linux-kernel - Re: For review: pidfd_send

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Message-ID: <CAKOZuetMK0eRxBrR8wXo_qCaQ7OGKQHqAy15cX437+Q+cvbbvA@mail.gmail.com>
Date:   Mon, 23 Sep 2019 04:31:09 -0700
From:   Daniel Colascione <dancol@...gle.com>
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>,
        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

On Mon, Sep 23, 2019 at 2:12 AM Michael Kerrisk (man-pages)
<mtk.manpages@...il.com> wrote:
>        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

The file *descriptor* remains valid even after the process to which it
refers exits. You can close(2) the file descriptor without getting
EBADF. I'd say, instead, that "a PID file descriptor is a stable
reference to a specific process; process-related operations on a PID
file descriptor fail after that process exits".