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From: "H.J. Lu" <hjl.tools@gmail.com>
To: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
Cc: Szabolcs Nagy <szabolcs.nagy@arm.com>,
	libc-alpha@sourceware.org,
	 Christian Brauner <brauner@kernel.org>
Subject: Re: [PATCH v2 4/9] aarch64: Add the clone3 wrapper
Date: Thu, 3 Nov 2022 14:58:28 -0700	[thread overview]
Message-ID: <CAMe9rOrGtrhfUawRoJJdjWcUd6A24RtjnqH8J1FFj+NLiMFgbQ@mail.gmail.com> (raw)
In-Reply-To: <1a5cc9ec-de78-cb4d-3bd3-7f37dc666f73@linaro.org>

On Thu, Nov 3, 2022 at 2:22 PM Adhemerval Zanella Netto
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> 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.

For the internal clone3, we can drop stack alignment adjustment.  The
internal users are responsible for correct stack alignment.  If there is
a public clone3 wrapper, it should adjust stack alignment.

-- 
H.J.

  reply	other threads:[~2022-11-03 21:59 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-30 19:26 [PATCH v2 0/9] Optimize posix_spawn signal setup with clone3 Adhemerval Zanella
2022-09-30 19:26 ` [PATCH v2 1/9] linux: Do not reset signal handler in posix_spawn if it is already SIG_DFL Adhemerval Zanella
2023-01-11 21:06   ` Carlos O'Donell
2022-09-30 19:26 ` [PATCH v2 2/9] linux: Add clone3 CLONE_CLEAR_SIGHAND optimization to posix_spawn Adhemerval Zanella
2023-01-11 21:06   ` Carlos O'Donell
2022-09-30 19:26 ` [PATCH v2 3/9] powerpc64le: Add the clone3 wrapper Adhemerval Zanella
2022-09-30 19:26 ` [PATCH v2 4/9] aarch64: " Adhemerval Zanella
2022-11-02 12:12   ` Szabolcs Nagy
2022-11-03 13:15     ` Adhemerval Zanella Netto
2022-11-03 14:01       ` Szabolcs Nagy
2022-11-03 16:22         ` Adhemerval Zanella Netto
2022-11-03 16:31           ` Szabolcs Nagy
2022-11-03 16:39             ` Adhemerval Zanella Netto
2022-11-03 16:52               ` Szabolcs Nagy
2022-11-03 16:55                 ` Adhemerval Zanella Netto
2022-11-03 20:55                   ` H.J. Lu
2022-11-03 21:28                     ` Adhemerval Zanella Netto
2022-11-03 21:22                   ` Adhemerval Zanella Netto
2022-11-03 21:58                     ` H.J. Lu [this message]
2022-11-04 12:32                       ` Adhemerval Zanella Netto
2022-09-30 19:26 ` [PATCH v2 5/9] s390x: " Adhemerval Zanella
2022-09-30 19:26 ` [PATCH v2 6/9] riscv: " Adhemerval Zanella
2022-09-30 19:26 ` [PATCH v2 7/9] arm: " Adhemerval Zanella
2022-09-30 19:26 ` [PATCH v2 8/9] mips: " Adhemerval Zanella
2022-09-30 19:26 ` [PATCH v2 9/9] Linux: optimize clone3 internal usage Adhemerval Zanella
2023-01-11 21:12   ` Carlos O'Donell
2022-10-27 16:48 ` [PATCH v2 0/9] Optimize posix_spawn signal setup with clone3 Adhemerval Zanella Netto
2023-01-11 21:11 ` Carlos O'Donell

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