Re: [PATCH] btrfs: Added null check to extent_root variable

[Qu Wenruo]

在 2024/9/5 03:16, David Sterba 写道:
> On Wed, Sep 04, 2024 at 03:21:34PM +0930, Qu Wenruo wrote:
> > 在 2024/9/4 12:07, Ghanshyam Agrawal 写道:
> > > Reported-by: syzbot+9c3e0cdfbfe351b0bc0e@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > Closes:https://syzkaller.appspot.com/bug?extid=9c3e0cdfbfe351b0bc0e
> > > Signed-off-by: Ghanshyam Agrawal <ghanshyam1898@xxxxxxxxx>
> > > ---
> > >    fs/btrfs/ref-verify.c | 3 +++
> > >    1 file changed, 3 insertions(+)
> > >
> > > diff --git a/fs/btrfs/ref-verify.c b/fs/btrfs/ref-verify.c
> > > index 9522a8b79d22..4e98ddf5e8df 100644
> > > --- a/fs/btrfs/ref-verify.c
> > > +++ b/fs/btrfs/ref-verify.c
> > > @@ -1002,6 +1002,9 @@ int btrfs_build_ref_tree(struct btrfs_fs_info *fs_info)
> > >    		return -ENOMEM;
> > >
> > >    	extent_root = btrfs_extent_root(fs_info, 0);
> > > +	if (!extent_root)
> > > +		return -EIO;
> > > +
> >
> > Can you reproduce the original bug and are sure it's an NULL extent tree
> > causing the problem?
> >
> > At least a quick glance into the console output shows there is no
> > special handling like rescue=ibadroots to ignore extent root, nor any
> > obvious corruption in the extent tree.
> >
> > If extent root is really empty, we should error out way earlier.
> >
> > Mind to explain the crash with more details?
>
> In the stack trace it looks the ref-verify mount option is enabled, I
> don't think we've tested that in combination with the rescue options as
> ref-verify is a debugging tool, must be built in config (by default is
> not and is not on distro configs).

Indeed it is the rescue=ibadroots mount option.
And unlike the regular mount options which all show a message line, none
of the rescue mount options will output an error message.

Thus that's why we're ignoring the missing extent root in
btrfs_read_block_groups().

In that case, yes the fix is correct.

Just need a small description on the combination to trigger the bug:

- Corrupted extent tree root

- rescue=ibadroots mount option

- Built-in ref-verify

Thanks,
Qu

> We should fix the bug where it crashes when run in syzkaller so we can
> allow it to continue coverage but otherwise I wouldn't put too much
> effort into that. I.e. if we can do a simple fallback and exit gracefully
> and not try to continue ref-verify + missint extent (or other trees).