commit-graph: check size of oid fanout chunk

We load the oid fanout chunk with pair_chunk(), which means we never see
the size of the chunk. We just assume the on-disk file uses the
appropriate size, and if it's too small we'll access random memory.

It's easy to check this up-front; the fanout always consists of 256
uint32's, since it is a fanout of the first byte of the hash pointing
into the oid index. These parameters can't be changed without
introducing a new chunk type.

This matches the similar check in the midx OIDF chunk (but note that
rather than checking for the error immediately, the graph code just
leaves parts of the struct NULL and checks for required fields later).

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

commit-graph.c

@@ -305,6 +305,16 @@ static int verify_commit_graph_lite(struct commit_graph *g)
 	return 0;
 }
 
+static int graph_read_oid_fanout(const unsigned char *chunk_start,
+				 size_t chunk_size, void *data)
+{
+	struct commit_graph *g = data;
+	if (chunk_size != 256 * sizeof(uint32_t))
+		return error("commit-graph oid fanout chunk is wrong size");
+	g->chunk_oid_fanout = (const uint32_t *)chunk_start;
+	return 0;
+}
+
 static int graph_read_oid_lookup(const unsigned char *chunk_start,
 				 size_t chunk_size, void *data)
 {
@@ -394,8 +404,7 @@ struct commit_graph *parse_commit_graph(struct repo_settings *s,
 				   GRAPH_HEADER_SIZE, graph->num_chunks))
 		goto free_and_return;
 
-	pair_chunk_unsafe(cf, GRAPH_CHUNKID_OIDFANOUT,
-		   (const unsigned char **)&graph->chunk_oid_fanout);
+	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);
 	pair_chunk_unsafe(cf, GRAPH_CHUNKID_EXTRAEDGES, &graph->chunk_extra_edges);

t/t5318-commit-graph.sh

@@ -2,6 +2,7 @@
 
 test_description='commit graph'
 . ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-chunk.sh
 
 GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS=0
 
@@ -821,4 +822,29 @@ test_expect_success 'overflow during generation version upgrade' '
 	)
 '
 
+corrupt_chunk () {
+	graph=full/.git/objects/info/commit-graph &&
+	test_when_finished "rm -rf $graph" &&
+	git -C full commit-graph write --reachable &&
+	corrupt_chunk_file $graph "$@"
+}
+
+check_corrupt_chunk () {
+	corrupt_chunk "$@" &&
+	git -C full -c core.commitGraph=false log >expect.out &&
+	git -C full -c core.commitGraph=true log >out 2>err &&
+	test_cmp expect.out out
+}
+
+test_expect_success 'reader notices too-small oid fanout chunk' '
+	# make it big enough that the graph file is plausible,
+	# otherwise we hit an earlier check
+	check_corrupt_chunk OIDF clear $(printf "000000%02x" $(test_seq 250)) &&
+	cat >expect.err <<-\EOF &&
+	error: commit-graph oid fanout chunk is wrong size
+	error: commit-graph is missing the OID Fanout chunk
+	EOF
+	test_cmp expect.err err
+'
+
 test_done