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From: Rasmus Andersson <rasmus@notion.se>
To: musl@lists.openwall.com
Subject: Re: [musl] waitpid (wait4) on Linux 5 returns invalid values
Date: Tue, 19 Jan 2021 12:35:26 -0800	[thread overview]
Message-ID: <CADFzTtf0C86gO4w1Y_THKa+JeR1iTSORLv9NoafX7ZajXypUjA@mail.gmail.com> (raw)
In-Reply-To: <CADFzTtd-47-iNo-YxqEH5UD3vCuCWd5c-EypK3XTQV3aqG94=Q@mail.gmail.com>

Relevant parts from disassembly: (objdump --disassemble --source
--line-numbers a.out)

0000000000401f61 <__syscall_cp_c>:
sccp():
  401f61: 48 89 f8              mov    %rdi,%rax
  401f64: 4d 89 c2              mov    %r8,%r10
  401f67: 48 89 f7              mov    %rsi,%rdi
  401f6a: 4d 89 c8              mov    %r9,%r8
  401f6d: 48 89 d6              mov    %rdx,%rsi
  401f70: 4c 8b 4c 24 08        mov    0x8(%rsp),%r9
  401f75: 48 89 ca              mov    %rcx,%rdx
  401f78: 0f 05                 syscall
  401f7a: c3                    retq

0000000000401f7b <__syscall_cp>:
__syscall_cp():
  401f7b: e9 e1 ff ff ff        jmpq   401f61 <__syscall_cp_c>

00000000004004fd <waitpid>:
waitpid():
  ...
  40051a: e8 5c 1a 00 00        callq  401f7b <__syscall_cp>
  40051f: 48 83 c4 18           add    $0x18,%rsp
  400523: c3                    retq

main():
  ...
  4002a2: e8 56 02 00 00        callq  4004fd <waitpid>

On Tue, Jan 19, 2021 at 12:28 PM Rasmus Andersson <rasmus@notion.se> wrote:
>
> In the meantime, in case you're curious, here's a program that
> reproduces the issue. I've attached a statically-compiled version too
> that can be disassembled. Running it:
> $ gcc -static a.c && ./a.out
> waitpid returned -10, wstat=0
>
> #include <sys/wait.h>
> #include <stdio.h>
> int main(int argc, char **argv) {
>         int wstat = 0;
>         pid_t child = waitpid(-1, &wstat, WNOHANG);
>         printf("waitpid returned %d, wstat=%d\n", child, wstat);
>         return 0;
> }
>
> On Tue, Jan 19, 2021 at 12:17 PM Rasmus Andersson <rasmus@notion.se> wrote:
> >
> > On Tue, Jan 19, 2021 at 11:33 AM Markus Wichmann <nullplan@gmx.net> wrote:
> > >
> > > On Tue, Jan 19, 2021 at 10:18:04AM -0800, Rasmus Andersson wrote:
> > > > However the wait4 syscall[3] in Linux 5 returns other values,
> > > > specifically it returns errors as negative values. The error that
> > > > trips up programs like runit's runsv is ECHILD (-10) which wait4
> > > > returns when there are no children (i.e. they have exited.)
> > > >
> > >
> > > Hmm... that is very weird. That should not happen. Because the
> > > syscall_cp macro already contains a call to __syscall_ret(), which does
> > > exactly what you propose. So that means, there is something else going
> > > on in your copy of the code. Did you change anything about the source
> > > code? Or can you disassemble the function to see what it does?
> > >
> > Strange! You are right of course; looking at src/internal/syscall.h I
> > indeed see that syscall_cp calls __syscall_ret(__syscall_cp(args))
> > The musl I'm building with comes from https://musl.cc/#binaries which
> > is created from these scripts according to the author:
> > https://git.zv.io/xstatic/mcm It doesn't seem to apply any patches.
> > Anyhow, I'm currently building musl & gcc myself using
> > musl-cross-make. I will see how that pans out and report back.
> > Thank you for your quick response Markus!

  reply	other threads:[~2021-01-19 20:35 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-19 18:18 Rasmus Andersson
2021-01-19 19:33 ` Markus Wichmann
2021-01-19 20:17   ` Rasmus Andersson
2021-01-19 20:28     ` Rasmus Andersson
2021-01-19 20:35       ` Rasmus Andersson [this message]
2021-01-19 20:56         ` Rasmus Andersson
2021-01-19 21:16         ` Rich Felker
2021-01-19 22:00           ` Rasmus Andersson
2021-01-19 22:02             ` Rasmus Andersson
2021-01-19 23:01               ` Zach van Rijn
2021-01-19 23:07                 ` Rasmus Andersson

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