Re: [PATCH 1/2] pid: add pidfd_open()
From: Christian Brauner
Date: Wed May 15 2019 - 11:32:36 EST
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
> 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
> 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
Thank you, Oleg! :)