Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences

[Jan Kara <jack@suse.cz>]

On Mon 01-02-16 17:02:12, Ross Zwisler wrote:
> On Thu, Jan 28, 2016 at 01:38:58PM -0800, Christoph Hellwig wrote:
> > On Thu, Jan 28, 2016 at 12:35:04PM -0700, Ross Zwisler wrote:
> > > There are a number of places in dax.c that look up the struct block_device
> > > associated with an inode.  Previously this was done by just using
> > > inode->i_sb->s_bdev.  This is correct for inodes that exist within the
> > > filesystems supported by DAX (ext2, ext4 & XFS), but when running DAX
> > > against raw block devices this value is NULL.  This causes NULL pointer
> > > dereferences when these block_device pointers are used.
> > 
> > It's also wrong for an XFS file system with a RT device..
> > 
> > > +#define DAX_BDEV(inode) (S_ISBLK(inode->i_mode) ? I_BDEV(inode) \
> > > +				: inode->i_sb->s_bdev)
> > 
> > .. but this isn't going to fix it.  You must use a bdev returned by
> > get_blocks or a similar file system method.
> 
> Jan & Dave,
> 
> Before I start in on a solution to this issue I just wanted to confirm that
> DAX can rely on the fact that the filesystem's get_block() call will reliably
> set bh->b_bdev for non-error returns.  From this conversation between Jan &
> Dave:
> 
> https://lkml.org/lkml/2016/1/7/723
> 
> "
>   > No. The real problem is a long-standing abuse of struct buffer_head to be
>   > used for passing block mapping information (it's on my todo list to remove
>   > that at least from DAX code and use cleaner block mapping interface but
>   > first I want basic DAX functionality to settle down to avoid unnecessary
>   > conflicts). Filesystem is not supposed to touch bh->b_bdev.
>   
>   That has not been true for a long, long time. e.g. XFS always
>   rewrites bh->b_bdev in get_blocks because the file may not reside on
>   the primary block device of the filesystem. i.e.:
>   
>           /*
>            * If this is a realtime file, data may be on a different device.
>            * to that pointed to from the buffer_head b_bdev currently.
>            */
>           bh_result->b_bdev = xfs_find_bdev_for_inode(inode);
>   > If you need
>   > that filled in, set it yourself in before passing bh to the block mapping
>   > function.
>   
>   That may be true, but we cannot assume that the bdev coming back
>   out of get_block is the same one that was passed in.
> "
> 
> It sounds like this is always true for XFS, and from looking at the ext4 code
> I think this is true there as well because bh->b_bdev is set in
> ext4_dax_mmap_get_block() via map_bh().
> 
> Relying on the bh->b_bdev returned by get_block() is correct, yea?

Yeah, sorry, I was confused. If the result is a mapped block (i.e. return
value of get_block callback is > 0), ext4 also sets bh->b_bdev via map_bh()
as you correctly point out. If the result is a hole or error, ext4 doesn't
set bh->b_bdev at all. So you can rely on bh->b_bdev.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR