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Message-ID: <Zdd8MAJJD3M11yeR@tycho.pizza>
Date: Thu, 22 Feb 2024 09:54:08 -0700
From: Tycho Andersen <tycho@...ho.pizza>
To: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
Cc: brauner@...nel.org, stgraber@...raber.org, cyphar@...har.com,
linux-kernel@...r.kernel.org,
Christian Brauner <christian.brauner@...ntu.com>
Subject: Re: [PATCH v1 2/2] tests/pid_namespace: add pid_max tests
On Thu, Feb 22, 2024 at 05:09:15PM +0100, Alexander Mikhalitsyn wrote:
> +static int pid_max_nested_limit_inner(void *data)
> +{
> + int fret = -1, nr_procs = 400;
> + int fd, ret;
> + pid_t pid;
> + pid_t pids[1000];
> +
> + ret = mount("", "/", NULL, MS_PRIVATE | MS_REC, 0);
> + if (ret) {
> + fprintf(stderr, "%m - Failed to make rootfs private mount\n");
> + return fret;
> + }
> +
> + umount2("/proc", MNT_DETACH);
> +
> + ret = mount("proc", "/proc", "proc", 0, NULL);
> + if (ret) {
> + fprintf(stderr, "%m - Failed to mount proc\n");
> + return fret;
> + }
> +
> + fd = open("/proc/sys/kernel/pid_max", O_RDWR | O_CLOEXEC | O_NOCTTY);
> + if (fd < 0) {
> + fprintf(stderr, "%m - Failed to open pid_max\n");
> + return fret;
> + }
> +
> + ret = write(fd, "500", sizeof("500") - 1);
> + close(fd);
> + if (ret < 0) {
> + fprintf(stderr, "%m - Failed to write pid_max\n");
> + return fret;
> + }
> +
> + for (nr_procs = 0; nr_procs < 500; nr_procs++) {
> + pid = fork();
> + if (pid < 0)
> + break;
> +
> + if (pid == 0)
> + exit(EXIT_SUCCESS);
> +
> + pids[nr_procs] = pid;
> + }
> +
> + if (nr_procs >= 400) {
> + fprintf(stderr, "Managed to create processes beyond the configured outer limit\n");
> + goto reap;
> + }
A small quibble, but I wonder about the semantics here. "You can write
whatever you want to this file, but we'll ignore it sometimes" seems
weird to me. What if someone (CRIU) wants to spawn a pid numbered 450
in this case? I suppose they read pid_max first, they'll be able to
tell it's impossible and can exit(1), but returning E2BIG from write()
might be more useful.
Tycho
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