Re: [RFC v3 03/23] sysdeps/wait: Use waitid if avaliable

[Alistair Francis <alistair23 at gmail dot com>]

On Thu, Jul 25, 2019 at 10:14 AM Eric W. Biederman
<ebiederm@xmission.com> wrote:
>
> Arnd Bergmann <arnd@arndb.de> writes:
>
> > On Thu, Jul 25, 2019 at 6:40 AM Rich Felker <dalias@libc.org> wrote:
> >> On Wed, Jul 24, 2019 at 05:04:53PM -0700, Alistair Francis wrote:
> >> > On Tue, Jul 23, 2019 at 1:45 AM Arnd Bergmann <arnd@arndb.de> wrote:
> >> > >
> >> > > Sounds good to me, the debate over what rusage to use should not hold
> >> > > up the review of the rest of that syscall.
> >> >
> >> > I'm unclear what the final decision is here. What is the solution are
> >> > we going to have wait4() or add P_PROCESS_PGID to waitid()?
> >> >
> >> > As well as that what is the solution to current implementations? If we
> >> > add wait4() then there isn't an issue (and I can drop this patch) but
> >> > if we add P_PROCESS_PGID then we will need a way to handle kernels
> >> > with waitid() but no P_PROCESS_PGID. Although my new plan is to only
> >> > use the waitid syscall if we don't have waitpid or wait4 so it seems
> >> > like this will only affect RV32 for the time being.
> >>
> >> I would really like some indication which solution will be taken,
> >> since it impacts choices that will need to be made in musl very soon.
> >> My favorite outcome would be bringing back wait4 for rv32 (and
> >> no-time32 archs in general) *and* adding P_PROCESS_PGID. In the short
> >> term, just using wait4 would be the simplest and cleanest for us (same
> >> as all other archs, no extra case to deal with), but in the long term
> >> there may be value in having rusage that can represent more than 68
> >> cpu-years spent by a process (seems plausible with large numbers of
> >> cores).
> >
> > Based on the feedback from Linus and Eric, the most likely outcome
> > at the moment seems to be an extension of waitid() to allow
> > P_PGID with id=0 like BSD does, and not bring back wait4() or
> > add P_PROCESS_PGID.
> >
> > So far, I don't think anyone has proposed an actual kernel patch.
> > I was hoping that Eric would do it, but I could also send it if he's
> > otherwise busy.
>
> So here is what I am looking at.  It still needs to be tested
> and the description needs to be improved so that it properly credits
> everyone.  However I think I have the major stroeks correct.
>
> From: "Eric W. Biederman" <ebiederm@xmission.com>
> Date: Tue, 23 Jul 2019 07:44:46 -0500
> Subject: [PATCH] waitid: Add support for waiting for the current process group
>
> It was recently discovered that the linux version of waitid is not a
> superset of the other wait functions because it does not include
> support for waiting for the current process group.  This has two
> downsides.  An extra system call is needed to get the current process
> group, and a signal could come in between the system call that
> retrieved the process gorup and the call to waitid that changes the
> current process group.
>
> Allow userspace to avoid both of those issues by defining
> idtype == P_PGID and id == 0 to mean wait for the caller's process
> group at the time of the call.
>
> Arguments can be made for using a different choice of idtype and id
> for this case but the BSDs already use this P_PGID and 0 to indicate
> waiting for the current process's process group.  So be nice to user
> space programmers and don't introduce an unnecessary incompatibility.
>
> Some people have noted that the posix description is that
> waitpid will wait for the current process group, and that in
> the presence of pthreads that process group can change.  To get
> clarity on this issue I looked at XNU, FreeBSD, and Luminos.  All of
> those flavors of unix waited for the current process group at the
> time of call and as written could not adapt to the process group
> changing after the call.
>
> At one point Linux did adapt to the current process group changing but
> that stopped in 161550d74c07 ("pid: sys_wait... fixes").  It has been
> over 11 years since Linux has that behavior, no programs that fail
> with the change in behavior have been reported, and I could not
> find any other unix that does this.  So I think it is safe to clarify
> the definition of current process group, to current process group
> at the time of the wait function.
>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

Will this land in 5.3? Also will it be back ported to stable kernels?

Do you mind keeping me in CC when you send the patch?

Alistair

> ---
>  kernel/exit.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/exit.c b/kernel/exit.c
> index a75b6a7f458a..3d86930f035e 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -1577,14 +1577,16 @@ static long kernel_waitid(int which, pid_t upid, struct waitid_info *infop,
>                 break;
>         case P_PGID:
>                 type = PIDTYPE_PGID;
> -               if (upid <= 0)
> +               if (upid < 0)
>                         return -EINVAL;
> +               if (upid == 0)
> +                       pid = get_pid(task_pgrp(current));
>                 break;
>         default:
>                 return -EINVAL;
>         }
>
> -       if (type < PIDTYPE_MAX)
> +       if ((type < PIDTYPE_MAX) && !pid)
>                 pid = find_get_pid(upid);
>
>         wo.wo_type      = type;
> --
> 2.21.0.dirty
>