From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1ECD383A0D for ; Fri, 23 Feb 2024 16:24:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1708705447; cv=none; b=Vl5B1lV3Bvy/rmwPDU44sU1+UsT5odweRrMEVKGIz3NJy4zyc9t1ZSyZGUs7GmOReYRJ08sbpQePWy/wafqUU3Wr7BGqKzZiULN+wJh6kPlLxvJQmD2tqKyggiHuroR1xCh3O129bgsBnd8ATi/alCaxcGp6IsLvCwGdWlaIIIM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1708705447; c=relaxed/simple; bh=jpd2uydbbK2quIVAUF2f9Jm7NYNaMdUuijZBqca6Nd8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=qYPaqdpdHm39eShJBpAJMTYuHCTul0Af0QYtJahaHCAvzpymxcbYMgDxpi5zwCdOtNi6E+MNLbo04w60Aecj3ZEs0HzfvqKp/e3MNJZC5hsK7O4peRQ1E4NAjTGTB5x4ZLGi1p+EayPH5OXOSkQ0kZHzBkZn0IsZRafiLPlqSp0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=nJM2oyYa; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="nJM2oyYa" Received: by smtp.kernel.org (Postfix) with ESMTPSA id AAD6FC433C7; Fri, 23 Feb 2024 16:24:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1708705446; bh=jpd2uydbbK2quIVAUF2f9Jm7NYNaMdUuijZBqca6Nd8=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=nJM2oyYa2uhQh7ht7Wgz9mA/FZAQ9kTpaAsRV9rzZbMGfzV9EwADGL8rI53rlbyP4 1eF7QTqIIoYx6Bc0AqEOV0PZnK4Xt6N4KEIFPqB0tMmW31U40Y8SGcrbh73RumILzY nTgXnOnRyaje6+d286YI0gk0r6TrEKHlC4sTmIexd87gwF/uHad27DvP4xC2FJaCSa +lZoNldBO4kLbufRa+5uV0A3teSZhZMaHh+oam0IATwTKbWofU82Gs70OXPW5rrKN6 N5WccSNcI+8r5FnlddIbsDiiWLmpQQc0EFRPU8ZKYMuOCztz2whPoPtHKD+Havy+Tp LkXlOb8aRvBnQ== Date: Fri, 23 Feb 2024 17:24:03 +0100 From: Christian Brauner To: Tycho Andersen Cc: Alexander Mikhalitsyn , stgraber@stgraber.org, cyphar@cyphar.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH v1 2/2] tests/pid_namespace: add pid_max tests Message-ID: <20240223-kantholz-knallen-558beba46c62@brauner> References: <20240222160915.315255-1-aleksandr.mikhalitsyn@canonical.com> <20240222160915.315255-3-aleksandr.mikhalitsyn@canonical.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: On Thu, Feb 22, 2024 at 09:54:08AM -0700, Tycho Andersen wrote: > 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. That's a good idea. But it's a bit tricky. The straightforward thing is to walk upwards through all ancestor pid namespaces and use the lowest pid_max value as the upper bound for the current pid namespace. This will guarantee that you get an error when you try to write a value that you would't be able to create. The same logic should probably apply to ns_last_pid as well. However, that still leaves cases where the current pid namespace writes a pid_max limit that is allowed (IOW, all ancestor pid namespaces are above that limit.). But then immediately afterwards an ancestor pid namespace lowers the pid_max limit. So you can always end up in a scenario like this.