From owner-freebsd-hackers@FreeBSD.ORG Tue Nov 3 17:40:54 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 388D81065676; Tue, 3 Nov 2009 17:40:54 +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 34F048FC12; Tue, 3 Nov 2009 17:40:52 +0000 (UTC) X-IronPort-AV: E=Sophos;i="4.44,675,1249250400"; d="scan'208";a="17524067" Received: from zivmaildisp1.uni-muenster.de (HELO ZIVMAILUSER04.UNI-MUENSTER.DE) ([128.176.188.85]) by zivm-relay3.uni-muenster.de with ESMTP; 03 Nov 2009 18:40:51 +0100 Received: by ZIVMAILUSER04.UNI-MUENSTER.DE (Postfix, from userid 149459) id EB12E1B07BE; Tue, 3 Nov 2009 18:40:51 +0100 (CET) Date: Tue, 03 Nov 2009 18:40:51 +0100 (CET) From: Alexander Best Sender: Organization: Westfaelische Wilhelms-Universitaet Muenster To: Alexander Best , 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 17:40:54 -0000 Alexander Best schrieb am 2009-11-03: > 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. ok. checked it. not being caused by your changes. maybe i missed something and in fact MAP_STACK requires addr to be non zero. couldn't find it in the mmap(2) manual though. > > alex