Re: [PATCH 1/6] hfsplus: prevent btree data loss on root split

["Ernesto A. Fernández" <ernesto.mnd.fernandez@gmail.com>]

On Thu, Oct 25, 2018 at 09:51:57AM -0700, Viacheslav Dubeyko wrote:
> On Tue, 2018-10-23 at 22:32 -0300, Ernesto A. Fernández wrote:
> > On Tue, Oct 23, 2018 at 06:23:53PM -0700, Viacheslav Dubeyko wrote:
> > > 
> > > On Fri, 2018-08-31 at 00:58 -0300, Ernesto A. Fernández wrote:
> > > > 
> > > > Creating, renaming or deleting a file may cause catalog
> > > > corruption and
> > > > data loss.  This bug is randomly triggered by xfstests
> > > > generic/027, but
> > > > here is a faster reproducer:
> > > > 
> > > >   truncate -s 50M fs.iso
> > > >   mkfs.hfsplus fs.iso
> > > >   mount fs.iso /mnt
> > > >   i=100
> > > >   while [ $i -le 150 ]; do
> > > >     touch /mnt/$i &>/dev/null
> > > >     ((++i))
> > > >   done
> > > >   i=100
> > > >   while [ $i -le 150 ]; do
> > > >     mv /mnt/$i /mnt/$(perl -e "print $i x82") &>/dev/null
> > > >     ((++i))
> > > >   done
> > > >   umount /mnt
> > > >   fsck.hfsplus -n fs.iso
> > > > 
> > > > The bug is triggered whenever hfs_brec_update_parent() needs to
> > > > split
> > > > the root node.  The height of the btree is not increased, which
> > > > leaves
> > > > the new node orphaned and its records lost.
> > > > 
> > > > Signed-off-by: Ernesto A. Fernández <ernesto.mnd.fernandez@gmail.
> > > > com>
> > > > ---
> > > >  fs/hfsplus/brec.c | 4 ++++
> > > >  1 file changed, 4 insertions(+)
> > > > 
> > > > diff --git a/fs/hfsplus/brec.c b/fs/hfsplus/brec.c
> > > > index ed8eacb34452..aa17a392b414 100644
> > > > --- a/fs/hfsplus/brec.c
> > > > +++ b/fs/hfsplus/brec.c
> > > > @@ -429,6 +429,10 @@ static int hfs_brec_update_parent(struct
> > > > hfs_find_data *fd)
> > > >  	if (new_node) {
> > > >  		__be32 cnid;
> > > >  
> > > > +		if (!new_node->parent) {
> > > > +			hfs_btree_inc_height(tree);
> > > > +			new_node->parent = tree->root;
> > > I worry about the case when we are adding the node on intermediate
> > > (not
> > > root) level. As far as I can see, we will be in trouble here
> > > because I
> > > don't see any processing of two possible cases: (1) root node; (2)
> > > node
> > > of intermediate level. Do I miss something here?
> > If 'new_node' had been the result of splitting a node other than
> > root,
> > then it would have a parent.
> > 
> 
> Could you please share the callstack or/and more detailed explanation
> that would show the correctness of the fix? Currently, it's not enough
> details in the comment for easy understanding the issue's environment
> and correctness of the fix. 

This patch is two months old now.  Why did you wait until the merge
window to give me your traditional "the patch is confusing" reply?

> I simply mean here that as implementation
> as concept of the HFS+ b-trees is not trivial. But everybody should
> easy understand the patch.

Those two sentences are not compatible.  Reviewers need to have some
understanding of the code they review.  No royal road...

> Thanks,
> Vyacheslav Dubeyko.
>