[PATCH 15/20] commit-graph: check size of generations chunk

[Jeff King <peff@peff.net>]

We neither check nor record the size of the generations chunk we parse
from a commit-graph file. This should have one uint32_t for each commit
in the file; if it is smaller (due to corruption, etc), we may read
outside the mapped memory.

The included test segfaults without this patch, as it shrinks the size
considerably (and the chunk is near the end of the file, so we read off
the end of the array rather than accidentally reading another chunk).

We can fix this by checking the size up front (like we do for other
fixed-size chunks, like CDAT).

Signed-off-by: Jeff King <peff@peff.net>
---
 commit-graph.c          | 14 ++++++++++++--
 t/t5318-commit-graph.sh |  8 ++++++++
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/commit-graph.c b/commit-graph.c
index 4377b547c8..ca26870d1b 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -350,6 +350,16 @@ static int graph_read_commit_data(const unsigned char *chunk_start,
 	return 0;
 }
 
+static int graph_read_generation_data(const unsigned char *chunk_start,
+				      size_t chunk_size, void *data)
+{
+	struct commit_graph *g = data;
+	if (chunk_size != g->num_commits * sizeof(uint32_t))
+		return error("commit-graph generations chunk is wrong size");
+	g->chunk_generation_data = chunk_start;
+	return 0;
+}
+
 static int graph_read_bloom_data(const unsigned char *chunk_start,
 				  size_t chunk_size, void *data)
 {
@@ -439,8 +449,8 @@ struct commit_graph *parse_commit_graph(struct repo_settings *s,
 		   &graph->chunk_base_graphs_size);
 
 	if (s->commit_graph_generation_version >= 2) {
-		pair_chunk_unsafe(cf, GRAPH_CHUNKID_GENERATION_DATA,
-			&graph->chunk_generation_data);
+		read_chunk(cf, GRAPH_CHUNKID_GENERATION_DATA,
+			   graph_read_generation_data, graph);
 		pair_chunk_unsafe(cf, GRAPH_CHUNKID_GENERATION_DATA_OVERFLOW,
 			&graph->chunk_generation_data_overflow);
 
diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
index 05bafcfe5f..6505ff595a 100755
--- a/t/t5318-commit-graph.sh
+++ b/t/t5318-commit-graph.sh
@@ -887,4 +887,12 @@ test_expect_success 'reader notices out-of-bounds extra edge' '
 	test_cmp expect.err err
 '
 
+test_expect_success 'reader notices too-small generations chunk' '
+	check_corrupt_chunk GDA2 clear 00000000 &&
+	cat >expect.err <<-\EOF &&
+	error: commit-graph generations chunk is wrong size
+	EOF
+	test_cmp expect.err err
+'
+
 test_done
-- 
2.42.0.884.g35e1fe1a6a