Re: [musl] Potentially infinite loop in posix_spawn'ed child

[Rich Felker <dalias@libc.org>]

On Tue, May 25, 2021 at 09:30:18AM +0300, Alexey Izbyshev wrote:
> On 2021-05-24 23:33, Rich Felker wrote:
> >On Mon, May 24, 2021 at 01:09:21PM +0300, Alexey Izbyshev wrote:
> >>Hi,
> >>
> >>I've noticed the following loop at https://git.musl-libc.org/cgit/musl/tree/src/process/posix_spawn.c#n159:
> >>
> >>    exec(args->path, args->argv, args->envp);
> >>    ret = -errno;
> >>
> >>fail:
> >>    /* Since sizeof errno < PIPE_BUF, the write is atomic. */
> >>    ret = -ret;
> >>    if (ret) while (__syscall(SYS_write, p, &ret, sizeof ret) < 0);
> >>    _exit(127);
> >>
> >>Is there any reason that write is done in a loop? If SIGPIPE is
> >>blocked or ignored and the parent dies before this point, the child
> >>will spin in it forever.
> >
> >I suppose the special case of EPIPE should be considered here as no
> >need to inform the parent. Are there any other errors that should be
> >treated specially?
> >
> I'm not aware of any other errors that would need treatment. Is this
> loop intended to be a detection/debugging aid in case of an
> unexpected error?

It's not a debugging aid so much as a guarantee against forward
progress doing the wrong thing (wrongly reporting success to the
parent when the execve failed). I don't think there are any errors
that should be able to happen here aside from EPIPE though, short of
munging with syscall semantics using seccomp or something which is
outside the scope of what could be expected to work correctly.

Rich