commit-graph: bounds-check generation overflow chunk

commit-graph: bounds-check generation overflow chunk

If the generation entry in a commit-graph doesn't fit, we instead insert
an offset into a generation overflow chunk. But since we don't record
the size of the chunk, we may read outside the chunk if the offset we
find on disk is malicious or corrupted.

We can't check the size of the chunk up-front; it will vary based on how
many entries need overflow. So instead, we'll do a bounds-check before
accessing the chunk memory. Unfortunately there is no error-return from
this function, so we'll just have to die(), which is what it does for
other forms of corruption.

As with other cases, we can drop the st_mult() call, since we know our
bounds-checked value will fit within a size_t.

Before this patch, the test here actually "works" because we read
garbage data from the next chunk. And since that garbage data happens
not to provide a generation number which changes the output, it appears
to work. We could construct a case that actually segfaults or produces
wrong output, but it would be a bit tricky. For our purposes its
sufficient to check that we've detected the bounds error.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>