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Date:   Mon, 10 Jun 2019 16:14:04 -0700
From:   Eric Biggers <ebiggers@kernel.org>
To:     Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>, linux-btrfs@vger.kernel.org
Cc:     Dmitry Vyukov <dvyukov@google.com>,
        Anand Jain <anand.jain@oracle.com>,
        syzbot <syzbot+5b658d997a83984507a6@syzkaller.appspotmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>
Subject: Re: kernel BUG at fs/btrfs/volumes.c:LINE!
Message-ID: <20190610231403.GZ63833@gmail.com>
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On Thu, Jun 07, 2018 at 06:52:13PM +0200, David Sterba wrote:
> On Thu, Jun 07, 2018 at 06:28:02PM +0200, Dmitry Vyukov wrote:
> > > Normally the GFP_NOFS allocations do not fail so I think the fuzzer
> > > environment is tuned to allow that, which is fine for coverage but does
> > > not happen in practice. This will be fixed eventually.
> > 
> > Isn't GFP_NOFS more restricted than normal allocations?  Are these
> > allocations accounted against memcg? It's easy to fail any allocation
> > within a memory container.
> 
> https://lwn.net/Articles/723317/ The 'too small to fail' and some
> unwritten semantics of GFP_NOFS but I think you're right about the
> memory controler that can fail any allocation though.
> 
> Error handling is being improved over time, the memory allocation
> failures are in some cases hard and this one would need to update some
> logic so it's not a oneliner.
> 

This bug is still there.  In btrfs_close_one_device():

	if (device->name) {
		name = rcu_string_strdup(device->name->str, GFP_NOFS);
		BUG_ON(!name); /* -ENOMEM */
		rcu_assign_pointer(new_device->name, name);
	}

It assumes that the memory allocation succeeded.

See syzbot report from v5.2-rc3 here: https://syzkaller.appspot.com/text?tag=CrashReport&x=16c839c1a00000

Is there any plan to fix this?

- Eric