• Jeff King's avatar
    pack-objects: fix tree_depth and layer invariants · bc35ac1a
    Jeff King authored
    Commit 108f5303 (pack-objects: move tree_depth into 'struct
    packing_data', 2018-08-16) dynamically manages a tree_depth array in
    packing_data that maintains one of these invariants:
      1. tree_depth is NULL (i.e., the requested options don't require us to
         track tree depths)
      2. tree_depth is non-NULL and has as many entries as the "objects"
    We maintain (2) by:
      a. When the objects array grows, grow tree_depth to the same size
         (unless it's NULL, in which case we can leave it).
      b. When a caller asks to set a depth via oe_set_tree_depth(), if
         tree_depth is NULL we allocate it.
    But in (b), we use the number of stored objects, _not_ the allocated
    size of the objects array. So we can run into a situation like this:
      1. packlist_alloc() needs to store the Nth object, so it grows the
         objects array to M, where M > N.
      2. oe_set_tree_depth() wants to store a depth, so it allocates an
         array of length N. Now we've violated our invariant.
      3. packlist_alloc() needs to store the N+1th object. But it _doesn't_
         grow the objects array, since N <= M still holds. We try to assign
         to tree_depth[N+1], which is out of bounds.
    That doesn't happen in our test scripts, because the repositories they
    use are so small, but it's easy to trigger by running:
      echo HEAD | git pack-objects --revs --delta-islands --stdout >/dev/null
    in any reasonably-sized repo (like git.git).
    We can fix it by always growing the array to match pack->nr_alloc, not
    pack->nr_objects. Likewise for the "layer" array from fe0ac2fb
    (pack-objects: move 'layer' into 'struct packing_data', 2018-08-16),
    which has the same bug.
    Signed-off-by: 's avatarJeff King <peff@peff.net>
    Signed-off-by: 's avatarJunio C Hamano <gitster@pobox.com>
pack-objects.h 10.6 KB