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

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

On 03/11/22 13:55, Adhemerval Zanella Netto 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).

All the internal usage of __clone_internal are done with all signal masked,
so aligning the stack is currently safe.  However, I still think moving out
the stack alignment of __clone3 is still a net gain: it remove an
implementation detail (block/unblock signals) and simplifies the arch-specific
code.

However it makes a possible libc wrap clunky, the caller will need to know
the ABI stack alignment prior to the call since kernel does not automatically
align the stack.