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

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Message-ID: <1308756565.2959.65.camel@bahia.local>
Date:	Wed, 22 Jun 2011 17:29:25 +0200
From:	Greg Kurz <gkurz@...ibm.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Bryan Donlan <bdonlan@...il.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:)

On Mon, 2011-06-20 at 15:44 -0700, Eric W. Biederman wrote:
> 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.
> 

Are you referring to Bryan's rt_sigqueueinfo_fd() syscall or to a
new /proc/self/sigqueue file ?

-- 
Gregory Kurz                                     gkurz@...ibm.com
Software Engineer @ IBM/Meiosys                  http://www.ibm.com
Tel +33 (0)534 638 479                           Fax +33 (0)561 400 420

"Anarchy is about taking complete responsibility for yourself."
        Alan Moore.

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