Magisk/native/jni/init/twostage.cpp

#include <sys/mount.h>

#include <magisk.hpp>
#include <utils.hpp>
#include <socket.hpp>

#include "init.hpp"

using namespace std;

void fstab_entry::to_file(FILE *fp) {
    fprintf(fp, "%s %s %s %s %s\n", dev.data(), mnt_point.data(),
            type.data(), mnt_flags.data(), fsmgr_flags.data());
}

#define set_info(val) \
line[val##1] = '\0'; \
entry.val = &line[val##0];

#define FSR "/first_stage_ramdisk"

extern uint32_t patch_verity(void *buf, uint32_t size);

void FirstStageInit::prepare() {
    if (cmd->force_normal_boot) {
        xmkdirs(FSR "/system/bin", 0755);
        rename("/init" /* magiskinit */, FSR "/system/bin/init");
        symlink("/system/bin/init", FSR "/init");
        rename("/.backup", FSR "/.backup");
        rename("/overlay.d", FSR "/overlay.d");
        xsymlink("/system/bin/init", "/init");

        chdir(FSR);
    } else {
        xmkdir("/system", 0755);
        xmkdir("/system/bin", 0755);
        rename("/init" /* magiskinit */ , "/system/bin/init");
        rename("/.backup/init", "/init");
    }

    char fstab_file[128];
    fstab_file[0] = '\0';

    // Find existing fstab file
    for (const char *suffix : { cmd->fstab_suffix, cmd->hardware, cmd->hardware_plat }) {
        if (suffix[0] == '\0')
            continue;
        for (const char *prefix: { "odm/etc/fstab", "vendor/etc/fstab", "fstab" }) {
            sprintf(fstab_file, "%s.%s", prefix, suffix);
            if (access(fstab_file, F_OK) != 0) {
                fstab_file[0] = '\0';
            } else {
                LOGD("Found fstab file: %s\n", fstab_file);
                goto exit_loop;
            }
        }
    }
exit_loop:

    // Try to load dt fstab
    vector<fstab_entry> fstab;
    read_dt_fstab(fstab);

    if (!fstab.empty()) {
        // Dump dt fstab to fstab file in rootfs and force init to use it instead

        // All dt fstab entries should be first_stage_mount
        for (auto &entry : fstab) {
            if (!str_contains(entry.fsmgr_flags, "first_stage_mount")) {
                if (!entry.fsmgr_flags.empty())
                    entry.fsmgr_flags += ',';
                entry.fsmgr_flags += "first_stage_mount";
            }
        }

        if (fstab_file[0] == '\0') {
            const char *suffix =
                    cmd->fstab_suffix[0] ? cmd->fstab_suffix :
                    (cmd->hardware[0] ? cmd->hardware :
                    (cmd->hardware_plat[0] ? cmd->hardware_plat : nullptr));
            if (suffix == nullptr) {
                LOGE("Cannot determine fstab suffix!\n");
                return;
            }
            sprintf(fstab_file, "fstab.%s", suffix);
        }

        // Patch init to force IsDtFstabCompatible() return false
        auto init = mmap_data::rw("/init");
        init.patch({ make_pair("android,fstab", "xxx") });
    } else {
        // Parse and load the fstab file
        file_readline(fstab_file, [&](string_view l) -> bool {
            if (l[0] == '#' || l.length() == 1)
                return true;
            char *line = (char *) l.data();

            int dev0, dev1, mnt_point0, mnt_point1, type0, type1,
                    mnt_flags0, mnt_flags1, fsmgr_flags0, fsmgr_flags1;

            sscanf(line, "%n%*s%n %n%*s%n %n%*s%n %n%*s%n %n%*s%n",
                   &dev0, &dev1, &mnt_point0, &mnt_point1, &type0, &type1,
                   &mnt_flags0, &mnt_flags1, &fsmgr_flags0, &fsmgr_flags1);

            fstab_entry entry;

            set_info(dev);
            set_info(mnt_point);
            set_info(type);
            set_info(mnt_flags);
            set_info(fsmgr_flags);

            fstab.emplace_back(std::move(entry));
            return true;
        });
    }

    {
        LOGD("Write fstab file: %s\n", fstab_file);
        auto fp = xopen_file(fstab_file, "we");
        for (auto &entry : fstab) {
            // Redirect system mnt_point so init won't switch root in first stage init
            if (entry.mnt_point == "/system")
                entry.mnt_point = "/system_root";

            // Force remove AVB for 2SI since it may bootloop some devices
            auto len = patch_verity(entry.fsmgr_flags.data(), entry.fsmgr_flags.length());
            entry.fsmgr_flags.resize(len);

            entry.to_file(fp.get());
        }
    }
    chmod(fstab_file, 0644);

    chdir("/");
}

#define INIT_PATH  "/system/bin/init"
#define REDIR_PATH "/system/bin/am"

void SARInit::first_stage_prep() {
    xmount("tmpfs", "/dev", "tmpfs", 0, "mode=755");

    // Patch init binary
    int src = xopen("/init", O_RDONLY);
    int dest = xopen("/dev/init", O_CREAT | O_WRONLY, 0);
    {
        auto init = mmap_data::ro("/init");
        init.patch({ make_pair(INIT_PATH, REDIR_PATH) });
        write(dest, init.buf, init.sz);
        fclone_attr(src, dest);
        close(dest);
    }

    // Replace redirect init with magiskinit
    dest = xopen("/dev/magiskinit", O_CREAT | O_WRONLY, 0);
    write(dest, self.buf, self.sz);
    fclone_attr(src, dest);
    close(src);
    close(dest);

    xmount("/dev/init", "/init", nullptr, MS_BIND, nullptr);
    xmount("/dev/magiskinit", REDIR_PATH, nullptr, MS_BIND, nullptr);
    xumount2("/dev", MNT_DETACH);

    // Block SIGUSR1
    sigset_t block, old;
    sigemptyset(&block);
    sigaddset(&block, SIGUSR1);
    sigprocmask(SIG_BLOCK, &block, &old);

    if (int child = xfork()) {
        LOGD("init daemon [%d]\n", child);
        // Wait for children signal
        int sig;
        sigwait(&block, &sig);

        // Restore sigmask
        sigprocmask(SIG_SETMASK, &old, nullptr);
    } else {
        // Establish socket for 2nd stage ack
        struct sockaddr_un sun;
        int sockfd = xsocket(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0);
        xbind(sockfd, (struct sockaddr*) &sun, setup_sockaddr(&sun, INIT_SOCKET));
        xlisten(sockfd, 1);

        // Resume parent
        kill(getppid(), SIGUSR1);

        // Wait for second stage ack
        int client = xaccept4(sockfd, nullptr, nullptr, SOCK_CLOEXEC);

        // Write backup files
        string tmp_dir = read_string(client);
        chdir(tmp_dir.data());
        int cfg = xopen(INTLROOT "/config", O_WRONLY | O_CREAT, 0);
        xwrite(cfg, config.buf, config.sz);
        close(cfg);
        restore_folder(ROOTOVL, overlays);

        // Ack and bail out!
        write_int(sockfd, 0);
        close(client);
        close(sockfd);

        exit(0);
    }
}