linux-kernel - Re: [PATCH] Introduce ActivePid: in /proc/self/status (v2, was Vpid:)

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Message-ID: <m1ei2o9l7a.fsf@fess.ebiederm.org>
Date:	Mon, 20 Jun 2011 15:44:57 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Bryan Donlan <bdonlan@...il.com>
Cc:	Greg Kurz <gkurz@...ibm.com>, akpm@...ux-foundation.org,
	containers@...ts.osdl.org, linux-kernel@...r.kernel.org,
	serge@...lyn.com, daniel.lezcano@...e.fr, oleg@...hat.com,
	xemul@...nvz.org, Cedric Le Goater <clg@...t.ibm.com>
Subject: Re: [PATCH] Introduce ActivePid: in /proc/self/status (v2, was Vpid:)

Bryan Donlan <bdonlan@...il.com> writes:

> On Mon, Jun 20, 2011 at 07:45, Greg Kurz <gkurz@...ibm.com> wrote:
>> On Thu, 2011-06-16 at 13:54 -0400, Bryan Donlan wrote:
>
>>> Although getting the in-namespace PID is a useful thing, wouldn't a
>>> truly race-free API be preferable? Any access by PID has the race
>>> condition in which the target process could die, and its PID get
>>> recycled between retrieving the PID and doing something with it.
>>
>> Well the PID is a racy construct when used by another task than the
>> parent... fortunately, most userland code can cope with it ! :)
>
> That doesn't mean we shouldn't try to fix the race! :)
>
>>> Perhaps a file-descriptor API would be better, such as something like
>>> this:
>>>
>>> int openpid(int id, int flags);
>>> int rt_sigqueueinfo_fd(int process_fd, int sig, siginfo_t *info);
>>> int sigqueue_fd(int process_fd, int sig, const union sigval value); //
>>> glibc wrapper
>>>
>>
>> The race still exists: openpid() is being passed a PID... Only the
>> parent can legitimately know that this PID identifies a specific
>> unwaited child.
>
> Yes, the idea would be either the parent process, or the target
> process itself would open the PID, then pass the resulting file
> descriptor to whatever process is actually doing the killing.
> Alternately, one could add additional calls to help identify whether
> the right process was opened (perhaps a call to get a directory handle
> to the corresponding /proc directory?)

fd = open("/proc/self/", O_DIRECTORY);
?

Doing something based on proc files seems like a reasonable direction to
head if we are working on a race free api.

I suspect all we need is a sigqueue file.

Eric

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