linux-kernel - Re: For review: pidfd_send

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Date:   Tue, 24 Sep 2019 21:42:35 +0200
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     mtk.manpages@...il.com, 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 9/23/19 1:31 PM, Daniel Colascione wrote:
> 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".

Thanks, Daniel. I like that rephrasing, but, since pidfd_send_signal()
is (so far as I know) currently the only relevant process-related
operation (and because this is the manual page describing that
syscall), I made it:

[[
By contrast, a PID file descriptor is a stable reference to a
specific process; if that process terminates, pidfd_send_signal()
fails with the error ESRCH.
]]

Thanks,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/