Re: [PATCH v3 4/6] blktrace: fix debugfs use after free

From: Christoph Hellwig <hch@infradead.org>
To: Luis Chamberlain <mcgrof@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>,
	axboe@kernel.dk, viro@zeniv.linux.org.uk, bvanassche@acm.org,
	gregkh@linuxfoundation.org, rostedt@goodmis.org,
	mingo@redhat.com, jack@suse.cz, ming.lei@redhat.com,
	nstange@suse.de, akpm@linux-foundation.org, mhocko@suse.com,
	yukuai3@huawei.com, linux-block@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, Omar Sandoval <osandov@fb.com>,
	Hannes Reinecke <hare@suse.com>, Michal Hocko <mhocko@kernel.org>,
	syzbot+603294af2d01acfdd6da@syzkaller.appspotmail.com
Subject: Re: [PATCH v3 4/6] blktrace: fix debugfs use after free
Date: Wed, 29 Apr 2020 04:50:51 -0700	[thread overview]
Message-ID: <20200429115051.GA27378@infradead.org> (raw)
In-Reply-To: <20200429114542.GJ11244@42.do-not-panic.com>

On Wed, Apr 29, 2020 at 11:45:42AM +0000, Luis Chamberlain wrote:
> On Wed, Apr 29, 2020 at 04:26:37AM -0700, Christoph Hellwig wrote:
> > I can't say I'm a fan of all these long backtraces in commit logs..
> > 
> > > +static struct dentry *blk_debugfs_dir_register(const char *name)
> > > +{
> > > +	return debugfs_create_dir(name, blk_debugfs_root);
> > > +}
> > 
> > I don't think we really need this helper.
> 
> We don't export blk_debugfs_root, didn't think we'd want to, and
> since only a few scew funky drivers would use the struct gendisk
> and also support BLKTRACE, I didn't think we'd want to export it
> now.
> 
> A new block private symbol namespace alright?

Err, that function is static and has two callers.

> > This could be simplified down to:
> > 
> > 	if (bdev && bdev != bdev->bd_contains)
> > 		return bdev->bd_part->debugfs_dir;
> > 	return q->debugfs_dir;
> >
> > Given that bd_part is in __blkdev_get very near bd_contains.
> 
> Ah neat.
> 
> > Also given that this patch completely rewrites blk_trace_debugfs_dir is
> > there any point in the previous patch?
> 
> Still think it helps with making this patch easier to read, but I don't
> care, lemme know if I should just fold it.

In fact I'm not even sure we need the helper.  Modulo the comment
this just becomes a:

	if (bdev && bdev != bdev->bd_contains)
 		dir = bdev->bd_part->debugfs_dir;
	else
	 	dir = q->debugfs_dir;

in do_blk_trace_setup.