Re: extending wait4(2) or waitid(2) linux syscall

[Arnd Bergmann <arnd@arndb.de>]

On Mon, Nov 26, 2018 at 4:18 PM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Arnd Bergmann:
>
> > On Thu, Nov 15, 2018 at 7:30 AM Dmitry V. Levin <ldv@altlinux.org> wrote:
> >> On Thu, Nov 15, 2018 at 06:39:03AM -0800, Arnd Bergmann wrote:
> >> > On Thu, Nov 15, 2018 at 6:05 AM Dmitry V. Levin wrote:
> >> > > On Thu, Apr 20, 2017 at 03:20:51PM +0200, Albert ARIBAUD wrote:
> >>
> >> 1. strace needs a race-free invocation of wait4(2) or waitid(2)
> >> with a different signal mask, this cannot be achieved without
> >> an extended version of syscall, similar to pselect6(2) extension
> >> over select(2) and ppoll(2) extension over poll(2).
> >>
> >> Signal mask specification in linux requires two parameters:
> >> "const sigset_t *sigmask" and "size_t sigsetsize".
> >> Creating pwait6(2) as an extension of wait4(2) with two arguments
> >> is straightforward.
> >> Creating pwaitid(2) as an extension of waitid(2) that already has 5
> >> arguments would require an indirection similar to pselect6(2).
> >
> > Getting back to this point: you could also do the same thing with
> > the CLONE_FD approach from Josh Triplett[1] or Casey Dahlin's
> > older waitfd() syscall, correct?
>
> A descriptor-based solution would not be useful to glibc because
> applications assume that glibc does not (persistently) open any file
> descriptors behind t heir back.

Right, makes sense. What about a temporary file descriptor as discussed
in the recent procfd() mail thread then? Would that work?

/* for illustration, needs error handling and more features */
int pwait(pid_t id, siginfo_t *infop)
{
      char waitfd_file[MAX_PROCFD_LEN];
      struct pollfd pfd[1] = { {.events = POLLIN }};

      snprintf(waitfd_file, MAX_PROCFD_LEN, "/proc/%d/wait", pid);
      pfd.fd = open(waitfd_file,  O_RDONLY);
      ppoll(&pfd, 1, NULL, sigmask);
      read(fd, infop, sizeof(*infop));
      close(fd);

     return 0;
}

       Arnd