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

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Message-ID: <20160131224413.GN20456@dastard>
Date:	Mon, 1 Feb 2016 09:44:13 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Christoph Hellwig <hch@...radead.org>,
	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>,
	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 Fri, Jan 29, 2016 at 04:34:30PM -0700, Ross Zwisler wrote:
> On Fri, Jan 29, 2016 at 11:28:15AM -0700, 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.
> > 
> > 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...
> 
> Well, RT devices are completely broken as far as I can see.  I've reported the
> breakage to the XFS list.  Anything I do that triggers a RT block allocation
> in XFS causes a lockdep splat + a kernel BUG - I've tried regular pwrite(),

Set CONFIG_XFS_DEBUG=n (assert failure that can be ignored causing
the bug, and lockdep simply has an annotation problem) and it should
work.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com