Re: [PATCH v6 6/6] blktrace: fix debugfs use after free

[Luis Chamberlain]

On Tue, Jun 09, 2020 at 10:32:18AM -0700, Christoph Hellwig wrote:
> On Tue, Jun 09, 2020 at 05:29:22PM +0000, Luis Chamberlain wrote:
> > Is scsi-generic is the only unwanted ugly child blktrace has to deal
> > with? For some reason I thought drivers/md/md.c was one but it seems
> > like it is not. Do we have an easy way to search for these? I think
> > this would just affect how we express the comment only.
> 
> grep for blk_trace_setup.  For all blk devices that setup comes in
> through the block device ioctl path, and that relies on having a
> struct block_device and queue.  sg on the other hand calls
> blk_trace_setup directly with a NULL bdev argument.

Alright, then we should be good.

> > >  		 */
> > > -		dir = q->sg_debugfs_dir;
> > > +		dir = debugfs_create_dir(buts->name, blk_debugfs_root);
> > > +		bt->dir = dir;
> > 
> > The other chicken and egg problem to consider at least in the comments
> > is that the debugfs directory for these types of devices *have* an
> > exposed path, but the data structure is rather opaque to the device and
> > even blktrace.  Fortunately given the recent set of changes around the
> > q->blk_trace and clarifications around its use we have made it clear now
> > that so long as hold the q->blk_trace_mutex *and* check q->blk_trace we
> > *should* not race against two separate creations of debugfs directories,
> > so I think this is safe, so long as these indpendent drivers don't end
> > up re-using the same path for some other things later in the future, and
> > since we have control over what goes under debugfsroot block / I think
> > we should be good.
> > 
> > But I think that the concern for race on names may still be worth
> > explaining a bit here.
> 
> Feel free to add more comments, but please try to keep them short
> and crisp.  At the some point long comments really distract from what
> is going on.

Sure.

Come to think of it, given the above, I think we can also do way with
the the partition stuff too, and rely on the buts->name too. I'll try
this out, and test it.

  Luis