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>

Makefile

@@ -757,6 +757,7 @@ ETAGS_TARGET = TAGS
 # runs in the future.
 FUZZ_OBJS += oss-fuzz/dummy-cmd-main.o
 FUZZ_OBJS += oss-fuzz/fuzz-commit-graph.o
+FUZZ_OBJS += oss-fuzz/fuzz-config.o
 FUZZ_OBJS += oss-fuzz/fuzz-date.o
 FUZZ_OBJS += oss-fuzz/fuzz-pack-headers.o
 FUZZ_OBJS += oss-fuzz/fuzz-pack-idx.o

ci/run-build-and-minimal-fuzzers.sh

@@ -12,7 +12,7 @@ group "Build fuzzers" make \
 	LIB_FUZZING_ENGINE="-fsanitize=fuzzer,address" \
 	fuzz-all
 
-for fuzzer in commit-graph date pack-headers pack-idx ; do
+for fuzzer in commit-graph config date pack-headers pack-idx ; do
 	begin_group "fuzz-$fuzzer"
 	./oss-fuzz/fuzz-$fuzzer -verbosity=0 -runs=1 || exit 1
 	end_group "fuzz-$fuzzer"

oss-fuzz/.gitignore

@@ -1,4 +1,5 @@
 fuzz-commit-graph
+fuzz-config
 fuzz-date
 fuzz-pack-headers
 fuzz-pack-idx

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;
+}