Reland: "init: chroot from recovery to /first_stage_ramdisk"

When using the recovery image as a trampoline to boot the system,
first chroot from the recovery image to /first_stage_ramdisk, to
minimize differences between these two boot paths.

Primary motivation is due to the fact that the basename of each mount
point is used by device-manager to name its nodes, and the previous
code that created used /system_recovery_mount as the mount point for
system.img broke AVB.  Instead of hacking around that issue, this
change unified mounting for the recovery trampoline and true first
stage ramdisk paths.

Change when relanding: the original change skipped the move mount from
/first_stage_ramdisk to / and only did a chroot instead.  This was a
mistake that resulted in the subsequent move mount of /system to / to
mount over the '/' directory instead of moving that mount.  This
change uses a bind mount of /first_stage_ramdisk to itself instead of
skipping the first move mount.

Bug: 114062208
Test: AVB works with blueline_mainline
Change-Id: I65207edfe98531892da2eafcbff19b438c9c64fe

1 files changed, 3 insertions, 55 deletions

diff --git a/init/switch_root.cpp b/init/switch_root.cpp
index 0e59b576b..575b67f38 100644
--- a/init/switch_root.cpp
+++ b/init/switch_root.cpp
@@ -16,7 +16,6 @@
 
 #include "switch_root.h"
 
-#include <dirent.h>
 #include <fcntl.h>
 #include <mntent.h>
 #include <sys/mount.h>
@@ -35,45 +34,6 @@ namespace init {
 
 namespace {
 
-void FreeRamdisk(DIR* dir, dev_t dev) {
-    int dfd = dirfd(dir);
-
-    dirent* de;
-    while ((de = readdir(dir)) != nullptr) {
-        if (de->d_name == "."s || de->d_name == ".."s) {
-            continue;
-        }
-
-        bool is_dir = false;
-
-        if (de->d_type == DT_DIR || de->d_type == DT_UNKNOWN) {
-            struct stat info;
-            if (fstatat(dfd, de->d_name, &info, AT_SYMLINK_NOFOLLOW) != 0) {
-                continue;
-            }
-
-            if (info.st_dev != dev) {
-                continue;
-            }
-
-            if (S_ISDIR(info.st_mode)) {
-                is_dir = true;
-                auto fd = openat(dfd, de->d_name, O_RDONLY | O_DIRECTORY);
-                if (fd >= 0) {
-                    auto subdir =
-                            std::unique_ptr<DIR, decltype(&closedir)>{fdopendir(fd), closedir};
-                    if (subdir) {
-                        FreeRamdisk(subdir.get(), dev);
-                    } else {
-                        close(fd);
-                    }
-                }
-            }
-        }
-        unlinkat(dfd, de->d_name, is_dir ? AT_REMOVEDIR : 0);
-    }
-}
-
 std::vector<std::string> GetMounts(const std::string& new_root) {
     auto fp = std::unique_ptr<std::FILE, decltype(&endmntent)>{setmntent("/proc/mounts", "re"),
                                                                endmntent};
@@ -112,24 +72,16 @@ std::vector<std::string> GetMounts(const std::string& new_root) {
 void SwitchRoot(const std::string& new_root) {
     auto mounts = GetMounts(new_root);
 
+    LOG(INFO) << "Switching root to '" << new_root << "'";
+
     for (const auto& mount_path : mounts) {
         auto new_mount_path = new_root + mount_path;
+        mkdir(new_mount_path.c_str(), 0755);
         if (mount(mount_path.c_str(), new_mount_path.c_str(), nullptr, MS_MOVE, nullptr) != 0) {
             PLOG(FATAL) << "Unable to move mount at '" << mount_path << "'";
         }
     }
 
-    auto old_root_dir = std::unique_ptr<DIR, decltype(&closedir)>{opendir("/"), closedir};
-    if (!old_root_dir) {
-        PLOG(ERROR) << "Could not opendir(\"/\"), not freeing ramdisk";
-    }
-
-    struct stat old_root_info;
-    if (stat("/", &old_root_info) != 0) {
-        PLOG(ERROR) << "Could not stat(\"/\"), not freeing ramdisk";
-        old_root_dir.reset();
-    }
-
     if (chdir(new_root.c_str()) != 0) {
         PLOG(FATAL) << "Could not chdir to new_root, '" << new_root << "'";
     }
@@ -141,10 +93,6 @@ void SwitchRoot(const std::string& new_root) {
     if (chroot(".") != 0) {
         PLOG(FATAL) << "Unable to chroot to new root";
     }
-
-    if (old_root_dir) {
-        FreeRamdisk(old_root_dir.get(), old_root_info.st_dev);
-    }
 }
 
 }  // namespace init