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

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

On 03/11/22 13:52, Szabolcs Nagy wrote:
> The 11/03/2022 13:39, Adhemerval Zanella Netto wrote:
>>
>>
>> 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).
> 
> if the api is not exposed, then i think the arg can be modified
> in place. (if clone3 api is exposed to users then we should not
> modify user structs unless the clone3 api contract explicitly
> allows this.)
> 
> either aligning in pthread_create or __clone_internal works for me,
> the target specific clone3 syscall should not in case that gets
> exposed to users.
> 

The arg modification would be done only internally by __clone_internal,
if we ever export __clone3 it will not mess with stack alignment (my
idea is to remove it from x86_64 as well).