From: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
To: Christian Brauner <christian.brauner@ubuntu.com>,
Florian Weimer <fw@deneb.enyo.de>
Cc: mtk.manpages@gmail.com, Oleg Nesterov <oleg@redhat.com>,
Christian Brauner <christian@brauner.io>,
Jann Horn <jannh@google.com>,
"Eric W. Biederman" <ebiederm@xmission.com>,
Daniel Colascione <dancol@google.com>,
Joel Fernandes <joel@joelfernandes.org>,
linux-man <linux-man@vger.kernel.org>,
Linux API <linux-api@vger.kernel.org>,
lkml <linux-kernel@vger.kernel.org>
Subject: Re: For review: pidfd_send_signal(2) manual page
Date: Tue, 24 Sep 2019 21:44:49 +0200 [thread overview]
Message-ID: <90dd38d5-34b3-b72f-8e5a-b51f944f22fb@gmail.com> (raw)
In-Reply-To: <20190923142325.jowzbnwjw7g7si7j@wittgenstein>
Hello Christian,
On 9/23/19 4:23 PM, Christian Brauner wrote:
> On Mon, Sep 23, 2019 at 01:26:34PM +0200, 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.
>
> Agreed.
>
>>
>>> 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.
>
> If by valid you mean "refers to a process/thread-group leader" aka is a
> pidfd then yes: Getting ESRCH means that the process has exited and has
> already been waited upon.
> If it had only exited but not waited upon aka is a zombie, then sending
> a signal will just work because that's currently how sending signals to
> zombies works, i.e. if you only send a signal and don't do any
> additional checks you won't notice a difference between a process being
> alive and a process being a zombie. The userspace visible behavior in
> terms of signaling them is identical.
(Thanks for the clarification. I added the text "(i.e., it has
terminated and been waited on)" to the ESRCH error.)
>>> 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?
>
> If you're the parent of the process you can do this without CLONE_PIDFD:
> pid = fork();
> pidfd = pidfd_open();
> ret = pidfd_send_signal(pidfd, 0, NULL, 0);
> if (ret < 0 && errno == ESRCH)
> /* pidfd refers to another, recycled process */
Although there is still the race between the fork() and the
pidfd_open(), right?
>>> 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.
Covered in another thread. I await some further feedback from Florian.
Thanks,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
next prev parent reply other threads:[~2019-09-24 19:44 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-23 9:12 For review: pidfd_send_signal(2) manual page Michael Kerrisk (man-pages)
2019-09-23 11:26 ` Florian Weimer
2019-09-23 14:23 ` Christian Brauner
2019-09-24 19:44 ` Michael Kerrisk (man-pages) [this message]
2019-09-24 19:57 ` Christian Brauner
2019-09-24 20:07 ` Christian Brauner
2019-09-24 21:00 ` Michael Kerrisk (man-pages)
2019-09-24 21:08 ` Daniel Colascione
2019-09-25 13:46 ` Michael Kerrisk (man-pages)
2019-09-24 21:53 ` Christian Brauner
2019-09-25 13:46 ` Michael Kerrisk (man-pages)
2019-09-25 13:51 ` Florian Weimer
2019-09-25 14:02 ` Michael Kerrisk (man-pages)
2019-09-25 13:53 ` Christian Brauner
2019-09-25 14:29 ` Michael Kerrisk (man-pages)
2019-09-24 19:43 ` Michael Kerrisk (man-pages)
2019-09-25 1:48 ` Jann Horn
2019-09-23 11:31 ` Daniel Colascione
2019-09-24 19:42 ` Michael Kerrisk (man-pages)
2019-09-23 14:29 ` Christian Brauner
2019-09-23 20:27 ` Michael Kerrisk (man-pages)
2019-09-23 21:27 ` Eric W. Biederman
2019-09-24 19:10 ` Michael Kerrisk (man-pages)
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=90dd38d5-34b3-b72f-8e5a-b51f944f22fb@gmail.com \
--to=mtk.manpages@gmail.com \
--cc=christian.brauner@ubuntu.com \
--cc=christian@brauner.io \
--cc=dancol@google.com \
--cc=ebiederm@xmission.com \
--cc=fw@deneb.enyo.de \
--cc=jannh@google.com \
--cc=joel@joelfernandes.org \
--cc=linux-api@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-man@vger.kernel.org \
--cc=oleg@redhat.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).