From owner-freebsd-hackers@FreeBSD.ORG Tue Nov 3 15:50:40 2009 Return-Path: Delivered-To: freebsd-hackers@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 78B641065670; Tue, 3 Nov 2009 15:50:40 +0000 (UTC) (envelope-from a_best01@uni-muenster.de) Received: from zivm-exrelay3.uni-muenster.de (ZIVM-EXRELAY3.UNI-MUENSTER.DE [128.176.192.20]) by mx1.freebsd.org (Postfix) with ESMTP id CFEBC8FC13; Tue, 3 Nov 2009 15:50:39 +0000 (UTC) X-IronPort-AV: E=Sophos;i="4.44,674,1249250400"; d="scan'208";a="17514856" Received: from zivmaildisp1.uni-muenster.de (HELO ZIVMAILUSER01.UNI-MUENSTER.DE) ([128.176.188.85]) by zivm-relay3.uni-muenster.de with ESMTP; 03 Nov 2009 16:50:38 +0100 Received: by ZIVMAILUSER01.UNI-MUENSTER.DE (Postfix, from userid 149459) id 303111B0766; Tue, 3 Nov 2009 16:50:38 +0100 (CET) Date: Tue, 03 Nov 2009 16:50:36 +0100 (CET) From: Alexander Best Sender: Organization: Westfaelische Wilhelms-Universitaet Muenster To: John Baldwin , Alexander Best Message-ID: In-Reply-To: <200911030932.24583.jhb@freebsd.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: freebsd-hackers@freebsd.org, Alan Cox Subject: Re: mmap(2) with MAP_ANON honouring offset although it shouldn't X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 03 Nov 2009 15:50:40 -0000 John Baldwin schrieb am 2009-11-03: > On Monday 02 November 2009 5:14:27 pm Alexander Best wrote: > > John Baldwin schrieb am 2009-11-02: > > > On Monday 02 November 2009 4:05:56 pm Alexander Best wrote: > > > > John Baldwin schrieb am 2009-11-02: > > > > > On Friday 30 October 2009 10:38:24 pm Alexander Best wrote: > > > > > > John Baldwin schrieb am 2009-10-21: > > > > > > > On Wednesday 21 October 2009 11:51:04 am Alexander Best > > > > > > > wrote: > > > > > > > > although the mmap(2) manual states in section MAP_ANON: > > > > > > > > "The offset argument is ignored." > > > > > > > > this doesn't seem to be true. running > > > > > > > > printf("%p\n", mmap((void*)0x1000, 0x1000, PROT_NONE, > > > > > > > > MAP_ANON, > > > > > > > > -1, > > > > > > > > 0x12345678)); > > > > > > > > and > > > > > > > > printf("%p\n", mmap((void*)0x1000, 0x1000, PROT_NONE, > > > > > > > > MAP_ANON, > > > > > > > > -1, > > > > > > > > 0)); > > > > > > > > produces different outputs. i've attached a patch to > > > > > > > > solve > > > > > > > > the > > > > > > > > problem. the > > > > > > > > patch is similar to the one proposed in this PR, but > > > > > > > > should > > > > > > > > apply > > > > > > > > cleanly to > > > > > > > > CURRENT: > > > > > > > > http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/71258 > > > > > > > A simpler patch would be to simply set pos = 0 below the > > > > > > > MAP_STACK > > > > > > > line if > > > > > > > MAP_ANON is set. > > > > > > how about the following patch. problem seems to be that pos > > > > > > = 0 > > > > > > needs to be > > > > > > set before pageoff is being calculated. > > > > > I think that that patch is fine, but will defer to alc@. I > > > > > think > > > > > he > > > > > argued > > > > > that any non-zero offset passed to MAP_ANON should fail with > > > > > EINVAL. > > > > thanks. if that's what the POSIX standard requests that's ok. > > > > however in that > > > > case we need to change the mmap(2) manual, because right now it > > > > says in > > > > section MAP_ANON: > > > > "The offset argument is ignored." > > > > which should be changed to something like: > > > > "The offset argument must be zero." > > > > also if the behaviour of MAP_ANON changes this also changes the > > > > semantics of > > > > MAP_STACK since it implies MAP_ANON. so we need to decide if > > > > MAP_STACK should > > > > silently reset any offset value to zero or like MAP_ANON should > > > > fail if offset > > > > isn't zero in which case the MAP_STACK section of the mmap(2) > > > > manual needs to > > > > be changed to someting like: > > > > "MAP_STACK implies MAP_ANON, and requires offset to be zero." > > > Right now MAP_STACK sets pos to 0 in the current code, and I > > > don't > > > expect we > > > would remove that if we decide to reject non-zero offsets for > > > MAP_ANON. I'd > > > probably rather err on the side of leniency and just ignore the > > > offset rather > > > than rejecting non-zero, but I'm a bit burned from the last round > > > of > > > mmap() > > > API changes. :) > > hmmm...i think this will require quite a few changes. if i remember > correctly > > MAP_STACK at some point does: > > flags =| MAP_ANON; > > so if we decide MAP_ANON and MAP_STACK should behave differently > > this will > > require some checks to distinguish between both flags further down > > in the > > code. > > let's see what alc@ thinks about this one then. API changes are a > > nasty > nasty > > business. ;) > Umm, if you revert your change and just add a simple clause that > does: > if (flags & MAP_ANON && pos != 0) > return (EINVAL); > after the MAP_STACK section then I think that would work fine. It > would > not require any further magic apart from that. oh. you're right. didn't think of that one. indeed this would let mmap fail with MAP_ANON and pos != 0, but would keep the current MAP_STACK behaviour (which is ignoring pos). sounds like a really clean and useful mmap API change. if alc@ agrees i could put your change in the form of a patch and together with a mmap(2) manual change, submit it as followup to kern/71258. it shouldn't be a big deal mfc'ing the changes to 8-stable (maybe even 8.0-release), 7-stable and 6-stable. well...better make that 8.1-release. ;) who knows what weird mmap calls are in the ports. ;) i'll try to build universe over the night to see if the changes break anything. alex