Use /bootstrap-apex for bootstrap APEXes

This new directory is bind-mounted to /apex in the bootstrap mount
namespace so that apexd-bootstrap mounts bootstrap APEXes there via
/apex.

The directory is shared between two mount namespaces, hence visible
in the default mount namespace.

Bug: 290148078
Test: VendorApexHostTestCases
Change-Id: I841480e41be8def5a4c6a4aa874c4e21465a71d3

init/init.cpp

@@ -832,6 +832,12 @@ static void MountExtraFilesystems() {
     CHECKCALL(mount("tmpfs", "/apex", "tmpfs", MS_NOEXEC | MS_NOSUID | MS_NODEV,
                     "mode=0755,uid=0,gid=0"));
 
+    if (NeedsTwoMountNamespaces()) {
+        // /bootstrap-apex is used to mount "bootstrap" APEXes.
+        CHECKCALL(mount("tmpfs", "/bootstrap-apex", "tmpfs", MS_NOEXEC | MS_NOSUID | MS_NODEV,
+                        "mode=0755,uid=0,gid=0"));
+    }
+
     // /linkerconfig is used to keep generated linker configuration
     CHECKCALL(mount("tmpfs", "/linkerconfig", "tmpfs", MS_NOEXEC | MS_NOSUID | MS_NODEV,
                     "mode=0755,uid=0,gid=0"));

init/mount_namespace.cpp

@@ -66,15 +66,6 @@ static std::string GetMountNamespaceId() {
     return ret;
 }
 
-// In case we have two sets of APEXes (non-updatable, updatable), we need two separate mount
-// namespaces.
-static bool NeedsTwoMountNamespaces() {
-    if (IsRecoveryMode()) return false;
-    // In microdroid, there's only one set of APEXes in built-in directories include block devices.
-    if (IsMicrodroid()) return false;
-    return true;
-}
-
 static android::base::unique_fd bootstrap_ns_fd;
 static android::base::unique_fd default_ns_fd;
 
@@ -83,6 +74,15 @@ static std::string default_ns_id;
 
 }  // namespace
 
+// In case we have two sets of APEXes (non-updatable, updatable), we need two separate mount
+// namespaces.
+bool NeedsTwoMountNamespaces() {
+    if (IsRecoveryMode()) return false;
+    // In microdroid, there's only one set of APEXes in built-in directories include block devices.
+    if (IsMicrodroid()) return false;
+    return true;
+}
+
 bool SetupMountNamespaces() {
     // Set the propagation type of / as shared so that any mounting event (e.g.
     // /data) is by default visible to all processes. When private mounting is
@@ -163,6 +163,23 @@ bool SetupMountNamespaces() {
             PLOG(ERROR) << "Cannot switch back to bootstrap mount namespace";
             return false;
         }
+
+        // Some components (e.g. servicemanager) need to access bootstrap
+        // APEXes from the default mount namespace. To achieve that, we bind-mount
+        // /apex to /bootstrap-apex in the bootstrap mount namespace. Since /bootstrap-apex
+        // is "shared", the mounts are visible in the default mount namespace as well.
+        //
+        // The end result will look like:
+        //   in the bootstrap mount namespace:
+        //     /apex  (== /bootstrap-apex)
+        //       {bootstrap APEXes from the read-only partition}
+        //
+        //   in the default mount namespace:
+        //     /bootstrap-apex
+        //       {bootstrap APEXes from the read-only partition}
+        //     /apex
+        //       {APEXes, can be from /data partition}
+        if (!(BindMount("/bootstrap-apex", "/apex"))) return false;
     } else {
         // Otherwise, default == bootstrap
         default_ns_fd.reset(OpenMountNamespace());

init/mount_namespace.h

@@ -24,9 +24,12 @@ namespace init {
 enum MountNamespace { NS_BOOTSTRAP, NS_DEFAULT };
 
 bool SetupMountNamespaces();
+
 base::Result<void> SwitchToMountNamespaceIfNeeded(MountNamespace target_mount_namespace);
 
 base::Result<MountNamespace> GetCurrentMountNamespace();
 
+bool NeedsTwoMountNamespaces();
+
 }  // namespace init
 }  // namespace android

init/selinux.cpp

@@ -757,7 +757,7 @@ void SelinuxRestoreContext() {
     selinux_android_restorecon("/dev/device-mapper", 0);
 
     selinux_android_restorecon("/apex", 0);
-
+    selinux_android_restorecon("/bootstrap-apex", 0);
     selinux_android_restorecon("/linkerconfig", 0);
 
     // adb remount, snapshot-based updates, and DSUs all create files during

rootdir/Android.mk

@@ -91,7 +91,7 @@ endif
 #
 # create some directories (some are mount points) and symlinks
 LOCAL_POST_INSTALL_CMD := mkdir -p $(addprefix $(TARGET_ROOT_OUT)/, \
-    dev proc sys system data data_mirror odm oem acct config storage mnt apex debug_ramdisk \
+    dev proc sys system data data_mirror odm oem acct config storage mnt apex bootstrap-apex debug_ramdisk \
     linkerconfig second_stage_resources postinstall $(BOARD_ROOT_EXTRA_FOLDERS)); \
     ln -sf /system/bin $(TARGET_ROOT_OUT)/bin; \
     ln -sf /system/etc $(TARGET_ROOT_OUT)/etc; \