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Date:   Mon, 22 Jun 2020 14:03:00 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Christian Brauner <christian.brauner@ubuntu.com>
Cc:     Dominique Martinet <asmadeus@codewreck.org>,
        Alexander Kapshuk <alexander.kapshuk@gmail.com>,
        linux-kernel@vger.kernel.org, ebiederm@xmission.com,
        akpm@linux-foundation.org, liuzhiqiang26@huawei.com,
        joel@joelfernandes.org, paulmck@linux.vnet.ibm.com,
        kernel test robot <lkp@intel.com>
Subject: Re: [PATCH] kernel/signal.c: Export symbol __lock_task_sighand
Message-ID: <20200622120259.GD6516@redhat.com>
References: <20200621133704.77896-1-alexander.kapshuk@gmail.com>
 <20200622062527.GA6516@redhat.com>
 <20200622083905.c3nurmkbo5yhd6lj@wittgenstein>
 <20200622102401.GA12377@nautica>
 <20200622113610.okzntx7jmnk6n7au@wittgenstein>
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On 06/22, Christian Brauner wrote:
>
> On Mon, Jun 22, 2020 at 12:24:01PM +0200, Dominique Martinet wrote:
> > Christian Brauner wrote on Mon, Jun 22, 2020:
> > > On Mon, Jun 22, 2020 at 08:25:28AM +0200, Oleg Nesterov wrote:
> > >> current->sighand is stable and can't go away. Unless "current" is exiting and
> > >> has already passed exit_notify(). So I don't think net/9p needs this helper.
> > >
> > > From what I can gather from the thread (cf. [1]) that is linked in the
> > > commit message the main motivation for all of this is sparse not being
> > > happy and not some bug. (Maybe I'm not seeing something though.)
> > >
> > > The patch itself linked here doesn't seem to buy anything. I agree with
> > > Oleg. Afaict, lock_task_sighand() would only be needed here if the task
> > > wouldn't be current. So maybe it should just be dropped from the series.
> >
> > Sure. I honestly have no idea on what guarantees we have from the task
> > being current here as opposed to any other task -- I guess that another
> > thread calling exit for exemple would have to wait?
>
> When a thread in a non-trivial thread-group (sorry for the math
> reference :)) execs it'll unshare its struct sighand.

Well, not really...

The execing threads will kill other other threads, then it will check
if ->sighand should be unshared. The latter is very unlikely, I don't
think CLONE_SIGHAND without CLONE_THREAD is actually used today.

But this doesn't really matter. I mean, even if you race with another
thread doing exec/exit/whatever, current->sighand is stable. Unless, again,
current has already exited (called exit_notify()).

> The new struct
> sighand will be assigned using rcu_assign_pointer() so afaik (Paul or
> Oleg can yell at me if I'm talking nonsense) any prior callers will see
> the prior sighand value.

Yes, but see above.

If tsk is not current, then (in general) it is not safe to use tsk->sighand
directly. It can can be changed by exec (as you explained), or you can hit
tsk->sighand == NULL if you race with exit.

Oleg.