Re: 3.0-rcX BUG at fs/btrfs/ioctl.c:432 - bisected

Sage Weil
2011-06-10T18:41:17Z

On Fri, 10 Jun 2011, Josef Bacik wrote:
> On 06/10/2011 02:35 PM, Sage Weil wrote:
> > On Fri, 10 Jun 2011, Josef Bacik wrote:
> >> On 06/10/2011 02:14 PM, Sage Weil wrote:
> >>> On Fri, 10 Jun 2011, Sage Weil wrote:
> >>>> On Fri, 10 Jun 2011, Chris Mason wrote:
> >>>>> Excerpts from Jim Schutt's message of 2011-06-10 13:06:22 -0400:
> >>>>>
> >>>>> [ two different btrfs crashes ]
> >>>>>
> >>>>> I think your two crashes in btrfs were from the uninit variables and
> >>>>> those should be fixed in rc2.
> >>>>>
> >>>>>> When I did my bisection, my criteria for success/failure was
> >>>>>> "did mkcephfs succeed?". When I apply this criteria to a recent
> >>>>>> linus kernel (e.g. 06e86849cf4019), which includes the fix you
> >>>>>> mentioned (aa0467d8d2a00e), I get still a different failure mode,
> >>>>>> which doesn't actually reference btrfs:
> >>>>>>
> >>>>>> [ 276.364178] BUG: unable to handle kernel NULL pointer dereference at 000000000000000a
> >>>>>> [ 276.365127] IP: [<ffffffffa05434b1>] journal_start+0x3e/0x9c [jbd]
> >>>>>
> >>>>> Looking at the resulting code in the oops, we're here in journal_start:
> >>>>>
> >>>>> if (handle) {
> >>>>> J_ASSERT(handle->h_transaction->t_journal == journal);
> >>>>>
> >>>>> handle comes from current->journal_info, and we're doing a deref on
> >>>>> handle->h_transaction, which is probably 0xa.
> >>>>>
> >>>>> So, we're leaving crud in current->journal_info and ext3 is finding it.
> >>>>>
> >>>>> Perhaps its from ceph starting a transaction but leaving it running?
> >>>>> The bug came with Josef's transaction performance fixes, but it is
> >>>>> probably a mixture of his code with the ioctls ceph is using.
> >>>>
> >>>> Ah, yeah, that's the problem. We saw a similar problem a while back with
> >>>> the start/stop transaction ioctls. In this case, create_snapshot is doing
> >>>>
> >>>> trans = btrfs_start_transaction(root->fs_info->extent_root, 5);
> >>>> if (IS_ERR(trans)) {
> >>>> ret = PTR_ERR(trans);
> >>>> goto fail;
> >>>> }
> >>>>
> >>>> which sets current->journal_info. Then
> >>>>
> >>>> ret = btrfs_snap_reserve_metadata(trans, pending_snapshot);
> >>>> BUG_ON(ret);
> >>>>
> >>>> list_add(&pending_snapshot->list,
> >>>> &trans->transaction->pending_snapshots);
> >>>> if (async_transid) {
> >>>> *async_transid = trans->transid;
> >>>> ret = btrfs_commit_transaction_async(trans,
> >>>> root->fs_info->extent_root, 1);
> >>>> } else {
> >>>> ret = btrfs_commit_transaction(trans,
> >>>> root->fs_info->extent_root);
> >>>> }
> >>>>
> >>>> but the async snap creation ioctl takes the async path, which runs
> >>>> btrfs_commit_transaction in a worker thread.
> >>>>
> >>>> I'm not sure what the right thing to do is here is... can whatever is in
> >>>> journal_info be attached to trans instead in
> >>>> btrfs_commit_transaction_async()?
> >>>
> >>> It looks like it's not used for anything in btrfs, actually; it's just set
> >>> and cleared. What's the point of that?
> >>>
> >>
> >> It is used now, check the beginning of start_transaction(). Thanks,
> >
> > Oh I see, okay.
> >
> > So clearing it in btrfs_commit_transaction_async should be fine then,
> > right? When btrfs_commit_transaction runs in the other thread it won't
> > care that current->journal_info is NULL.
> >
>
> Oh yeah your patch is good :),

Okay cool. Here's the fix with a proper changelog and a little
use-after-free paranoia.

Thanks!
sage

>From 9881c0752293769d5133c01dff3ab04c0c24c61b Mon Sep 17 00:00:00 2001
From: Sage Weil <sage@newdream.net>
Date: Fri, 10 Jun 2011 11:41:25 -0700
Subject: [PATCH] Btrfs: clear current->journal_info on async transaction commit

Normally current->jouranl_info is cleared by commit_transaction. For an
async snap or subvol creation, though, it runs in a work queue. Clear
it in btrfs_commit_transaction_async() to avoid leaking a non-NULL
journal_info when we return to userspace. When the actual commit runs in
the other thread it won't care that it's current->journal_info is already
NULL.

Signed-off-by: Sage Weil <sage@newdream.net>
---
fs/btrfs/transaction.c | 5 ++++-
1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index dd71966..9d516ed 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -1118,8 +1118,11 @@ int btrfs_commit_transaction_async(struct btrfs_trans_handle *trans,
wait_current_trans_commit_start_and_unblock(root, cur_trans);
else
wait_current_trans_commit_start(root, cur_trans);
- put_transaction(cur_trans);

+ if (current->journal_info == trans)
+ current->journal_info = NULL;
+
+ put_transaction(cur_trans);
return 0;
}

--
1.7.0