recovery: Allow "Mount /system" for system_root_image.

When system images contain the root directory, there is no entry of
"/system" in the fstab. Change it to look for "/" instead if
ro.build.system_root_image is true. We actually mount the partition
to /system_root instead, and create a symlink to /system_root/system
for /system. This allows "adb shell" to work properly.

Bug: 22855115
Change-Id: I91864444950dc3229fda3cc133ddbadeb8817fb8
(cherry picked from commit abb8f7785e)

recovery.cpp

@@ -862,9 +862,24 @@ prompt_and_wait(Device* device, int status) {
                 break;
 
             case Device::MOUNT_SYSTEM:
-                if (ensure_path_mounted("/system") != -1) {
-                    ui->Print("Mounted /system.\n");
+                char system_root_image[PROPERTY_VALUE_MAX];
+                property_get("ro.build.system_root_image", system_root_image, "");
+
+                // For a system image built with the root directory (i.e.
+                // system_root_image == "true"), we mount it to /system_root, and symlink /system
+                // to /system_root/system to make adb shell work (the symlink is created through
+                // the build system).
+                // Bug: 22855115
+                if (strcmp(system_root_image, "true") == 0) {
+                    if (ensure_path_mounted_at("/", "/system_root") != -1) {
+                        ui->Print("Mounted /system.\n");
+                    }
+                } else {
+                    if (ensure_path_mounted("/system") != -1) {
+                        ui->Print("Mounted /system.\n");
+                    }
                 }
+
                 break;
         }
     }

roots.cpp

@@ -70,7 +70,8 @@ Volume* volume_for_path(const char* path) {
     return fs_mgr_get_entry_for_mount_point(fstab, path);
 }
 
-int ensure_path_mounted(const char* path) {
+// Mount the volume specified by path at the given mount_point.
+int ensure_path_mounted_at(const char* path, const char* mount_point) {
     Volume* v = volume_for_path(path);
     if (v == NULL) {
         LOGE("unknown volume for path [%s]\n", path);
@@ -88,14 +89,18 @@ int ensure_path_mounted(const char* path) {
         return -1;
     }
 
+    if (!mount_point) {
+        mount_point = v->mount_point;
+    }
+
     const MountedVolume* mv =
-        find_mounted_volume_by_mount_point(v->mount_point);
+        find_mounted_volume_by_mount_point(mount_point);
     if (mv) {
         // volume is already mounted
         return 0;
     }
 
-    mkdir(v->mount_point, 0755);  // in case it doesn't already exist
+    mkdir(mount_point, 0755);  // in case it doesn't already exist
 
     if (strcmp(v->fs_type, "yaffs2") == 0) {
         // mount an MTD partition as a YAFFS2 filesystem.
@@ -104,25 +109,30 @@ int ensure_path_mounted(const char* path) {
         partition = mtd_find_partition_by_name(v->blk_device);
         if (partition == NULL) {
             LOGE("failed to find \"%s\" partition to mount at \"%s\"\n",
-                 v->blk_device, v->mount_point);
+                 v->blk_device, mount_point);
             return -1;
         }
-        return mtd_mount_partition(partition, v->mount_point, v->fs_type, 0);
+        return mtd_mount_partition(partition, mount_point, v->fs_type, 0);
     } else if (strcmp(v->fs_type, "ext4") == 0 ||
                strcmp(v->fs_type, "squashfs") == 0 ||
                strcmp(v->fs_type, "vfat") == 0) {
-        result = mount(v->blk_device, v->mount_point, v->fs_type,
+        result = mount(v->blk_device, mount_point, v->fs_type,
                        v->flags, v->fs_options);
         if (result == 0) return 0;
 
-        LOGE("failed to mount %s (%s)\n", v->mount_point, strerror(errno));
+        LOGE("failed to mount %s (%s)\n", mount_point, strerror(errno));
         return -1;
     }
 
-    LOGE("unknown fs_type \"%s\" for %s\n", v->fs_type, v->mount_point);
+    LOGE("unknown fs_type \"%s\" for %s\n", v->fs_type, mount_point);
     return -1;
 }
 
+int ensure_path_mounted(const char* path) {
+    // Mount at the default mount point.
+    return ensure_path_mounted_at(path, nullptr);
+}
+
 int ensure_path_unmounted(const char* path) {
     Volume* v = volume_for_path(path);
     if (v == NULL) {

roots.h

@@ -19,10 +19,6 @@
 
 #include "common.h"
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 // Load and parse volume data from /etc/recovery.fstab.
 void load_volume_table();
 
@@ -33,7 +29,10 @@ Volume* volume_for_path(const char* path);
 // success (volume is mounted).
 int ensure_path_mounted(const char* path);
 
-// Make sure that the volume 'path' is on is mounted.  Returns 0 on
+// Similar to ensure_path_mounted, but allows one to specify the mount_point.
+int ensure_path_mounted_at(const char* path, const char* mount_point);
+
+// Make sure that the volume 'path' is on is unmounted.  Returns 0 on
 // success (volume is unmounted);
 int ensure_path_unmounted(const char* path);
 
@@ -46,8 +45,4 @@ int format_volume(const char* volume);
 // mounted (/tmp and /cache) are mounted.  Returns 0 on success.
 int setup_install_mounts();
 
-#ifdef __cplusplus
-}
-#endif
-
 #endif  // RECOVERY_ROOTS_H_