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

[Adhemerval Zanella Netto]

On 03/11/22 17:55, H.J. Lu wrote:
> On Thu, Nov 3, 2022 at 9:55 AM Adhemerval Zanella Netto
> <adhemerval.zanella@linaro.org> wrote:
>>
>>
>>
>> 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).
> 
> Is there a bug for the signal handling issue?

No with current usage, on both pthread_create and posix_spawn glibc mask
all internal signals (including internal ones).