linux-kernel - Re: For review: pidfd_send

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Date:   Mon, 23 Sep 2019 13:26:34 +0200
From:   Florian Weimer <fw@...eb.enyo.de>
To:     "Michael Kerrisk \(man-pages\)" <mtk.manpages@...il.com>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Christian Brauner <christian@...uner.io>,
        Jann Horn <jannh@...gle.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Daniel Colascione <dancol@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        linux-man <linux-man@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: For review: pidfd_send_signal(2) manual page

* Michael Kerrisk:

> SYNOPSIS
>        int pidfd_send_signal(int pidfd, int sig, siginfo_t info,
>                              unsigned int flags);

This probably should reference a header for siginfo_t.

>        ESRCH  The target process does not exist.

If the descriptor is valid, does this mean the process has been waited
for?  Maybe this can be made more explicit.

>        The  pidfd_send_signal()  system call allows the avoidance of race
>        conditions that occur when using traditional interfaces  (such  as
>        kill(2)) to signal a process.  The problem is that the traditional
>        interfaces specify the target process via a process ID (PID), with
>        the  result  that the sender may accidentally send a signal to the
>        wrong process if the originally intended target process has termi‐
>        nated  and its PID has been recycled for another process.  By con‐
>        trast, a PID file descriptor is a stable reference to  a  specific
>        process;  if  that  process  terminates,  then the file descriptor
>        ceases to be  valid  and  the  caller  of  pidfd_send_signal()  is
>        informed of this fact via an ESRCH error.

It would be nice to explain somewhere how you can avoid the race using
a PID descriptor.  Is there anything else besides CLONE_PIDFD?

>        static
>        int pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
>                unsigned int flags)
>        {
>            return syscall(__NR_pidfd_send_signal, pidfd, sig, info, flags);
>        }

Please use a different function name.  Thanks.