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

From: Ming Lei <ming.lei@redhat.com>
To: Luis Chamberlain <mcgrof@kernel.org>
Cc: axboe@kernel.dk, viro@zeniv.linux.org.uk, bvanassche@acm.org,
	gregkh@linuxfoundation.org, rostedt@goodmis.org,
	mingo@redhat.com, jack@suse.cz, 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 2/5] blktrace: fix debugfs use after free
Date: Thu, 16 Apr 2020 14:09:21 +0800	[thread overview]
Message-ID: <20200416060921.GB2723777@T590> (raw)
In-Reply-To: <20200416054750.GA2723777@T590>

On Thu, Apr 16, 2020 at 01:47:50PM +0800, Ming Lei wrote:
> 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.

Just test the 1st two patches via "blktrace /dev/sda2", follows the
result, so this way can't be accepted.

[root@ktest-01 ~]# blktrace /dev/sda2
Thread 0 failed open /sys/kernel/debug/block/sda2/trace0: 2/No such file or directory
Thread 4 failed open /sys/kernel/debug/block/sda2/trace4: 2/No such file or directory
Thread 1 failed open /sys/kernel/debug/block/sda2/trace1: 2/No such file or directory
Thread 2 failed open /sys/kernel/debug/block/sda2/trace2: 2/No such file or directory
Thread 5 failed open /sys/kernel/debug/block/sda2/trace5: 2/No such file or directory
Thread 3 failed open /sys/kernel/debug/block/sda2/trace3: 2/No such file or directory
Thread 6 failed open /sys/kernel/debug/block/sda2/trace6: 2/No such file or directory
Thread 7 failed open /sys/kernel/debug/block/sda2/trace7: 2/No such file or directory
FAILED to start thread on CPU 0: 1/Operation not permitted
FAILED to start thread on CPU 1: 1/Operation not permitted
FAILED to start thread on CPU 2: 1/Operation not permitted
FAILED to start thread on CPU 3: 1/Operation not permitted
FAILED to start thread on CPU 4: 1/Operation not permitted
FAILED to start thread on CPU 5: 1/Operation not permitted
FAILED to start thread on CPU 6: 1/Operation not permitted
FAILED to start thread on CPU 7: 1/Operation not permitted

Thanks, 
Ming