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

[Ross Zwisler]

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?