Avoid large eval inputs in fuzzer

While we limit the size of the main compilation input, the size
of eval inputs was not limited. This could result in stack
overflows, e.g. oss-fuzz #25464.

sapi/fuzzer/fuzzer-execute.c

@@ -20,13 +20,14 @@
 #include "fuzzer-sapi.h"
 
 #define MAX_STEPS 1000
+#define MAX_SIZE (16 * 1024)
 static uint32_t steps_left;
 
 /* Because the fuzzer is always compiled with clang,
  * we can assume that we don't use global registers / hybrid VM. */
 typedef int (ZEND_FASTCALL *opcode_handler_t)(zend_execute_data *);
 
-void fuzzer_execute_ex(zend_execute_data *execute_data) {
+static void fuzzer_execute_ex(zend_execute_data *execute_data) {
 	while (1) {
 		int ret;
 		if (--steps_left == 0) {
@@ -46,8 +47,19 @@ void fuzzer_execute_ex(zend_execute_data *execute_data) {
 	}
 }
 
+static zend_op_array *(*orig_compile_string)(zend_string *source_string, const char *filename);
+
+static zend_op_array *fuzzer_compile_string(zend_string *str, const char *filename) {
+	if (ZSTR_LEN(str) > MAX_SIZE) {
+		/* Avoid compiling huge inputs via eval(). */
+		zend_bailout();
+	}
+
+	return orig_compile_string(str, filename);
+}
+
 int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
-	if (Size > 16 * 1024) {
+	if (Size > MAX_SIZE) {
 		/* Large inputs have a large impact on fuzzer performance,
 		 * but are unlikely to be necessary to reach new codepaths. */
 		return 0;
@@ -68,7 +80,10 @@ int LLVMFuzzerInitialize(int *argc, char ***argv) {
 	signal(SIGPIPE, SIG_IGN);
 
 	fuzzer_init_php();
+
 	zend_execute_ex = fuzzer_execute_ex;
+	orig_compile_string = zend_compile_string;
+	zend_compile_string = fuzzer_compile_string;
 
 	/* fuzzer_shutdown_php(); */
 	return 0;