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From: Christian Brauner <christian@brauner.io>
To: Oleg Nesterov <oleg@redhat.com>
Cc: jannh@google.com, viro@zeniv.linux.org.uk,
	torvalds@linux-foundation.org, linux-kernel@vger.kernel.org,
	arnd@arndb.de, dhowells@redhat.com, akpm@linux-foundation.org,
	cyphar@cyphar.com, ebiederm@xmission.com,
	elena.reshetova@intel.com, keescook@chromium.org,
	luto@amacapital.net, luto@kernel.org, tglx@linutronix.de,
	linux-alpha@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org,
	linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org,
	linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	linux-s390@vger.kernel.org, linux-sh@vger.kernel.org,
	sparclinux@vger.kernel.org, linux-xtensa@linux-xtensa.org,
	linux-api@vger.kernel.org, linux-arch@vger.kernel.org,
	linux-kselftest@vger.kernel.org
Subject: Re: [PATCH 1/2] pid: add pidfd_open()
Date: Wed, 15 May 2019 17:30:42 +0200
Message-ID: <20190515153041.cshzaj7xhf2p4zv7@brauner.io> (raw)
In-Reply-To: <20190515151912.GE18892@redhat.com>

On Wed, May 15, 2019 at 05:19:13PM +0200, Oleg Nesterov wrote:
> On 05/15, Christian Brauner wrote:
> >
> > On Wed, May 15, 2019 at 04:38:58PM +0200, Oleg Nesterov wrote:
> > >
> > > it seems that you can do a single check
> > >
> > > 	tsk = pid_task(p, PIDTYPE_TGID);
> > > 	if (!tsk)
> > > 		ret = -ESRCH;
> > >
> > > this even looks more correct if we race with exec changing the leader.
> >
> > The logic here being that you can only reach the thread_group leader
> > from struct pid if PIDTYPE_PID == PIDTYPE_TGID for this struct pid?
> 
> Not exactly... it is not that PIDTYPE_PID == PIDTYPE_TGID for this pid,
> struct pid has no "type" or something like this.
> 
> The logic is that pid->tasks[PIDTYPE_XXX] is the list of task which use
> this pid as "XXX" type.
> 
> For example, clone(CLONE_THREAD) creates a pid which has a single non-
> empty list, pid->tasks[PIDTYPE_PID]. This pid can't be used as TGID or
> SID.
> 
> So if pid_task(PIDTYPE_TGID) returns non-NULL we know that this pid was
> used for a group-leader, see copy_process() which does

Ah, this was what I was asking myself when I worked on thread-specific
signal sending. This clarifies quite a lot of things!

Though I wonder how one reliably gets a the PGID or SID from a
PIDTYPE_PID.

> 
> 	if (thread_group_leader(p))
> 		attach_pid(p, PIDTYPE_TGID);
> 
> 
> If we race with exec which changes the leader pid_task(TGID) can return
> the old leader. We do not care, but this means that we should not check
> thread_group_leader().

Nice!

Thank you, Oleg! :)
Christian

  reply index

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-15 10:03 Christian Brauner
2019-05-15 10:04 ` [PATCH 2/2] tests: add pidfd_open() tests Christian Brauner
2019-05-15 12:29 ` [PATCH 1/2] pid: add pidfd_open() Geert Uytterhoeven
2019-05-15 14:00 ` Yann Droneaud
2019-05-15 14:16   ` Christian Brauner
2019-05-15 14:51     ` Aleksa Sarai
2019-05-15 15:29     ` Yann Droneaud
2019-05-15 14:38 ` Oleg Nesterov
2019-05-15 14:49   ` Christian Brauner
2019-05-15 15:19     ` Oleg Nesterov
2019-05-15 15:30       ` Christian Brauner [this message]
2019-05-15 15:35   ` Oleg Nesterov
2019-05-15 15:40     ` Christian Brauner
2019-05-15 17:45 ` Daniel Colascione
2019-05-16 13:08   ` Christian Brauner
2019-05-16 14:03     ` Jann Horn
2019-05-16 14:05       ` Christian Brauner
2019-05-16 14:53     ` Aleksa Sarai

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