Re: [PATCH] save/restore errno around pthread_atfork handlers

[Bobby Bingham <koorogi@koorogi.info>]

On Fri, Nov 10, 2017 at 06:31:34PM -0500, Rich Felker wrote:
> On Fri, Nov 10, 2017 at 02:58:29PM -0600, Bobby Bingham wrote:
> > If the syscall fails, errno must be preserved for the caller. There's no
> > guarantee that the handlers registered with pthread_atfork won't clobber
> > errno.
> > ---
> >  src/process/fork.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> >
> > diff --git a/src/process/fork.c b/src/process/fork.c
> > index b96f0024..6602eafc 100644
> > --- a/src/process/fork.c
> > +++ b/src/process/fork.c
> > @@ -15,6 +15,7 @@ pid_t fork(void)
> >  {
> >  	pid_t ret;
> >  	sigset_t set;
> > +	int olderr;
> >  	__fork_handler(-1);
> >  	__block_all_sigs(&set);
> >  #ifdef SYS_fork
> > @@ -30,6 +31,10 @@ pid_t fork(void)
> >  		libc.threads_minus_1 = 0;
> >  	}
> >  	__restore_sigs(&set);
> > +
> > +	olderr = errno;
> >  	__fork_handler(!ret);
> > +	errno = olderr;
> > +
> >  	return ret;
> >  }
> > --
> > 2.15.0
>
> I think the patch as written is incorrect, because it can set errno to
> 0 after application code in the atfork handler set it to something
> nonzero; doing so is non-conforming.

Good point.  It does make me wonder though: when libc invokes a callback
and that callback sets errno to zero, is that a violation of the
prohibition on library functions setting errno to zero?

>
> It would be possible to special-case to avoid this, but it probably
> makes more sense to just call SYS_fork/SYS_clone with __syscall rather
> than syscall, then return __syscall_ret(ret) instead of return ret.
> Does that sound correct?

Yes, and it's also probably simpler.  I'll send a new patch.

>
> Rich