Add the clone3 wrapper

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 03/11/22 13:31, Szabolcs Nagy wrote:
> The 11/03/2022 13:22, Adhemerval Zanella Netto wrote:
>>
>>
>> On 03/11/22 11:01, Szabolcs Nagy wrote:
>>> The 11/03/2022 10:15, Adhemerval Zanella Netto wrote:
>>>> On 02/11/22 09:12, Szabolcs Nagy wrote:
>>>>> The 09/30/2022 16:26, Adhemerval Zanella via Libc-alpha wrote:
>>>>>> It follow the internal signature:
>>>>>>
>>>>>>   extern int clone3 (struct clone_args *__cl_args, size_t __size,
>>>>>>  int (*__func) (void *__arg), void *__arg);
>>>>>>
>>>>>> And x86_64 semantics to return EINVAL if either cl_args or func
>>>>>> is NULL.  The stack is 16-byte aligned prior executing func.
>>>>>
>>>>> "x86_64 semantics" sounds wrong: maybe this should be documented?
>>>>> i'd expect 0 cl_args/func to be UB like in most posix apis.
>>>>
>>>> Right, I think it is worth to document the function semantic
>>>> properly at least on its internal header (include/clone_internal.h).
>>>> H.J also added a new clone3.h headers, which is not currently installed
>>>> that I am inclined to just remove it from now.  We might reinstate 
>>>> if/when we decide to provide the clone3 as an ABI.
>>>>
>>>> And returning EINVAL for 0 cl_args/func aligns with our exported clone
>>>> interface, where EINVAL is also returned for 0 function argument.
>>>
>>> ok.
>>>
>>>>>
>>>>> and aligning sp in the child fails if signals are allowed there
>>>>> (pthreads does not allow signals now, direct callers might).
>>>>> i dont know if that's a concert (or if unaligned stack is
>>>>> something we should fix up in clone3).
>>>>
>>>> It was overlooked on initial x86_64 clone3 implementation as well.  I
>>>> think it better to just return EINVAL for unaligned stacks and avoid
>>>> to change the stack pointer in the created thread.
>>>
>>> long time ago linux did that on aarch64, but it was removed:
>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e6d9a52543338603e25e71e0e4942f05dae0dd8a
>>>
>>> i think in clone3 the kernel should have aligned (it knows
>>> the bounds now), doing it in the userspace wrapper is weird
>>> (should we adjust the stack size?). and not doing it at all
>>> makes clone3 hard to use portably (user has to know target
>>> specific pcs requirements).
>>>
>>> not sure what's the best way forward.
>>
>> I think the stack size won't matter much here, at least not from
>> kernel point of view (the resulting stack size will most likely
>> be page aligned anyway).  But I think this kernel commit makes a good
>> point that silently adjusting the stack in userland is not the
>> correct approach, I think H.J has done to make it consistent with
>> glibc clone implementation which does it.
>>
>> IMHO the best approach would to just remove the stack alignment,
>> since it incurs the signal handling issue.
> 
> current generic clone callers dont align the stack and
> e.g. unaligned pthread custom stack should work.
> 
> so we have to do arch specific stack alignment somewhere,
> maybe in pthread_create?

I am thinking on __clone_internal, where if an unaligned stack is
used it creates a new clone_args struct with adjust arguments.  It
can adjust the struct in place (not sure which is better).