commit-graph: handle overflow in chunk_size checks

commit-graph: handle overflow in chunk_size checks

We check the size of chunks with fixed records by multiplying the width
of each record by the number of commits in the file. Like:

  if (chunk_size != g->num_commits * GRAPH_DATA_WIDTH)

If this multiplication overflows, we may not notice a chunk is too small
(which could later lead to out-of-bound reads).

In the current code this is only possible for the CDAT chunk, but the
reasons are quite subtle. We compute g->num_commits by dividing the size
of the OIDL chunk by the hash length (since it consists of a bunch of
hashes). So we know that any size_t multiplication that uses a value
smaller than the hash length cannot overflow. And the CDAT records are
the only ones that are larger (the others are just 4-byte records). So
it's worth fixing all of these, to make it clear that they're not
subject to overflow (without having to reason about seemingly unrelated
code).

The obvious thing to do is add an st_mult(), like:

  if (chunk_size != st_mult(g->num_commits, GRAPH_DATA_WIDTH))

And that certainly works, but it has one downside: if we detect an
overflow, we'll immediately die(). But the commit graph is an optional
file; if we run into other problems loading it, we'll generally return
an error and fall back to accessing the full objects. Using st_mult()
means a malformed file will abort the whole process.

So instead, we can do a division like this:

  if (chunk_size / GRAPH_DATA_WIDTH != g->num_commits)

where there's no possibility of overflow. We do lose a little bit of
precision; due to integer division truncation we'd allow up to an extra
GRAPH_DATA_WIDTH-1 bytes of data in the chunk. That's OK. Our main goal
here is making sure we don't have too _few_ bytes, which would cause an
out-of-bounds read (we could actually replace our "!=" with "<", but I
think it's worth being a little pedantic, as a large mismatch could be a
sign of other problems).

I didn't add a test here. We'd need to generate a very large graph file
in order to get g->num_commits large enough to cause an overflow. And a
later patch in this series will use this same division technique in a
way that is much easier to trigger in the tests.

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