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

["H.J. Lu" <hjl.tools@gmail.com>]

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?


-- 
H.J.