We expect a commit-graph file to have a fixed-size data record for each
commit in the file (and we know the number of commits to expct from the
size of the lookup table). If we encounter a file where this is too
small, we'll look past the end of the chunk (and possibly even off the
mapped memory).

We can fix this by checking the size up front when we record the
pointer.

The included test doesn't segfault, since it ends up reading bytes
from another chunk. But it produces nonsense results, since the values
it reads are garbage. Our test notices this by comparing the output to a
non-corrupted run of the same command (and of course we also check that
the expected error is printed to stderr).

Signed-off-by: Jeff King <redacted>
---
 commit-graph.c          | 12 +++++++++++-
 t/t5318-commit-graph.sh |  9 +++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/commit-graph.c b/commit-graph.c
index 472332f603..9b80bbd75b 100644
--- a/commit-graph.c
+++ b/commit-graph.c

@@ -340,6 +340,16 @@ static int graph_read_oid_lookup(const unsigned char *chunk_start,
 	return 0;
 }

+static int graph_read_commit_data(const unsigned char *chunk_start,
+				  size_t chunk_size, void *data)
+{
+	struct commit_graph *g = data;
+	if (chunk_size != g->num_commits * GRAPH_DATA_WIDTH)

+		return error("commit-graph commit data chunk is wrong size");
+	g->chunk_commit_data = chunk_start;
+	return 0;
+}
+
 static int graph_read_bloom_data(const unsigned char *chunk_start,
 				  size_t chunk_size, void *data)
 {

@@ -422,7 +432,7 @@ struct commit_graph *parse_commit_graph(struct repo_settings *s,

 	read_chunk(cf, GRAPH_CHUNKID_OIDFANOUT, graph_read_oid_fanout, graph);
 	read_chunk(cf, GRAPH_CHUNKID_OIDLOOKUP, graph_read_oid_lookup, graph);
-	pair_chunk_unsafe(cf, GRAPH_CHUNKID_DATA, &graph->chunk_commit_data);
+	read_chunk(cf, GRAPH_CHUNKID_DATA, graph_read_commit_data, graph);