Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752577AbdLBWTO (ORCPT ); Sat, 2 Dec 2017 17:19:14 -0500 Received: from bombadil.infradead.org ([65.50.211.133]:40032 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752500AbdLBWTM (ORCPT ); Sat, 2 Dec 2017 17:19:12 -0500 Date: Sat, 2 Dec 2017 14:19:10 -0800 From: Matthew Wilcox To: Jann Horn Cc: john.hubbard@gmail.com, Michael Kerrisk , linux-man , Linux API , Michael Ellerman , linux-mm@kvack.org, LKML , linux-arch , Michal Hocko , John Hubbard Subject: Re: [PATCH] mmap.2: MAP_FIXED is no longer discouraged Message-ID: <20171202221910.GA8228@bombadil.infradead.org> References: <20171202021626.26478-1-jhubbard@nvidia.com> <20171202150554.GA30203@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.1 (2017-09-22) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2105 Lines: 40 On Sat, Dec 02, 2017 at 07:49:20PM +0100, Jann Horn wrote: > On Sat, Dec 2, 2017 at 4:05 PM, Matthew Wilcox wrote: > > On Fri, Dec 01, 2017 at 06:16:26PM -0800, john.hubbard@gmail.com wrote: > >> MAP_FIXED has been widely used for a very long time, yet the man > >> page still claims that "the use of this option is discouraged". > > > > I think we should continue to discourage the use of this option, but > > I'm going to include some of your text in my replacement paragraph ... > > > > -Because requiring a fixed address for a mapping is less portable, > > -the use of this option is discouraged. > > +The use of this option is discouraged because it forcibly unmaps any > > +existing mapping at that address. Programs which use this option need > > +to be aware that their memory map may change significantly from one run to > > +the next, depending on library versions, kernel versions and random numbers. > > How about adding something explicit about when it's okay to use MAP_FIXED? > "This option should only be used to displace an existing mapping that is > controlled by the caller, or part of such a mapping." or something like that? > > > +In a threaded process, checking the existing mappings can race against > > +a new dynamic library being loaded > > malloc() and its various callers can also cause mmap() calls, which is probably > more relevant than library loading. That's a bit more expected though. "I called malloc and my address space changed". Well, yeah. But "I called getpwnam and my address space changed" is a bit more surprising. Don't you think? Maybe that should be up front rather than buried at the end of the sentence. "In a multi-threaded process, the address space can change in response to virtually any library call. This is because almost any library call may be implemented by using dlopen(3) to load another shared library, which will be mapped into the process's address space. The PAM libraries are an excellent example, as well as more obvious examples like brk(2), malloc(3) and even pthread_create(3)." What do you think?