linux-kernel - Re: [PATCH 2/5] blktrace: fix debugfs use after free

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Date:   Thu, 16 Apr 2020 13:47:50 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     axboe@...nel.dk, viro@...iv.linux.org.uk, bvanassche@....org,
        gregkh@...uxfoundation.org, rostedt@...dmis.org, mingo@...hat.com,
        jack@...e.cz, 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 2/5] blktrace: fix debugfs use after free

On Thu, Apr 16, 2020 at 05:25:24AM +0000, Luis Chamberlain wrote:
> On Thu, Apr 16, 2020 at 10:10:36AM +0800, Ming Lei wrote:
> > In theory, multiple partitions can be traced concurrently, but looks
> > it never works, so it won't cause trouble for multiple partition trace.
> > 
> > One userspace visible change is that blktrace debugfs dir name is switched 
> > to disk name from partition name in case of partition trace, will it
> > break some utilities?
> 
> How is this possible, its not clear to me, we go from:
> 
> -	q->debugfs_dir = debugfs_create_dir(kobject_name(q->kobj.parent),
> -					    blk_debugfs_root);
> 
> To this:
> 
> +	q->debugfs_dir = debugfs_create_dir(kobject_name(q->kobj.parent),
> +					    blk_debugfs_root);
> 
> 
> Maybe I am overlooking something.

Your patch removes the blktrace debugfs dir:

do_blk_trace_setup()

-       dir = debugfs_lookup(buts->name, blk_debugfs_root);
-       if (!dir)
-               bt->dir = dir = debugfs_create_dir(buts->name, blk_debugfs_root);
-

Then create blktrace attributes under the dir of q->debugfs_dir.

However, buts->name could be one partition device name, but
q->debugfs_dir has to be disk name.

This change is visible to blktrace utilities.

Thanks,
Ming