Re: [PATCH v8 1/9] dax: fix NULL pointer dereference in __dax_dbg()

[Jan Kara]

On Wed 13-01-16 00:08:29, Ross Zwisler wrote:
> On Tue, Jan 12, 2016 at 10:34:58AM +0100, Jan Kara wrote:
> > On Thu 07-01-16 22:27:51, Ross Zwisler wrote:
> > > In __dax_pmd_fault() we currently assume that get_block() will always set
> > > bh.b_bdev and we unconditionally dereference it in __dax_dbg().  This
> > > assumption isn't always true - when called for reads of holes
> > > ext4_dax_mmap_get_block() returns a buffer head where bh->b_bdev is never
> > > set.  I hit this BUG while testing the DAX PMD fault path.
> > > 
> > > Instead, initialize bh.b_bdev before passing bh into get_block().  It is
> > > possible that the filesystem's get_block() will update bh.b_bdev, and this
> > > is fine - we just want to initialize bh.b_bdev to something reasonable so
> > > that the calls to __dax_dbg() work and print something useful.
> > > 
> > > Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> > > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> > 
> > Looks good. But don't you need to do the same for __dax_fault(),
> > dax_zero_page_range() and similar places passing bh to dax functions?
> > 
> > 								Honza
> 
> I don't think we need it anywhere else.  The only reason that we need to
> initialize the bh.b_bdev manually in the __dax_pmd_fault() path is that if the
> get_block() call ends up finding a hole (so doesn't fill out b_bdev) we still
> go through the dax_pmd_dbg() path to print an error message, which uses
> b_bdev.  I believe that in the other paths where we hit a hole, such as
> __dax_fault(), we don't use b_bdev because we don't have the same error path
> prints, and the regular code for handling holes doesn't use b_bdev.
> 
> That being said, if you feel like it's cleaner to initialize it
> everywhere so everything is consistent and we don't have to worry about
> it, I'm fine to make the change.

Well, it seems more futureproof to me. In case someone decides to add some
debug message later on...

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR