Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932370AbaKEPV4 (ORCPT ); Wed, 5 Nov 2014 10:21:56 -0500 Received: from mail-lb0-f170.google.com ([209.85.217.170]:44487 "EHLO mail-lb0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754468AbaKEPVy (ORCPT ); Wed, 5 Nov 2014 10:21:54 -0500 MIME-Version: 1.0 In-Reply-To: <20141105082721.GB19607@infradead.org> References: <20141031084220.GA29085@infradead.org> <20141103083447.GA8617@infradead.org> <20141105082721.GB19607@infradead.org> From: Eric Rannaud Date: Wed, 5 Nov 2014 07:21:28 -0800 Message-ID: Subject: Re: [RFC PATCH] fs: allow open(dir, O_TMPFILE|..., 0) with mode 0 To: Christoph Hellwig Cc: Linus Torvalds , Andy Lutomirski , "linux-kernel@vger.kernel.org" , Andrew Morton , Al Viro , linux-fsdevel Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 5, 2014 at 12:27 AM, Christoph Hellwig wrote: > On Mon, Nov 03, 2014 at 11:04:27AM -0800, Linus Torvalds wrote: >> Oh, so you don't actually need any file contents at all? >> >> If that is actually a real usage, then maybe we should just say that >> "O_TMPFILE|O_RDONLY" is fine, and remove the check that it has to be >> writable. > > Wasn't this disallowed to prevent problems on old kernels that don't use > O_TMPFILE? In that case we'd ignore the flag and would just get a file > handle for the directory instead. Yes, that was the idea at first, as discussed at http://article.gmane.org/gmane.linux.file-systems/76273, in commit bb458c644. But soon after that, O_WRONLY was allowed (ba57ea64cb1), and O_RDWR was removed from O_TMPFILE. Now only remain the explicit checks in build_open_flags() that Linus mentioned. If we allow fd = open("/tmp", O_TMPFILE|O_RDONLY, 0600) it would be seen by an old kernel as fd = open("/tmp", O_DIRECTORY|O_RDONLY, 0600) which will succeed. But unlike the other cases the creative definition of O_TMPFILE was meant to prevent, this does not create a security risk for anyone implementing a secure tmpfile, as they would be asking for a writable fd. To implement an atomic open() with O_TMPFILE+flink, if neither O_WRONLY nor O_RDWR is in flags, you would have to manually check with fstat that fd is indeed a regular file and not a directory. At least if you need to run on old kernels. If such a changes goes in, the man page for open(2) should talk about what happens on old kernels (it already has an explanation for the writable case). -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/