linux-kernel - Re: [PATCH v3 1/2] seccomp: Add wait_killable semantic to seccomp user notifier

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Date:   Fri, 29 Apr 2022 11:42:15 +0200
From:   Rodrigo Campos <rodrigo@...volk.io>
To:     Sargun Dhillon <sargun@...gun.me>
Cc:     Kees Cook <keescook@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Will Drewry <wad@...omium.org>,
        Andy Lutomirski <luto@...capital.net>,
        Alban Crequy <alban@...volk.io>
Subject: Re: [PATCH v3 1/2] seccomp: Add wait_killable semantic to seccomp
 user notifier

On Fri, Apr 29, 2022 at 4:32 AM Sargun Dhillon <sargun@...gun.me> wrote:
> the concept is searchable. If the notifying process is signaled prior
> to the notification being received by the userspace agent, it will
> be handled as normal.

Why is that? Why not always handle in the same way (if wait killable
is set, wait like that)

> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index db10e73d06e0..9291b0843cb2 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -1081,6 +1088,12 @@ static void seccomp_handle_addfd(struct seccomp_kaddfd *addfd, struct seccomp_kn
>         complete(&addfd->completion);
>  }
>
> +static bool should_sleep_killable(struct seccomp_filter *match,
> +                                 struct seccomp_knotif *n)
> +{
> +       return match->wait_killable_recv && n->state == SECCOMP_NOTIFY_SENT;

Here for some reason we check the notification state to be SENT.

> +}
> +
>  static int seccomp_do_user_notification(int this_syscall,
>                                         struct seccomp_filter *match,
>                                         const struct seccomp_data *sd)
> @@ -1111,11 +1124,25 @@ static int seccomp_do_user_notification(int this_syscall,
>          * This is where we wait for a reply from userspace.
>          */
>         do {
> +               bool wait_killable = should_sleep_killable(match, &n);
> +

So here, the first time this runs this will be false even if the
wait_killable flag was used in the filter (because that function
checks the notification state to be sent, that is not true the first
time)

Why not just do wait_for_completion_killable if match->wait_killable
and wait_for_completion_interruptible otherwise? Am I missing
something?



Best,
Rodrigo