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

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Date:	Fri, 29 Jan 2016 11:28:15 -0700
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	linux-fsdevel@...r.kernel.org, linux-nvdimm@...1.01.org
Subject: Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences

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.

I guess I need to go off and understand if we can have DAX mappings on such a
device.  If we can, we may have a problem - we can get the block_device from
get_block() in I/O path and the various fault paths, but we don't have access
to get_block() when flushing via dax_writeback_mapping_range().  We avoid
needing it the normal case by storing the sector results from get_block() in
the radix tree.

/me is off to play with RT devices...