first_stage_init: add console

Start and wait on a console if androidboot.first_stage_console=1 is
present on the kernel command line. This only works on eng and
userdebug builds.

Change-Id: I978e9390a89509431b399ea58b284736b27eeb1b

init/Android.bp

@@ -26,6 +26,7 @@ cc_defaults {
         "-Wextra",
         "-Wno-unused-parameter",
         "-Werror",
+        "-DALLOW_FIRST_STAGE_CONSOLE=0",
         "-DALLOW_LOCAL_PROP_OVERRIDE=0",
         "-DALLOW_PERMISSIVE_SELINUX=0",
         "-DREBOOT_BOOTLOADER_ON_PANIC=0",
@@ -36,6 +37,8 @@ cc_defaults {
     product_variables: {
         debuggable: {
             cppflags: [
+                "-UALLOW_FIRST_STAGE_CONSOLE",
+                "-DALLOW_FIRST_STAGE_CONSOLE=1",
                 "-UALLOW_LOCAL_PROP_OVERRIDE",
                 "-DALLOW_LOCAL_PROP_OVERRIDE=1",
                 "-UALLOW_PERMISSIVE_SELINUX",

init/Android.mk

@@ -8,6 +8,7 @@ LOCAL_PATH:= $(call my-dir)
 
 ifneq (,$(filter userdebug eng,$(TARGET_BUILD_VARIANT)))
 init_options += \
+    -DALLOW_FIRST_STAGE_CONSOLE=1 \
     -DALLOW_LOCAL_PROP_OVERRIDE=1 \
     -DALLOW_PERMISSIVE_SELINUX=1 \
     -DREBOOT_BOOTLOADER_ON_PANIC=1 \
@@ -15,6 +16,7 @@ init_options += \
     -DDUMP_ON_UMOUNT_FAILURE=1
 else
 init_options += \
+    -DALLOW_FIRST_STAGE_CONSOLE=0 \
     -DALLOW_LOCAL_PROP_OVERRIDE=0 \
     -DALLOW_PERMISSIVE_SELINUX=0 \
     -DREBOOT_BOOTLOADER_ON_PANIC=0 \

init/first_stage_init.cpp

@@ -24,10 +24,12 @@
 #include <sys/stat.h>
 #include <sys/sysmacros.h>
 #include <sys/types.h>
+#include <sys/wait.h>
 #include <unistd.h>
 
 #include <filesystem>
 #include <string>
+#include <thread>
 #include <vector>
 
 #include <android-base/chrono_utils.h>
@@ -92,9 +94,50 @@ void FreeRamdisk(DIR* dir, dev_t dev) {
     }
 }
 
-bool ForceNormalBoot() {
-    std::string cmdline;
-    android::base::ReadFileToString("/proc/cmdline", &cmdline);
+void StartConsole() {
+    if (mknod("/dev/console", S_IFCHR | 0600, makedev(5, 1))) {
+        PLOG(ERROR) << "unable to create /dev/console";
+        return;
+    }
+    pid_t pid = fork();
+    if (pid != 0) {
+        int status;
+        waitpid(pid, &status, 0);
+        LOG(ERROR) << "console shell exited with status " << status;
+        return;
+    }
+    int fd = -1;
+    int tries = 10;
+    // The device driver for console may not be ready yet so retry for a while in case of failure.
+    while (tries--) {
+        fd = open("/dev/console", O_RDWR);
+        if (fd != -1) {
+            break;
+        }
+        std::this_thread::sleep_for(100ms);
+    }
+    if (fd == -1) {
+        LOG(ERROR) << "Could not open /dev/console, errno = " << errno;
+        _exit(127);
+    }
+    ioctl(fd, TIOCSCTTY, 0);
+    dup2(fd, 0);
+    dup2(fd, 1);
+    dup2(fd, 2);
+    close(fd);
+
+    const char* path = "/system/bin/sh";
+    const char* args[] = {path, nullptr};
+    int rv = execv(path, const_cast<char**>(args));
+    LOG(ERROR) << "unable to execv, returned " << rv << " errno " << errno;
+    _exit(127);
+}
+
+bool FirstStageConsole(const std::string& cmdline) {
+    return cmdline.find("androidboot.first_stage_console=1") != std::string::npos;
+}
+
+bool ForceNormalBoot(const std::string& cmdline) {
     return cmdline.find("androidboot.force_normal_boot=1") != std::string::npos;
 }
 
@@ -127,6 +170,8 @@ int FirstStageMain(int argc, char** argv) {
 #undef MAKE_STR
     // Don't expose the raw commandline to unprivileged processes.
     CHECKCALL(chmod("/proc/cmdline", 0440));
+    std::string cmdline;
+    android::base::ReadFileToString("/proc/cmdline", &cmdline);
     gid_t groups[] = {AID_READPROC};
     CHECKCALL(setgroups(arraysize(groups), groups));
     CHECKCALL(mount("sysfs", "/sys", "sysfs", 0, NULL));
@@ -198,7 +243,11 @@ int FirstStageMain(int argc, char** argv) {
         LOG(FATAL) << "Failed to load kernel modules";
     }
 
-    if (ForceNormalBoot()) {
+    if (ALLOW_FIRST_STAGE_CONSOLE && FirstStageConsole(cmdline)) {
+        StartConsole();
+    }
+
+    if (ForceNormalBoot(cmdline)) {
         mkdir("/first_stage_ramdisk", 0755);
         // SwitchRoot() must be called with a mount point as the target, so we bind mount the
         // target directory to itself here.