linux-kernel - Re: [PATCH v3 1/1] waitid: Add support for waiting for the current process group

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Date:   Wed, 14 Aug 2019 10:06:19 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Rich Felker <dalias@...c.org>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        GNU C Library <libc-alpha@...rceware.org>,
        Alistair Francis <alistair23@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Arnd Bergmann <arnd@...db.de>,
        Adhemerval Zanella <adhemerval.zanella@...aro.org>,
        Florian Weimer <fweimer@...hat.com>,
        Palmer Dabbelt <palmer@...ive.com>, macro@....com,
        Zong Li <zongbox@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>, Peter Anvin <hpa@...or.com>
Subject: Re: [PATCH v3 1/1] waitid: Add support for waiting for the current
 process group

On Wed, Aug 14, 2019 at 9:55 AM Rich Felker <dalias@...c.org> wrote:
>
> I don't think "downsides" sufficiently conveys that this is hard
> breakage of a requirement for waitpid.

Well, let's be honest here. Who has _ever_ seen a signal handler
changing the current process group?

In fact, the SYSV version of setpgid() takes a process ID to set it
*for somebody else*, so the signal safety is not even necessarily
relevant, since it might be racing with _another_ thread doing it
(which even the kernel side won't fix - it's just user space doing odd
things).

So yes - it's technically true that it's impossible to emulate
properly in user space.

But I doubt it makes _any_ difference what-so-ever, and glibc might as
well do something like

     ret = waitid(P_PGID, 0, ..);
     if (ret == -EINVAL) { do the emulation }

which makes it work with older kernels, and has zero downside in practice.

Hmm?

              Linus