fuzz: add fuzzer for config parsing

Add a new fuzz target that exercises the parsing of git configs.
The existing git_config_from_mem function is a perfect entry point
for fuzzing as it exercises the same code paths as the rest of the
config parsing functions and offers an easily fuzzable interface.

Config parsing is a useful thing to fuzz because it operates on user
controlled data and is a central component of many git operations.

Signed-off-by: Brian C Tracy <brian.tracy33@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

oss-fuzz/fuzz-config.c

@@ -0,0 +1,33 @@
+#include "git-compat-util.h"
+#include "config.h"
+
+int LLVMFuzzerTestOneInput(const uint8_t *, size_t);
+static int config_parser_callback(const char *, const char *,
+					const struct config_context *, void *);
+
+static int config_parser_callback(const char *key, const char *value,
+					const struct config_context *ctx UNUSED,
+					void *data UNUSED)
+{
+	/*
+	 * Visit every byte of memory we are given to make sure the parser
+	 * gave it to us appropriately. We need to unconditionally return 0,
+	 * but we also want to prevent the strlen from being optimized away.
+	 */
+	size_t c = strlen(key);
+
+	if (value)
+		c += strlen(value);
+	return c == SIZE_MAX;
+}
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, const size_t size)
+{
+	struct config_options config_opts = { 0 };
+
+	config_opts.error_action = CONFIG_ERROR_SILENT;
+	git_config_from_mem(config_parser_callback, CONFIG_ORIGIN_BLOB,
+				"fuzztest-config", (const char *)data, size, NULL,
+				CONFIG_SCOPE_UNKNOWN, &config_opts);
+	return 0;
+}