From: "Thompson, David" <dthompson2@worcester.edu>
To: "Ludovic Courtès" <ludo@gnu.org>
Cc: 21694@debbugs.gnu.org, David Thompson <davet@gnu.org>
Subject: bug#21694: 'clone' syscall binding unreliable
Date: Fri, 16 Oct 2015 19:12:53 -0400 [thread overview]
Message-ID: <CAJ=RwfZvF3p1xHJ3nvPnS0ULTjL2rVnOPJXHJKEpiE=e7VPLzw@mail.gmail.com> (raw)
In-Reply-To: <87zizio8dc.fsf@gnu.org>
On Fri, Oct 16, 2015 at 4:39 PM, Ludovic Courtès <ludo@gnu.org> wrote:
> I’m reporting the problem and (hopefully) the solution, but I think we’d
> better double-check this.
>
> The problem: Running the test below in a loop sometimes gets a SIGSEGV
> in the child process (on x86_64, libc 2.22.)
>
> --8<---------------cut here---------------start------------->8---
> (use-modules (guix build syscalls) (ice-9 match))
>
> (match (clone (logior CLONE_NEWUSER
> CLONE_CHILD_SETTID
> CLONE_CHILD_CLEARTID
> SIGCHLD))
> (0
> (throw 'x)) ;XXX: sometimes segfaults
> (pid
> (match (waitpid pid)
> ((_ . status)
> (pk 'status status)
> (exit (not (status:term-sig status)))))))
> --8<---------------cut here---------------end--------------->8---
>
> Looking at (guix build syscalls) though, I see an ABI mismatch between
> our definition and the actual ‘syscall’ C function, and between our
> ‘clone’ definition and the actual C function.
>
> This leads to the attached patch, which also fixes the above problem for me.
>
> Could you test this patch?
The patch looks good. Thanks for catching this!
> Now, there remains the question of CLONE_CHILD_SETTID and
> CLONE_CHILD_CLEARTID. Since we’re passing NULL for ‘ctid’, I expect
> that these flags have no effect at all.
I added those flags in commit ee78d02 because they solved a real issue
I ran into. Adding those flags made 'clone' look like a
'primitive-fork' call when examined with strace.
> Conversely, libc uses these flags to update the thread ID in the child
> process (x86_64/arch-fork.h):
>
> --8<---------------cut here---------------start------------->8---
> #define ARCH_FORK() \
> INLINE_SYSCALL (clone, 4, \
> CLONE_CHILD_SETTID | CLONE_CHILD_CLEARTID | SIGCHLD, 0, \
> NULL, &THREAD_SELF->tid)
> --8<---------------cut here---------------end--------------->8---
>
> This is certainly useful, but we’d have troubles doing it from the FFI…
> It may that this is fine if the process doesn’t use threads.
Right, so here's what 'primitive-fork' does:
clone(child_stack=0,
flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD,
child_tidptr=0x7fc5398cea10) = 13247
Here's what 'clone' does:
clone(child_stack=0,
flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0)
= 14038
In practice it may not be a problem since most of the time you'd
'exec' after cloning. Is there any reliable way to get a hold of
whatever THREAD_SELF is? I wish the libc 'clone' function didn't have
that silly callback and behaved like 'fork', then we could have
avoided these issues altogether.
- Dave
next prev parent reply other threads:[~2015-10-16 23:13 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-10-16 20:39 bug#21694: 'clone' syscall binding unreliable Ludovic Courtès
2015-10-16 23:12 ` Thompson, David [this message]
2015-10-17 10:14 ` Ludovic Courtès
2015-10-28 14:39 ` Ludovic Courtès
2015-10-22 14:38 ` Mark H Weaver
2015-10-25 20:59 ` Ludovic Courtès
2015-10-28 4:53 ` Mark H Weaver
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