linux-kernel - Re: For review: pidfd_send

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Message-ID: <f4330496-f34e-59f2-6de4-0aafdf639c7a@gmail.com>
Date:   Tue, 24 Sep 2019 21:43:38 +0200
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Florian Weimer <fw@...eb.enyo.de>
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>,
        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

Hello Florian,

On 9/23/19 1:26 PM, Florian Weimer wrote:
> * 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.

Thanks. I added: #include <signal.h>

>>        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.

Yes. I added "(i.e., it has terminated and been waited on)".

>>        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?

Please see my comment in reply to Christian (which will be sent just
after this).

>>        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.

Please see my open question in the thread on pidfd_open().

Thanks for the review, Florian.

Cheers,

Michael

Michael

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