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From:   Jann Horn <jannh@google.com>
Date:   Thu, 28 May 2020 06:04:48 +0200
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Subject: Re: [PATCH 1/2] seccomp: notify user trap about unused filter
To:     Christian Brauner <christian.brauner@ubuntu.com>
Cc:     kernel list <linux-kernel@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Andy Lutomirski <luto@kernel.org>,
        Tycho Andersen <tycho@tycho.ws>,
        Matt Denton <mpdenton@google.com>,
        Sargun Dhillon <sargun@sargun.me>,
        Chris Palmer <palmer@google.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Robert Sesek <rsesek@google.com>,
        Jeffrey Vander Stoep <jeffv@google.com>,
        Linux Containers <containers@lists.linux-foundation.org>
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On Wed, May 27, 2020 at 1:19 PM Christian Brauner
<christian.brauner@ubuntu.com> wrote:
> We've been making heavy use of the seccomp notifier to intercept and
> handle certain syscalls for containers. This patch allows a syscall
> supervisor listening on a given notifier to be notified when a seccomp
> filter has become unused.
[...]
> To fix this, we introduce a new "live" reference counter that tracks the
> live tasks making use of a given filter and when a notifier is
> registered waiting tasks will be notified that the filter is now empty
> by receiving a (E)POLLHUP event.
> The concept in this patch introduces is the same as for signal_struct,
> i.e. reference counting for life-cycle management is decoupled from
> reference counting live taks using the object.
[...]
> + * @live: tasks that actually use this filter, only to be altered
> + *          during fork(), exit()/free_task(), and filter installation

This comment is a bit off. Actually, @live counts the number of tasks
that use the filter directly plus the number of dependent filters that
have non-zero @live.

[...]
> +void seccomp_filter_notify(const struct task_struct *tsk)
> +{
> +       struct seccomp_filter *orig = tsk->seccomp.filter;
> +
> +       while (orig && refcount_dec_and_test(&orig->live)) {
> +               if (waitqueue_active(&orig->wqh))
> +                       wake_up_poll(&orig->wqh, EPOLLHUP);
> +               orig = orig->prev;
> +       }
> +}

/me fetches the paint bucket

Maybe name this seccomp_filter_unuse() or
seccomp_filter_unuse_notify() or something like that? The current name
isn't very descriptive.