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From: Rasmus Andersson <rasmus@notion.se>
Date: Tue, 19 Jan 2021 14:02:50 -0800
Message-ID: <CADFzTteNPP3uVE43gk+mL+jQETr0O9AcoMbMb=GNYGz9ZaYyiw@mail.gmail.com>
To: Rich Felker <dalias@libc.org>
Cc: musl@lists.openwall.com
Content-Type: text/plain; charset="UTF-8"
Subject: Re: [musl] waitpid (wait4) on Linux 5 returns invalid values

The program compiles correctly with
https://more.musl.cc/9/x86_64-linux-musl/x86_64-linux-musl-native.tgz
(The GCC 9 version) Perhaps an issue with GCC 10?

On Tue, Jan 19, 2021 at 2:00 PM Rasmus Andersson <rasmus@notion.se> wrote:
>
> That was compiled using
> https://more.musl.cc/10/x86_64-linux-musl/x86_64-linux-musl-native.tgz
> on Alpine Linux in a docker container (alpine:3.12)
>
> Full repro:
> $ wget https://more.musl.cc/10/x86_64-linux-musl/x86_64-linux-musl-native.tgz
> $ tar -xf x86_64-linux-musl-native.tgz
> $ cat << EOF > a.c
> #include <sys/wait.h>
> #include <stdio.h>
> #include <errno.h>
> #include <string.h>
> int main(int argc, char **argv) {
>   int wstat = 0;
>   pid_t child = waitpid(-1, &wstat, WNOHANG);
>   printf("waitpid returned %d, wstat=%d, errno=%d (%s)\n", child,
> wstat, errno, strerror(errno));
>   return 0;
> }
> EOF
> $ x86_64-linux-musl-native/bin/gcc -static a.c
> $ ./a.out
> waitpid returned -10, wstat=0, errno=0 (No error information)
>
> On Tue, Jan 19, 2021 at 1:16 PM Rich Felker <dalias@libc.org> wrote:
> >
> > On Tue, Jan 19, 2021 at 12:35:26PM -0800, Rasmus Andersson wrote:
> > > 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
> >
> > This disassembly shows a miscompiled (or compiled with wrong patches)
> > waitpid. It should be a tail call to __syscall_ret, not a retq.
> >
> > Rich