linux-kernel - Re: [PATCH 2/2] selftests/seccomp: Ensure that notifications come in FIFO order

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Date:   Thu, 28 Apr 2022 07:15:51 -0600
From:   Tycho Andersen <tycho@...ho.pizza>
To:     Sargun Dhillon <sargun@...gun.me>
Cc:     Kees Cook <keescook@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 2/2] selftests/seccomp: Ensure that notifications come in
 FIFO order

On Wed, Apr 27, 2022 at 06:54:47PM -0700, Sargun Dhillon wrote:
> +	/* Start children, and them generate notifications */
                           ^^ - they maybe?

> +	for (i = 0; i < ARRAY_SIZE(pids); i++) {
> +		pid = fork();
> +		if (pid == 0) {
> +			ret = syscall(__NR_getppid);
> +			exit(ret != USER_NOTIF_MAGIC);
> +		}
> +		pids[i] = pid;
> +	}
> +
> +	/* This spins until all of the children are sleeping */
> +restart_wait:
> +	for (i = 0; i < ARRAY_SIZE(pids); i++) {
> +		if (get_proc_stat(pids[i]) != 'S') {
> +			nanosleep(&delay, NULL);
> +			goto restart_wait;
> +		}
> +	}

I wonder if we should/can combine this loop with the previous one, and
wait for the child to sleep in getppid() before we fork the next one.
Otherwise isn't racy in the case that your loop continues to the next
iteration before the child processes are scheduled, so things might be
out of order? Maybe I'm missing something.

In any case, this change seems reasonable to me.

Tycho