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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 14 Aug 2019 10:06:19 -0700
Message-ID: <CAHk-=wgpoeAnhscv9+fKNLLJF0tvypGPAxyzBCa0rp5hppRDRQ@mail.gmail.com>
Subject: Re: [PATCH v3 1/1] waitid: Add support for waiting for the current
 process group
To:     Rich Felker <dalias@libc.org>
Cc:     Christian Brauner <christian.brauner@ubuntu.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        GNU C Library <libc-alpha@sourceware.org>,
        Alistair Francis <alistair23@gmail.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Adhemerval Zanella <adhemerval.zanella@linaro.org>,
        Florian Weimer <fweimer@redhat.com>,
        Palmer Dabbelt <palmer@sifive.com>, macro@wdc.com,
        Zong Li <zongbox@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>, Peter Anvin <hpa@zytor.com>
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On Wed, Aug 14, 2019 at 9:55 AM Rich Felker <dalias@libc.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