commit-graph, fuzz: add fuzzer for commit-graph

Break load_commit_graph_one() into a new function, parse_commit_graph().
The latter function operates on arbitrary buffers, which makes it
suitable as a fuzzing target. Since parse_commit_graph() is only called
by load_commit_graph_one() (and the fuzzer described below), we omit
error messages that would be duplicated by the caller.

Adds fuzz-commit-graph.c, which provides a fuzzing entry point
compatible with libFuzzer (and possibly other fuzzing engines).

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.gitignore

@@ -1,3 +1,4 @@
+/fuzz-commit-graph
 /fuzz_corpora
 /fuzz-pack-headers
 /fuzz-pack-idx

Makefile

@@ -684,6 +684,7 @@ SCRIPTS = $(SCRIPT_SH_INS) \
 
 ETAGS_TARGET = TAGS
 
+FUZZ_OBJS += fuzz-commit-graph.o
 FUZZ_OBJS += fuzz-pack-headers.o
 FUZZ_OBJS += fuzz-pack-idx.o
 

commit-graph.c

@@ -84,16 +84,10 @@ static int commit_graph_compatible(struct repository *r)
 struct commit_graph *load_commit_graph_one(const char *graph_file)
 {
 	void *graph_map;
-	const unsigned char *data, *chunk_lookup;
 	size_t graph_size;
 	struct stat st;
-	uint32_t i;
-	struct commit_graph *graph;
+	struct commit_graph *ret;
 	int fd = git_open(graph_file);
-	uint64_t last_chunk_offset;
-	uint32_t last_chunk_id;
-	uint32_t graph_signature;
-	unsigned char graph_version, hash_version;
 
 	if (fd < 0)
 		return NULL;
@@ -108,27 +102,55 @@ struct commit_graph *load_commit_graph_one(const char *graph_file)
 		die(_("graph file %s is too small"), graph_file);
 	}
 	graph_map = xmmap(NULL, graph_size, PROT_READ, MAP_PRIVATE, fd, 0);
+	ret = parse_commit_graph(graph_map, fd, graph_size);
+
+	if (!ret) {
+		munmap(graph_map, graph_size);
+		close(fd);
+		exit(1);
+	}
+
+	return ret;
+}
+
+struct commit_graph *parse_commit_graph(void *graph_map, int fd,
+					size_t graph_size)
+{
+	const unsigned char *data, *chunk_lookup;
+	uint32_t i;
+	struct commit_graph *graph;
+	uint64_t last_chunk_offset;
+	uint32_t last_chunk_id;
+	uint32_t graph_signature;
+	unsigned char graph_version, hash_version;
+
+	if (!graph_map)
+		return NULL;
+
+	if (graph_size < GRAPH_MIN_SIZE)
+		return NULL;
+
 	data = (const unsigned char *)graph_map;
 
 	graph_signature = get_be32(data);
 	if (graph_signature != GRAPH_SIGNATURE) {
 		error(_("graph signature %X does not match signature %X"),
 		      graph_signature, GRAPH_SIGNATURE);
-		goto cleanup_fail;
+		return NULL;
 	}
 
 	graph_version = *(unsigned char*)(data + 4);
 	if (graph_version != GRAPH_VERSION) {
 		error(_("graph version %X does not match version %X"),
 		      graph_version, GRAPH_VERSION);
-		goto cleanup_fail;
+		return NULL;
 	}
 
 	hash_version = *(unsigned char*)(data + 5);
 	if (hash_version != GRAPH_OID_VERSION) {
 		error(_("hash version %X does not match version %X"),
 		      hash_version, GRAPH_OID_VERSION);
-		goto cleanup_fail;
+		return NULL;
 	}
 
 	graph = alloc_commit_graph();
@@ -152,7 +174,8 @@ struct commit_graph *load_commit_graph_one(const char *graph_file)
 		if (chunk_offset > graph_size - GIT_MAX_RAWSZ) {
 			error(_("improper chunk offset %08x%08x"), (uint32_t)(chunk_offset >> 32),
 			      (uint32_t)chunk_offset);
-			goto cleanup_fail;
+			free(graph);
+			return NULL;
 		}
 
 		switch (chunk_id) {
@@ -187,7 +210,8 @@ struct commit_graph *load_commit_graph_one(const char *graph_file)
 
 		if (chunk_repeated) {
 			error(_("chunk id %08x appears multiple times"), chunk_id);
-			goto cleanup_fail;
+			free(graph);
+			return NULL;
 		}
 
 		if (last_chunk_id == GRAPH_CHUNKID_OIDLOOKUP)
@@ -201,11 +225,6 @@ struct commit_graph *load_commit_graph_one(const char *graph_file)
 	}
 
 	return graph;
-
-cleanup_fail:
-	munmap(graph_map, graph_size);
-	close(fd);
-	exit(1);
 }
 
 static void prepare_commit_graph_one(struct repository *r, const char *obj_dir)

commit-graph.h

@@ -54,6 +54,9 @@ struct commit_graph {
 
 struct commit_graph *load_commit_graph_one(const char *graph_file);
 
+struct commit_graph *parse_commit_graph(void *graph_map, int fd,
+					size_t graph_size);
+
 /*
  * Return 1 if and only if the repository has a commit-graph
  * file and generation numbers are computed in that file.

fuzz-commit-graph.c

@@ -0,0 +1,16 @@
+#include "commit-graph.h"
+
+struct commit_graph *parse_commit_graph(void *graph_map, int fd,
+					size_t graph_size);
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size);
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
+{
+	struct commit_graph *g;
+
+	g = parse_commit_graph((void *)data, -1, size);
+	free(g);
+
+	return 0;
+}