Date: Tue, 03 Nov 2009 18:40:51 +0100 (CET) From: Alexander Best <alexbestms@math.uni-muenster.de> To: Alexander Best <alexbestms@math.uni-muenster.de>, Alexander Best <alexbestms@math.uni-muenster.de>, John Baldwin <jhb@freebsd.org>, Alexander Best <alexbestms@math.uni-muenster.de> Cc: freebsd-hackers@freebsd.org, Alan Cox <alc@cs.rice.edu> Subject: Re: mmap(2) with MAP_ANON honouring offset although it shouldn't Message-ID: <permail-2009110317405180e26a0b00007260-a_best01@message-id.uni-muenster.de> In-Reply-To: <permail-20091103171812f0889e8400000d8e-a_best01@message-id.uni-muenster.de>
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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 <sys/types.h>
> #include <sys/mman.h>
> #include <stdio.h>
> 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 <sys/types.h>
> #include <sys/mman.h>
> #include <stdio.h>
> 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
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