Re: [PATCH 2/2] Btrfs: report and handle error on unexpected first key on extent buffer

[Filipe Manana <fdmanana@kernel.org>]

On Tue, Feb 19, 2019 at 12:54 AM Qu Wenruo <quwenruo.btrfs@gmx.com> wrote:
>
>
>
> On 2019/2/19 上午12:58, fdmanana@kernel.org wrote:
> > From: Filipe Manana <fdmanana@suse.com>
> >
> > When there is a kind of corruption in an extent buffer such that its first
> > key does not match the key at the respective parent slot, one of two things
> > happens:
>
> Isn't that handled by read_tree_block() already?

It is, but only at the time we read a node/leaf from disk.
By doing the check here we can actually catch other types of bugs and
memory corruption.

To be honest I missed that since this is motivated by a report on
older kernel (SLE12 SP3).
So I still find it useful to have due to the reason pointed above,
however I'm not against simply removing the check from key_search().

> Thanks,
> Qu
>
> >
> > 1) When assertions are enabled, we effectively hit a BUG_ON() which
> >    requires rebooting the machine later. This also does not tell any
> >    information about which extent buffer is affected, from which root,
> >    the expected and found keys, etc.
> >
> > 2) When assertions are disabled, we just ignore the mismatch and assume
> >    everything is ok, which can potentially lead to all sorts of unexpected
> >    problems later after a tree search (in the worst case, could lead to
> >    further silent corruption).
> >
> > So improve this by always checking if the first key of an extent buffer is
> > what it's supposed to be, when doing a key search at key_search(), and
> > report and return an appropriate error. The overhead is just comparing one
> > key, which is minimal and is anyway just done in a special case where we
> > skip the more expensive binary search (the binary search in the parent
> > node returned 0, exact key match).
> >
> > Signed-off-by: Filipe Manana <fdmanana@suse.com>
> > ---
> >  fs/btrfs/ctree.c | 38 +++++++++++++++++---------------------
> >  1 file changed, 17 insertions(+), 21 deletions(-)
> >
> > diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
> > index 5b9f602fb9e2..a0bd0278208d 100644
> > --- a/fs/btrfs/ctree.c
> > +++ b/fs/btrfs/ctree.c
> > @@ -2529,35 +2529,31 @@ setup_nodes_for_search(struct btrfs_trans_handle *trans,
> >       return ret;
> >  }
> >
> > -static void key_search_validate(struct extent_buffer *b,
> > -                             const struct btrfs_key *key,
> > -                             int level)
> > -{
> > -#ifdef CONFIG_BTRFS_ASSERT
> > -     struct btrfs_disk_key disk_key;
> > -
> > -     btrfs_cpu_key_to_disk(&disk_key, key);
> > -
> > -     if (level == 0)
> > -             ASSERT(!memcmp_extent_buffer(b, &disk_key,
> > -                 offsetof(struct btrfs_leaf, items[0].key),
> > -                 sizeof(disk_key)));
> > -     else
> > -             ASSERT(!memcmp_extent_buffer(b, &disk_key,
> > -                 offsetof(struct btrfs_node, ptrs[0].key),
> > -                 sizeof(disk_key)));
> > -#endif
> > -}
> > -
> >  static int key_search(struct extent_buffer *b, const struct btrfs_key *key,
> >                     int level, int *prev_cmp, int *slot)
> >  {
> > +     struct btrfs_key found_key;
> > +
> >       if (*prev_cmp != 0) {
> >               *prev_cmp = btrfs_bin_search(b, key, level, slot);
> >               return *prev_cmp;
> >       }
> >
> > -     key_search_validate(b, key, level);
> > +     if (level == 0)
> > +             btrfs_item_key_to_cpu(b, &found_key, 0);
> > +     else
> > +             btrfs_node_key_to_cpu(b, &found_key, 0);
> > +
> > +     if (btrfs_comp_cpu_keys(&found_key, key) != 0) {
> > +             btrfs_crit(b->fs_info,
> > +"unexpected first key for extent buffer: bytenr=%llu level=%d root=%llu expected key=(%llu %u %llu) found key=(%llu %u %llu)",
> > +                        btrfs_header_bytenr(b), level, btrfs_header_owner(b),
> > +                        key->objectid, key->type, key->offset,
> > +                        found_key.objectid, found_key.type,
> > +                        found_key.offset);
> > +             return -EUCLEAN;
> > +     }
> > +
> >       *slot = 0;
> >
> >       return 0;
> >
>