On Tue, Aug 16, 2016 at 06:50:00PM +0200, David Sterba wrote:

On Wed, Aug 03, 2016 at 12:57:28PM -0700, Liu Bo wrote:

quoted

When btree node (level = 1) has nritems which equals to zero,
we can end up with panic due to insert_ptr()'s

BUG_ON(slot > nritems);

where slot is 1 and nritems is 0, as copy_for_split() calls
insert_ptr(.., path->slots[1] + 1, ...);

A invalid value results in the whole mess, this adds the check
for btree's node nritems so that we stop reading block when
when something is wrong.

Signed-off-by: Liu Bo <redacted>
---
 fs/btrfs/disk-io.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 37d1780..a5a22be 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c

@@ -612,6 +612,20 @@ static noinline int check_leaf(struct btrfs_root *root,
 	return 0;
 }
 
+static noinline int check_node(struct btrfs_root *root,
+			       struct extent_buffer *node)
+{
+	unsigned long nr = btrfs_header_nritems(node);
+
+	if (nr <= 0 || nr >= BTRFS_NODEPTRS_PER_BLOCK(root)) {

nr is unsigned, so it's just  "== 0"

and the BTRFS_NODEPTRS_PER_BLOCK value is inclusive, which should
explain Holger's findings.

493 * sizeof (btrfs_key_ptr) + sizeof (btrfs_header) + slack = nodesize

493 * 33 + 101 + slack = 16k (the closest value)

gives slack = 14 (smaller than sizeof (btrfs_key_ptr))

Oh, right, I made a mistake when getting the assumption from insert_ptr(), it should be nr > BTRFS_NODEPTRS_PER_BLOCK(root).

Thanks,

-liubo