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

[John Baldwin]

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. :)

-- 
John Baldwin