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

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Date:   Wed, 29 Apr 2020 12:02:30 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     axboe@...nel.dk, viro@...iv.linux.org.uk, bvanassche@....org,
        gregkh@...uxfoundation.org, rostedt@...dmis.org, mingo@...hat.com,
        jack@...e.cz, ming.lei@...hat.com, nstange@...e.de,
        akpm@...ux-foundation.org, mhocko@...e.com, yukuai3@...wei.com,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Omar Sandoval <osandov@...com>,
        Hannes Reinecke <hare@...e.com>,
        Michal Hocko <mhocko@...nel.org>,
        syzbot+603294af2d01acfdd6da@...kaller.appspotmail.com
Subject: Re: [PATCH v3 4/6] blktrace: fix debugfs use after free

On Wed, Apr 29, 2020 at 04:50:51AM -0700, Christoph Hellwig wrote:
> 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.

Yes but that is to make it easier to look for who is creating the
debugfs_dir for either the request_queue or partition. I'll export
blk_debugfs_root and we'll open code all this.

> > > 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.

True, alright will remove that patch.

  Luis