From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751888AbbIOFTt (ORCPT ); Tue, 15 Sep 2015 01:19:49 -0400 Received: from relay5-d.mail.gandi.net ([217.70.183.197]:41990 "EHLO relay5-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750750AbbIOFTr (ORCPT ); Tue, 15 Sep 2015 01:19:47 -0400 X-Originating-IP: 50.43.43.179 Date: Mon, 14 Sep 2015 22:19:19 -0700 From: Josh Triplett To: "Kirill A. Shutemov" Cc: Palmer Dabbelt , arnd@arndb.de, dhowells@redhat.com, viro@zeniv.linux.org.uk, ast@plumgrid.com, aishchuk@linux.vnet.ibm.com, aarcange@redhat.com, akpm@linux-foundation.org, luto@kernel.org, acme@kernel.org, bhe@redhat.com, 3chas3@gmail.com, chris@zankel.net, dave@sr71.net, dyoung@redhat.com, drysdale@google.com, davem@davemloft.net, ebiederm@xmission.com, geoff@infradead.org, gregkh@linuxfoundation.org, hpa@zytor.com, mingo@kernel.org, iulia.manda21@gmail.com, plagnioj@jcrosoft.com, jikos@kernel.org, kexec@lists.infradead.org, linux-api@vger.kernel.org, linux-arch@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-xtensa@linux-xtensa.org, mathieu.desnoyers@efficios.com, jcmvbkbc@gmail.com, paulmck@linux.vnet.ibm.com, a.p.zijlstra@chello.nl, tglx@linutronix.de, tomi.valkeinen@ti.com, vgoyal@redhat.com, x86@kernel.org Subject: Re: [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace Message-ID: <20150915051919.GB4091@x> References: <1441832902-28993-1-git-send-email-palmer@dabbelt.com> <1442271047-4908-1-git-send-email-palmer@dabbelt.com> <1442271047-4908-5-git-send-email-palmer@dabbelt.com> <20150915002358.GA12618@node.dhcp.inet.fi> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150915002358.GA12618@node.dhcp.inet.fi> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 15, 2015 at 03:23:58AM +0300, Kirill A. Shutemov wrote: > On Mon, Sep 14, 2015 at 03:50:38PM -0700, Palmer Dabbelt wrote: > > This used to be hidden behind CONFIG_MMAP_ALLOW_UNINITIALIZED, so > > userspace wouldn't actually ever see it be non-zero. While I could > > have kept hiding it, the man pages seem to indicate that > > MAP_UNINITIALIZED should be visible: > > > > mmap(2) > > MAP_UNINITIALIZED (since Linux 2.6.33) > > Don't clear anonymous pages. This flag is intended to improve > > performance on embedded devices. This flag is honored only if the > > kernel was configured with the CONFIG_MMAP_ALLOW_UNINITIALIZED > > option. Because of the security implications, that option is > > normally enabled only on embedded devices (i.e., devices where one > > has complete control of the contents of user memory). > > > > and since the only time it shows up in my /usr/include is in this > > header I believe this should have been visible to userspace (as > > non-zero, which wouldn't do anything when or'd into the flags) all > > along. > > Are you sure about "wouldn't do anything"? > Suspiciously, 0x4000000 is also (1 << MAP_HUGE_SHIFT). I'm not sure if any > architecture has order-1 huge pages, but still looks like we have conflict > here. > > I think it's harmful to expose non-zero MAP_UNINITIALIZED to system which > potentially can handle multiple users. Or non-trivial user space in > general. The flag should always exist. If it was defined to conflict with something else, that's a serious ABI problem. But the flag should always exist, even if the kernel ends up ignoring it. > Should we leave it at least under '#ifndef CONFIG_MMU'? I don't think it's > possible to have single ABI for MMU and MMU-less systems anyway. And we > can avoid conflict with MAP_HUGE_SHIFT this way. No; even if you have an MMU (which is useful for things like fork()), a system without user separation (for instance, without CONFIG_MULTIUSER) can reasonably use MAP_UNINITIALIZED. > P.S. MAP_UNINITIALIZED itself looks very broken to me. I probably need dig > mailing list on why it was allowed. That's what the config option *and* explicit flag are for; there are more than enough warnings about the implications. - Josh Triplett