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Date:   Tue, 10 Mar 2020 13:57:55 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Bernd Edlinger <bernd.edlinger@hotmail.de>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Jann Horn <jannh@google.com>, Jonathan Corbet <corbet@lwn.net>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Andrei Vagin <avagin@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Yuyang Du <duyuyang@gmail.com>,
        David Hildenbrand <david@redhat.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        David Howells <dhowells@redhat.com>,
        James Morris <jamorris@linux.microsoft.com>,
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        Aleksa Sarai <cyphar@cyphar.com>,
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Subject: Re: [PATCH v2 2/5] exec: Factor unshare_sighand out of de_thread and
 call it separately
Message-ID: <202003101356.A2B9885F17@keescook>
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On Tue, Mar 10, 2020 at 09:34:03PM +0100, Bernd Edlinger wrote:
> On 3/10/20 9:29 PM, Kees Cook wrote:
> > On Sun, Mar 08, 2020 at 04:36:17PM -0500, Eric W. Biederman wrote:
> >>
> >> This makes the code clearer and makes it easier to implement a mutex
> >> that is not taken over any locations that may block indefinitely waiting
> >> for userspace.
> >>
> >> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> >> ---
> >>  fs/exec.c | 39 ++++++++++++++++++++++++++-------------
> >>  1 file changed, 26 insertions(+), 13 deletions(-)
> >>
> >> diff --git a/fs/exec.c b/fs/exec.c
> >> index c3f34791f2f0..ff74b9a74d34 100644
> >> --- a/fs/exec.c
> >> +++ b/fs/exec.c
> >> @@ -1194,6 +1194,23 @@ static int de_thread(struct task_struct *tsk)
> >>  	flush_itimer_signals();
> >>  #endif
> > 
> > Semi-related (existing behavior): in de_thread(), what keeps the thread
> > group from changing? i.e.:
> > 
> >         if (thread_group_empty(tsk))
> >                 goto no_thread_group;
> > 
> >         /*
> >          * Kill all other threads in the thread group.
> >          */
> >         spin_lock_irq(lock);
> > 	... kill other threads under lock ...
> > 
> > Why is the thread_group_emtpy() test not under lock?
> > 
> 
> A new thread cannot created when only one thread is executing,
> right?

*face palm* Yes, of course. :) I'm thinking too hard.

-- 
Kees Cook