> On Mon, 23 Jun 2008 00:28:20 +0200 Michael Buesch <mb@bu3sch.de> wrote:
quoted text
> 
> > On Monday 23 June 2008 00:05:51 Michael Buesch wrote:
> > > > Note: r9 and r3 are both NULL pointers. r3 is the value returned from alloc_page_buffers.
> > > > R9 is a copy of that, which gets accessed.
> > > 
> > > Hm, yeah. I looked at that code already, but I can't see how it could return
> > > a NULL pointer.
> > 
> > Well, actually, it can return a NULL pointer.
> > 
> >  928         head = NULL;
> >  929         offset = PAGE_SIZE;
> >  930         while ((offset -= size) >= 0) {
> > ...
> >  949         }
> >  950         return head;
> > 
> > So if size, which is a passed in as parameter, is > PAGE_SIZE it will return NULL.
> > 
> > The size parameter is calculated by doing
> > blocksize = 1 << inode->i_blkbits;
> > in an earlier function in the callchain.
> 
> Yes, that's a more likely scenario.  isofs has a history of passing
> garbage into the VFS.
> 
> > So, well. I dunno what i_blkbits is. There's no docs in struct inode.
> 
> It's log2 of the filesystem blocksize.  It'd be interesting to work out
> what value isofs is setting it to, and why.
  Well, yes, that looks as a reason at the first sight. But what I don't
get is, how can isofs possibly set such a blocksize when it uses
sb_set_blocksize() which checks whether blocksize isn't larger than page
size... So it must be something less obvious.
  bd_set_size() can possibly set blocksize larger than PAGE_SIZE and
it's called from do_open() but it uses bdev_hardsect_size() and that
shouldn't be larger than PAGE_SIZE either (at least drivers seem to take
care of this).
  I have seen one more report of this Oops for SLES10 kernel and also in that
case an IO error happened so probably that is a trigger... But so far I
don't get the details.

									Honza
-- 
Jan Kara <jack@suse.cz>
SuSE CR Labs
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