Re: [PATCH v5 1/3] commit-graph, fuzz: Add fuzzer for commit-graph

> 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>
> ---
>  .gitignore          |  1 +
>  Makefile            |  1 +
>  commit-graph.c      | 53 ++++++++++++++++++++++++++++++---------------
>  commit-graph.h      |  3 +++
>  fuzz-commit-graph.c | 16 ++++++++++++++
>  5 files changed, 57 insertions(+), 17 deletions(-)
>  create mode 100644 fuzz-commit-graph.c
>
> diff --git a/.gitignore b/.gitignore
> index 0d77ea5894..8bcf153ed9 100644
> --- a/.gitignore
> +++ b/.gitignore
> @@ -1,3 +1,4 @@
> +/fuzz-commit-graph
>  /fuzz_corpora
>  /fuzz-pack-headers
>  /fuzz-pack-idx
> diff --git a/Makefile b/Makefile
> index 1a44c811aa..6b72f37c29 100644
> --- a/Makefile
> +++ b/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
>  
> diff --git a/commit-graph.c b/commit-graph.c
> index 40c855f185..07dd410f3c 100644
> --- a/commit-graph.c
> +++ b/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)
> diff --git a/commit-graph.h b/commit-graph.h
> index 9db40b4d3a..813e7c19f1 100644
> --- a/commit-graph.h
> +++ b/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.
> diff --git a/fuzz-commit-graph.c b/fuzz-commit-graph.c
> new file mode 100644
> index 0000000000..cf790c9d04
> --- /dev/null
> +++ b/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;
> +}