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

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Date:   Wed, 29 Apr 2020 04:50:51 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     Christoph Hellwig <hch@...radead.org>, 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 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.