From owner-freebsd-hackers@FreeBSD.ORG Tue Nov 3 17:18:15 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 0134710656A3; Tue, 3 Nov 2009 17:18:15 +0000 (UTC) (envelope-from a_best01@uni-muenster.de) Received: from zivm-exrelay2.uni-muenster.de (ZIVM-EXRELAY2.UNI-MUENSTER.DE [128.176.192.15]) by mx1.freebsd.org (Postfix) with ESMTP id 134A48FC08; Tue, 3 Nov 2009 17:18:13 +0000 (UTC) X-IronPort-AV: E=Sophos;i="4.44,675,1249250400"; d="scan'208";a="227984190" Received: from zivmaildisp1.uni-muenster.de (HELO ZIVMAILUSER01.UNI-MUENSTER.DE) ([128.176.188.85]) by zivm-relay2.uni-muenster.de with ESMTP; 03 Nov 2009 18:18:13 +0100 Received: by ZIVMAILUSER01.UNI-MUENSTER.DE (Postfix, from userid 149459) id 076891B0766; Tue, 3 Nov 2009 18:18:12 +0100 (CET) Date: Tue, 03 Nov 2009 18:18:12 +0100 (CET) From: Alexander Best Sender: Organization: Westfaelische Wilhelms-Universitaet Muenster To: Alexander Best , John Baldwin , Alexander Best Message-ID: In-Reply-To: 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 17:18:15 -0000 Alexander Best schrieb am 2009-11-03: > 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. just realised that building universe or only world is pretty useless since the API changes only affect apps during runtime and at compilation time. :) i've run a few tests. the following app: #include #include #include main() { printf("%p\n", mmap((void*)0x1000, 0x1000, PROT_READ|PROT_WRITE, MAP_STACK, -1, 1)); printf("%p\n", mmap((void*)0x1000, 0x1000, PROT_READ|PROT_WRITE, MAP_ANON, -1, 1)); } outputs: 0x1000 0xffffffff as expected. #include #include #include main() { printf("%p\n", mmap((void*)0, 0x1000, PROT_READ|PROT_WRITE, MAP_STACK, -1, 0)); printf("%p\n", mmap((void*)0, 0x1000, PROT_READ|PROT_WRITE, MAP_ANON, -1, 0)); } however produces this output: 0xffffffff 0x28195000 which seems a bit odd. the mmap(2) manual doesn't say anything about MAP_STACK not working when addr is zero. i'll see if this is caused by the changes jhb@ suggested or not. > alex