Detect volume down key combo for safe mode

It is possible that a module is breaking the device so bad that zygote
cannot even be started. In this case, system_server cannot start and
detect the safe mode key combo, set the persist property, and reboot.

Also on old Android versions, the system directly goes to safe mode
after detecting a key combo without rebooting, defeating the purpose of
Magisk's safe mode protection if we only check for the persist property.

Directly adding key combo check natively in magiskd allows us to enter
Magisk safe mode before the system is even aware of it.

native/jni/core/bootstages.cpp

@@ -1,11 +1,7 @@
 #include <sys/mount.h>
 #include <sys/wait.h>
-#include <stdlib.h>
-#include <stdio.h>
-#include <unistd.h>
-#include <fcntl.h>
-#include <string.h>
-#include <dirent.h>
+#include <sys/sysmacros.h>
+#include <linux/input.h>
 #include <libgen.h>
 #include <vector>
 #include <string>
@@ -220,6 +216,53 @@ static void collect_logs(bool reset) {
 	});
 }
 
+#define test_bit(bit, array) (array[bit / 8] & (1 << (bit % 8)))
+
+static bool check_key_combo() {
+	uint8_t bitmask[(KEY_MAX + 1) / 8];
+	vector<int> events;
+	constexpr char name[] = "/dev/.ev";
+
+	// First collect candidate events that accepts volume down
+	for (int minor = 64; minor < 96; ++minor) {
+		if (xmknod(name, S_IFCHR | 0444, makedev(13, minor)))
+			continue;
+		int fd = open(name, O_RDONLY | O_CLOEXEC);
+		unlink(name);
+		if (fd < 0)
+			continue;
+		memset(bitmask, 0, sizeof(bitmask));
+		ioctl(fd, EVIOCGBIT(EV_KEY, sizeof(bitmask)), bitmask);
+		if (test_bit(KEY_VOLUMEDOWN, bitmask))
+			events.push_back(fd);
+		else
+			close(fd);
+	}
+	if (events.empty())
+		return false;
+
+	run_finally fin([&]{ std::for_each(events.begin(), events.end(), close); });
+
+	// Check if volume down key is held continuously for more than 3 seconds
+	for (int i = 0; i < 300; ++i) {
+		bool pressed = false;
+		for (const int &fd : events) {
+			memset(bitmask, 0, sizeof(bitmask));
+			ioctl(fd, EVIOCGKEY(sizeof(bitmask)), bitmask);
+			if (test_bit(KEY_VOLUMEDOWN, bitmask)) {
+				pressed = true;
+				break;
+			}
+		}
+		if (!pressed)
+			return false;
+		// Check every 10ms
+		usleep(10000);
+	}
+	LOGD("KEY_VOLUMEDOWN detected: enter safe mode\n");
+	return true;
+}
+
 /****************
  * Entry points *
  ****************/
@@ -260,7 +303,7 @@ void post_fs_data(int client) {
 		goto unblock_init;
 	}
 
-	if (getprop("persist.sys.safemode", true) == "1") {
+	if (getprop("persist.sys.safemode", true) == "1" || check_key_combo()) {
 		safe_mode = true;
 		// Disable all modules and magiskhide so next boot will be clean
 		foreach_modules("disable");

native/jni/init/getinfo.cpp

@@ -48,10 +48,8 @@ static bool check_key_combo() {
 	constexpr const char *name = "/event";
 
 	for (int minor = 64; minor < 96; ++minor) {
-		if (mknod(name, S_IFCHR | 0444, makedev(13, minor))) {
-			PLOGE("mknod");
+		if (xmknod(name, S_IFCHR | 0444, makedev(13, minor)))
 			continue;
-		}
 		int fd = open(name, O_RDONLY | O_CLOEXEC);
 		unlink(name);
 		if (fd < 0)
@@ -60,17 +58,15 @@ static bool check_key_combo() {
 		ioctl(fd, EVIOCGBIT(EV_KEY, sizeof(bitmask)), bitmask);
 		if (test_bit(KEY_VOLUMEUP, bitmask))
 			events.push_back(fd);
+		else
+			close(fd);
 	}
-
 	if (events.empty())
 		return false;
 
-	run_finally fin([&]() -> void {
-		for (const int &fd : events)
-			close(fd);
-	});
+	run_finally fin([&]{ std::for_each(events.begin(), events.end(), close); });
 
-	// Return true if volume key up is hold for more than 3 seconds
+	// Return true if volume up key is held for more than 3 seconds
 	int count = 0;
 	for (int i = 0; i < 500; ++i) {
 		for (const int &fd : events) {