On Mon Jul 30 2007 at 10:18:19 -0500, Eric Haszlakiewicz wrote:
> On Mon, Jul 30, 2007 at 11:14:33AM +0200, Joerg Sonnenberger wrote:
> > On Tue, Jul 24, 2007 at 05:15:31PM -0500, Eric Haszlakiewicz wrote:
> > >  if you map a 10 byte file (for example), the mmap man page says that
> > > the mapped region may be extended up to the page size.
> > >   I suppose there's a bit of a performance hit with zeroing out the page
> > > every time, but it seems like a necessary thing to do.  A couple other
> > > OSes I've tried it on do so.
> > 
> > This discussion is lame. There are four possible approaches to this
> > problem:
> > (1) Discontinue UCB. Inacceptible as it introduces far more issues than
> > it is worth.
> > (2) Zero the page at random times (e.g. scheduling). Doesn't fix the
> > issue, just makes the window smaller.
> > (3) Trap on write. Horrible performance, MP issues.
> > (4) Disallow mmap for write on not page-sized files. Standard compliance
> > issues.
> 
> 5) Zero the page at mmap time.  Fixes all cases except two concurrently
> running processes reading and writing to the area beyond the end of the
> file.  Some performance drop, but less than the other options.

Some?  If you zero the pages of e.g. /bin/ls every time you run it, I'd
assume a little more than "some" performance drop.  And if you don't do
it in the exec path, I would be very careful to make sure it can't be
exploited to circumvent the zeroing protection.

.. but I'm still just trying to cause some trouble around here ;)

-- 
Antti Kantee <pooka@iki.fi>                     Of course he runs NetBSD
http://www.iki.fi/pooka/                          http://www.NetBSD.org/
    "la qualité la plus indispensable du cuisinier est l'exactitude"