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

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

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.