Magisk/native/jni/init/init.cpp

#include <sys/stat.h>
#include <sys/types.h>
#include <sys/mount.h>
#include <sys/sysmacros.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <dirent.h>
#include <fcntl.h>
#include <libgen.h>

#include <xz.h>
#include <magisk.h>
#include <selinux.h>
#include <cpio.h>
#include <utils.h>
#include <flags.h>

#include "binaries.h"
#ifdef USE_64BIT
#include "binaries_arch64.h"
#else
#include "binaries_arch.h"
#endif

#include "init.h"

using namespace std;

#ifdef MAGISK_DEBUG
static FILE *kmsg;
static int vprintk(const char *fmt, va_list ap) {
	fprintf(kmsg, "magiskinit: ");
	return vfprintf(kmsg, fmt, ap);
}

static void setup_klog() {
	mknod("/kmsg", S_IFCHR | 0666, makedev(1, 11));
	int fd = xopen("/kmsg", O_WRONLY | O_CLOEXEC);
	kmsg = fdopen(fd, "w");
	setbuf(kmsg, nullptr);
	unlink("/kmsg");
	log_cb.d = log_cb.i = log_cb.w = log_cb.e = vprintk;
	log_cb.ex = nop_ex;
}
#else
#define setup_klog(...)
#endif

static int test_main(int argc, char *argv[]);

constexpr const char *init_applet[] =
		{ "magiskpolicy", "supolicy", "init_test", nullptr };
constexpr int (*init_applet_main[])(int, char *[]) =
		{ magiskpolicy_main, magiskpolicy_main, test_main, nullptr };

static bool unxz(int fd, const uint8_t *buf, size_t size) {
	uint8_t out[8192];
	xz_crc32_init();
	struct xz_dec *dec = xz_dec_init(XZ_DYNALLOC, 1 << 26);
	struct xz_buf b = {
			.in = buf,
			.in_pos = 0,
			.in_size = size,
			.out = out,
			.out_pos = 0,
			.out_size = sizeof(out)
	};
	enum xz_ret ret;
	do {
		ret = xz_dec_run(dec, &b);
		if (ret != XZ_OK && ret != XZ_STREAM_END)
			return false;
		write(fd, out, b.out_pos);
		b.out_pos = 0;
	} while (b.in_pos != size);
	return true;
}

static void decompress_ramdisk() {
	constexpr char tmp[] = "tmp.cpio";
	constexpr char ramdisk_xz[] = "ramdisk.cpio.xz";
	if (access(ramdisk_xz, F_OK))
		return;
	LOGD("Decompressing ramdisk from %s\n", ramdisk_xz);
	uint8_t *buf;
	size_t sz;
	mmap_ro(ramdisk_xz, buf, sz);
	int fd = open(tmp, O_WRONLY | O_CREAT | O_TRUNC | O_CLOEXEC);
	unxz(fd, buf, sz);
	munmap(buf, sz);
	close(fd);
	cpio_mmap cpio(tmp);
	cpio.extract();
	unlink(tmp);
	unlink(ramdisk_xz);
}

int dump_magisk(const char *path, mode_t mode) {
	int fd = open(path, O_WRONLY | O_CREAT | O_TRUNC | O_CLOEXEC, mode);
	if (fd < 0)
		return 1;
	if (!unxz(fd, magisk_xz, sizeof(magisk_xz)))
		return 1;
	close(fd);
	return 0;
}

static int dump_manager(const char *path, mode_t mode) {
	int fd = open(path, O_WRONLY | O_CREAT | O_TRUNC | O_CLOEXEC, mode);
	if (fd < 0)
		return 1;
	if (!unxz(fd, manager_xz, sizeof(manager_xz)))
		return 1;
	close(fd);
	return 0;
}

void MagiskInit::preset() {
	root = open("/", O_RDONLY | O_CLOEXEC);

	if (cmd->system_as_root) {
		// Clear rootfs
		LOGD("Cleaning rootfs\n");
		frm_rf(root, { "overlay", "proc", "sys" });
	} else {
		decompress_ramdisk();

		// Revert original init binary
		rename("/.backup/init", "/init");
		rm_rf("/.backup");

		// Do not go further if device is booting into recovery
		if (access("/sbin/recovery", F_OK) == 0) {
			LOGD("Ramdisk is recovery, abort\n");
			re_exec_init();
		}
	}
}

#define umount_root(name) \
if (mnt_##name) \
	umount("/" #name);

void MagiskInit::cleanup() {
	umount(SELINUX_MNT);
	umount("/sys");
	umount("/proc");
	umount_root(system);
	umount_root(vendor);
	umount_root(product);
	umount_root(odm);
}

void MagiskInit::re_exec_init() {
	LOGD("Re-exec /init\n");
	cleanup();
	execv("/init", argv);
	exit(1);
}

void MagiskInit::start() {
	// Prevent file descriptor confusion
	mknod("/null", S_IFCHR | 0666, makedev(1, 3));
	int null = open("/null", O_RDWR | O_CLOEXEC);
	unlink("/null");
	xdup3(null, STDIN_FILENO, O_CLOEXEC);
	xdup3(null, STDOUT_FILENO, O_CLOEXEC);
	xdup3(null, STDERR_FILENO, O_CLOEXEC);
	if (null > STDERR_FILENO)
		close(null);

	full_read("/init", &self.buf, &self.sz);
	full_read("/.backup/.magisk", &config.buf, &config.sz);

	preset();
	early_mount();
	setup_rootfs();
	re_exec_init();
}

void MagiskInit::test() {
	cmdline_logging();
	log_cb.ex = nop_ex;

	chdir(dirname(argv[0]));
	chroot(".");
	chdir("/");

	preset();
	early_mount();
	setup_rootfs();
	cleanup();
}

static int test_main(int, char *argv[]) {
	cmdline cmd{};
	load_kernel_info(&cmd);
	MagiskInit init(argv, &cmd);
	init.test();
	return 0;
}

int main(int argc, char *argv[]) {
	umask(0);

	for (int i = 0; init_applet[i]; ++i) {
		if (strcmp(basename(argv[0]), init_applet[i]) == 0)
			return (*init_applet_main[i])(argc, argv);
	}

	if (argc > 1 && strcmp(argv[1], "-x") == 0) {
		if (strcmp(argv[2], "magisk") == 0)
			return dump_magisk(argv[3], 0755);
		else if (strcmp(argv[2], "manager") == 0)
			return dump_manager(argv[3], 0644);
	}

	if (getpid() != 1)
		return 1;

	setup_klog();

	cmdline cmd{};
	load_kernel_info(&cmd);

	MagiskInit init(argv, &cmd);

	// Run the main routine
	init.start();
}