Re: [PATCH 1/2] commit-graph, fuzz: Add fuzzer for commit-graph

[Junio C Hamano <gitster@pobox.com>]

Josh Steadmon <steadmon@google.com> writes:

> @@ -108,27 +106,61 @@ 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);

OK, assuming that the new helper returns NULL from all places in the
original that would have jumped to the cleanup-fail label, this would
do the same thing (it may not be the right thing to exit, but that
is OK for the purpose of this change).

> +	if (ret == NULL) {
> +		munmap(graph_map, graph_size);
> +		close(fd);
> +		exit(1);
> +	}
> +
> +	return ret;
> +}
> +
> +/*
> + * This function is intended to be used only from load_commit_graph_one() or in
> + * fuzz tests.
> + */

Hmph, is that necessary to say?  

"Right now, it has only these two callers" is sometimes handy for
those without good static analysis tools, like "git grep" ;-), but I
do not think of a reason why a new caller we'll add in the future,
who happens to have the data of the graph file in-core, should not
be allowed to call this function.


> +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;
> +
> +	/*
> +	 * This should already be checked in load_commit_graph_one, but we still
> +	 * need a check here for when we're calling parse_commit_graph directly
> +	 * from fuzz tests. We can omit the error message in that case.
> +	 */

In the same spirit, I am dubious of the longer-term value of this
comment.  As an explanation of why this conversion is correct in the
proposed log message for this change, it perfectly makes sense,
though.

> +	if (graph_size < GRAPH_MIN_SIZE)
> +		return NULL;
> +

The load_commit_graph_one() grabbed graph_map out of xmmap() so it
is guaranteed to be non-NULL, but we need to check graph_map != NULL
when we're calling this directly from the fuzz tests, exactly in the
same spirit that we check graph_size above.  Besides, these are to
make sure future callers won't misuse the API.

>  	data = (const unsigned char *)graph_map;

And the reset of the function is the same as the original modulo
jumping to the cleanup_fail label has been replaced with returning
NULL.

Looks good.

> ...
> -
> -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/fuzz-commit-graph.c b/fuzz-commit-graph.c
> new file mode 100644
> index 0000000000..420851d0d2
> --- /dev/null
> +++ b/fuzz-commit-graph.c
> @@ -0,0 +1,18 @@
> +#include "object-store.h"
> +#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);
> +	if (g)
> +		free(g);
> +
> +	return 0;
> +}