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

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

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.