Introduce more randomness

- Use C++ random generator instead of old and broken rand()
- Randomize string length to piss off stupid detectors

native/jni/core/socket.cpp

@@ -22,7 +22,7 @@ socklen_t setup_sockaddr(struct sockaddr_un *sun, const char *name) {
 int create_rand_socket(struct sockaddr_un *sun) {
 	memset(sun, 0, sizeof(*sun));
 	sun->sun_family = AF_LOCAL;
-	gen_rand_str(sun->sun_path + 1, 9);
+	gen_rand_str(sun->sun_path + 1, sizeof(sun->sun_path) - 1);
 	int fd = xsocket(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0);
 	xbind(fd, (struct sockaddr*) sun, ABS_SOCKET_LEN(sun));
 	xlisten(fd, 1);

native/jni/init/rootdir.cpp

@@ -19,15 +19,13 @@
 using namespace std;
 
 static void patch_socket_name(const char *path) {
-	uint8_t *buf;
-	char name[sizeof(MAIN_SOCKET)];
+	char *buf;
 	size_t size;
 	mmap_rw(path, buf, size);
 	for (int i = 0; i < size; ++i) {
 		if (memcmp(buf + i, MAIN_SOCKET, sizeof(MAIN_SOCKET)) == 0) {
-			gen_rand_str(name, sizeof(name));
-			memcpy(buf + i, name, sizeof(name));
-			i += sizeof(name);
+			gen_rand_str(buf + i, sizeof(MAIN_SOCKET));
+			i += sizeof(MAIN_SOCKET);
 		}
 	}
 	munmap(buf, size);
@@ -50,14 +48,10 @@ static void patch_init_rc(FILE *rc) {
 		fprintf(rc, "%s", line.data());
 		return true;
 	});
-	char pfd_svc[8], ls_svc[8], bc_svc[8];
-	// Make sure to be unique
-	pfd_svc[0] = 'a';
-	ls_svc[0] = '0';
-	bc_svc[0] = 'A';
-	gen_rand_str(pfd_svc + 1, sizeof(pfd_svc) - 1);
-	gen_rand_str(ls_svc + 1, sizeof(ls_svc) - 1);
-	gen_rand_str(bc_svc + 1, sizeof(bc_svc) - 1);
+	char pfd_svc[32], ls_svc[32], bc_svc[32];
+	gen_rand_str(pfd_svc, sizeof(pfd_svc));
+	gen_rand_str(ls_svc, sizeof(ls_svc));
+	gen_rand_str(bc_svc, sizeof(bc_svc));
 	LOGD("Inject magisk services: [%s] [%s] [%s]\n", pfd_svc, ls_svc, bc_svc);
 	fprintf(rc, magiskrc, pfd_svc, pfd_svc, ls_svc, bc_svc, bc_svc);
 }

native/jni/utils/include/misc.h

@@ -10,7 +10,6 @@ extern "C" {
 unsigned get_shell_uid();
 int fork_dont_care();
 int fork_no_zombie();
-void gen_rand_str(char *buf, int len);
 int strend(const char *s1, const char *s2);
 char *rtrim(char *str);
 void init_argv0(int argc, char **argv);
@@ -24,6 +23,8 @@ int parse_int(const char *s);
 #include <functional>
 #include <string_view>
 
+void gen_rand_str(char *buf, int len, bool varlen = true);
+
 #define str_contains(s, ss) ((ss) != nullptr && (s).find(ss) != std::string::npos)
 #define str_starts(s, ss) ((ss) != nullptr && (s).compare(0, strlen(ss), ss) == 0)
 

native/jni/utils/misc.cpp

@@ -1,6 +1,10 @@
 /* misc.cpp - Store all functions that are unable to be catagorized clearly
  */
 
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <sys/prctl.h>
+#include <sys/sysmacros.h>
 #include <string.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -8,10 +12,7 @@
 #include <pwd.h>
 #include <unistd.h>
 #include <syscall.h>
-#include <sys/types.h>
-#include <sys/wait.h>
-#include <sys/prctl.h>
-#include <sys/sysmacros.h>
+#include <random>
 
 #include <logging.h>
 #include <utils.h>
@@ -49,17 +50,27 @@ int fork_no_zombie() {
 	return 0;
 }
 
-static bool rand_init = false;
-
-void gen_rand_str(char *buf, int len) {
-	constexpr const char base[] = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
-	if (!rand_init) {
-		srand(time(nullptr));
-		rand_init = true;
+constexpr char ALPHANUM[] = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
+static bool seeded = false;
+static std::mt19937 gen;
+static std::uniform_int_distribution<int> dist(0, sizeof(ALPHANUM) - 1);
+void gen_rand_str(char *buf, int len, bool varlen) {
+	if (!seeded) {
+		if (access("/dev/urandom", F_OK) == 0) {
+			std::random_device rdev;
+			gen.seed(rdev());
+		} else {
+			// In magiskinit
+			gen.seed(time(nullptr));
 		}
-	for (int i = 0; i < len - 1; ++i) {
-		buf[i] = base[rand() % (sizeof(base) - 1)];
+		seeded = true;
 	}
+	if (varlen) {
+		std::uniform_int_distribution<int> len_dist(len / 2, len);
+		len = len_dist(gen);
+	}
+	for (int i = 0; i < len - 1; ++i)
+		buf[i] = ALPHANUM[dist(gen)];
 	buf[len - 1] = '\0';
 }