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[209.132.180.67]) by mx.google.com with ESMTP id v5-v6si7780015plg.99.2018.10.30.15.34.27; Tue, 30 Oct 2018 15:34:43 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; dkim=pass header.i=@joelfernandes.org header.s=google header.b=x5HZC1i2; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728505AbeJaH3I (ORCPT + 99 others); Wed, 31 Oct 2018 03:29:08 -0400 Received: from mail-pf1-f196.google.com ([209.85.210.196]:38568 "EHLO mail-pf1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726021AbeJaH3I (ORCPT ); Wed, 31 Oct 2018 03:29:08 -0400 Received: by mail-pf1-f196.google.com with SMTP id b11-v6so6579886pfi.5 for ; Tue, 30 Oct 2018 15:33:47 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=ItDKaBtL3bx62voZ9YLIuhAKcnRMtm18NkRhFzvA0OI=; b=x5HZC1i2LSgoJvAUy3G/6rWn/M3kxeUU5npE6QkjHOzN6K5PgOkUJ2GXTajyGr2k+r S3CM2e/oCro/LcWswo/K1vtMd5Z33vvIDUpk71HgmyUEIvyykMYHY6xmPjI7b0xW81hm giAaCirtzce6uTaxRylBQcejAfw702qenUNMM= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=ItDKaBtL3bx62voZ9YLIuhAKcnRMtm18NkRhFzvA0OI=; b=thV6tnCnzwpK2NzBjId5qKjayqpCQz2EiiPG0juqdGOr2O1+bxxDDvNXxIHIDYVrVV H3R2jweAzUdHopV0US3ctFpcRE45b56GCdcCMYilE8WTOOaLJItGSb9gHoX44DPCk63T L6vktr84cqufSlXe31I8ZLillPPuW2TQPD+MFWArhfhGXRRL6kkP6F+OfP4AYBYCgv+n 6+DORy3S194cYyzmpqWUGB5m6aMmO2fmhGdzpLW3AGxyJJ8FETwkev1LUyDsSB5CmGfZ msBxpGykQVNZGhM4MmbXFctPi04ltW6T75t85VshHwPNkkAyCSvhKzoQCse5geH9i0X2 zjxg== X-Gm-Message-State: AGRZ1gJ5X9n3qCsaeHlW2T4YDnwjUzJ9YsKbJ2y4HBgzMjJrYiFCn929 g1xYrJXCcmP4acDC/YJGtuqV6w== X-Received: by 2002:a62:7982:: with SMTP id u124-v6mr557870pfc.95.1540938826324; Tue, 30 Oct 2018 15:33:46 -0700 (PDT) Received: from localhost ([2620:0:1000:1601:3aef:314f:b9ea:889f]) by smtp.gmail.com with ESMTPSA id u69-v6sm37403340pfk.68.2018.10.30.15.33.44 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Tue, 30 Oct 2018 15:33:45 -0700 (PDT) Date: Tue, 30 Oct 2018 15:33:43 -0700 From: Joel Fernandes To: Aleksa Sarai Cc: Daniel Colascione , linux-kernel , Tim Murray , Suren Baghdasaryan Subject: Re: [RFC PATCH] Implement /proc/pid/kill Message-ID: <20181030223343.GB105735@joelaf.mtv.corp.google.com> References: <20181029221037.87724-1-dancol@google.com> <20181030050012.u43lcvydy6nom3ul@yavin> <20181030204501.jnbe7dyqui47hd2x@yavin> <20181030214243.GB32621@google.com> <20181030222339.ud4wfp75tidowuo4@yavin> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181030222339.ud4wfp75tidowuo4@yavin> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 31, 2018 at 09:23:39AM +1100, Aleksa Sarai wrote: > On 2018-10-30, Joel Fernandes wrote: > > On Wed, Oct 31, 2018 at 07:45:01AM +1100, Aleksa Sarai wrote: > > [...] > > > > > (Unfortunately > > > > > there are lots of things that make it a bit difficult to use /proc/$pid > > > > > exclusively for introspection of a process -- especially in the context > > > > > of containers.) > > > > > > > > Tons of things already break without a working /proc. What do you have in mind? > > > > > > Heh, if only that was the only blocker. :P > > > > > > The basic problem is that currently container runtimes either depend on > > > some non-transient on-disk state (which becomes invalid on machine > > > reboots or dead processes and so on), or on long-running processes that > > > keep file descriptors required for administration of a container alive > > > (think O_PATH to /dev/pts/ptmx to avoid malicious container filesystem > > > attacks). Usually both. > > > > > > What would be really useful would be having some way of "hiding away" a > > > mount namespace (of the pid1 of the container) that has all of the > > > information and bind-mounts-to-file-descriptors that are necessary for > > > administration. If the container's pid1 dies all of the transient state > > > has disappeared automatically -- because the stashed mount namespace has > > > died. In addition, if this was done the way I'm thinking with (and this > > > is the contentious bit) hierarchical mount namespaces you could make it > > > so that the pid1 could not manipulate its current mount namespace to > > > confuse the administrative process. You would also then create an > > > intermediate user namespace to help with several race conditions (that > > > have caused security bugs like CVE-2016-9962) we've seen when joining > > > containers. > > > > > > Unfortunately this all depends on hierarchical mount namespaces (and > > > note that this would just be that NS_GET_PARENT gives you the mount > > > namespace that it was created in -- I'm not suggesting we redesign peers > > > or anything like that). This makes it basically a non-starter. > > > > > > But if, on top of this ground-work, we then referenced containers > > > entirely via an fd to /proc/$pid then you could also avoid PID reuse > > > races (as well as being able to find out implicitly whether a container > > > has died thanks to the error semantics of /proc/$pid). And that's the > > > way I would suggest doing it (if we had these other things in place). > > > > I didn't fully follow exactly what you mean. If you can explain for the > > layman who doesn't know much experience with containers.. > > > > Are you saying that keeping open a /proc/$pid directory handle is not > > sufficient to prevent PID reuse while the proc entries under /proc/$pid are > > being looked into? If its not sufficient, then isn't that a bug? If it is > > sufficient, then can we not just keep the handle open while we do whatever we > > want under /proc/$pid ? > > Sorry, I went on a bit of a tangent about various internals of container > runtimes. My main point is that I would love to use /proc/$pid because > it makes reuse handling very trivial and is always correct, but that > there are things which stop us from being able to use it for everything > (which is what my incoherent rambling was on about). Ok thanks. So I am guessing if the following sequence works, then Dan's patch is not needed. 1. open /proc/ directory 2. inspect /proc/ or do whatever with 3. Issue the kill on 4. Close the /proc/ directory opened in step 1. So unless I missed something, the above sequence will not cause any PID reuse races. - Joel