mmap(2) with MAP_ANON honouring offset although it shouldn't

[John Baldwin]

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 at .  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.

-- 
John Baldwin