linux-kernel - Re: [PATCH v1 1/2] Add polling support to pidfd

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20190429163259.GA201155@google.com>
Date:   Mon, 29 Apr 2019 12:32:59 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Christian Brauner <christian@...uner.io>,
        linux-kernel@...r.kernel.org, luto@...capital.net,
        rostedt@...dmis.org, dancol@...gle.com, sspatil@...gle.com,
        jannh@...gle.com, surenb@...gle.com, timmurray@...gle.com,
        Jonathan Kowalski <bl0pbl33p@...il.com>,
        torvalds@...ux-foundation.org, kernel-team@...roid.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...nel.org>, Jann Horn <jann@...jh.net>,
        linux-kselftest@...r.kernel.org, Michal Hocko <mhocko@...e.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Serge Hallyn <serge@...lyn.com>, Shuah Khan <shuah@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tycho Andersen <tycho@...ho.ws>, viro@...iv.linux.org.uk,
        linux-api@...r.kernel.org
Subject: Re: [PATCH v1 1/2] Add polling support to pidfd

On Mon, Apr 29, 2019 at 04:20:30PM +0200, Oleg Nesterov wrote:
> On 04/29, Joel Fernandes wrote:
> >
> > However, in your code above, it is avoided because we get:
> >
> > Task A (poller)		Task B (exiting task being polled)
> > ------------            ----------------
> > poll() called
> > add_wait_queue()
> > 			exit_state is set to non-zero
> > read exit_state
> > remove_wait_queue()
> > 			wake_up_all()
> 
> just to clarify... No, sys_poll() path doesn't do remove_wait_queue() until
> it returns to user mode, and that is why we can't race with set-exit_code +
> wake_up().

I didn't follow what you mean, the removal from the waitqueue happens in
free_poll_entry() called from poll_freewait() which happens from
do_sys_poll() which is before the syscall returns to user mode. Could you
explain more?

> pidfd_poll() can race with the exiting task, miss exit_code != 0, and return
> zero. However, do_poll() won't block after that and pidfd_poll() will be called
> again.

Here also I didn't follow what you mean. If exit_code is read as 0 in
pidfd_poll(), then in do_poll() the count will be 0 and it will block in
poll_schedule_timeout(). Right? But above you're saying it wont block.
Also if you could show a timing diagram of this different race you're talking
about, that will make things clear. It is a bit hard for me to picture
otherwise.

Also, I will use task_pid() for getting the pid from the task, as you suggest
in the other thread.

thanks,

- Joel