Android U user+root实现方案

 背景

       由于项目（MTK平台）上要实现user+root的版本，供特殊用户使用。Android T上的方案无效，经历了各种搜索查看资料，和bsp大佬一起通宵奋战，整出了方案。梳理记录下，有需要的同学可以参考。

Root代码实现原理

      系统判断是否有root权限的地方在system/packages/modules/adb/daemon/main.cpp里面，

should_drop_privileges()函数返回false，表明可以root。

auth_required 是否需要adb鉴权（adb弹框确认），false时默认授权adb鉴权权限，不需要弹框确认。

userdebug 版本可以root是因为ro.secure = 0,代码路径build/core/main.mk 在should_drop_privileges()里面有判断

else # !user_variant
   # Turn on checkjni for non-user builds.
   ADDITIONAL_SYSTEM_PROPERTIES += ro.kernel.android.checkjni=1
   # Set device insecure for non-user builds.
   ADDITIONAL_SYSTEM_PROPERTIES += ro.secure=0

static bool should_drop_privileges() {
    // The properties that affect `adb root` and `adb unroot` are ro.secure and
    // ro.debuggable. In this context the names don't make the expected behavior
    // particularly obvious.
    //
    // ro.debuggable:
    //   Allowed to become root, but not necessarily the default. Set to 1 on
    //   eng and userdebug builds.
    //
    // ro.secure:
    //   Drop privileges by default. Set to 1 on userdebug and user builds.
    bool ro_secure = android::base::GetBoolProperty("ro.secure", true);
    bool ro_debuggable = __android_log_is_debuggable();
 
    // Drop privileges if ro.secure is set...
    bool drop = ro_secure;
 
    // ... except "adb root" lets you keep privileges in a debuggable build.
    std::string prop = android::base::GetProperty("service.adb.root", "");
    bool adb_root = (prop == "1");
    bool adb_unroot = (prop == "0");
    if (ro_debuggable && adb_root) {
        drop = false;
    }
    // ... and "adb unroot" lets you explicitly drop privileges.
    if (adb_unroot) {
        drop = true;
    }
 
    return drop;
}

实现中遇到的问题

      1，selinux问题（主要解决问题）；

      2，缺少su 和remount执行的bin文件；

具体实现方案和步骤

       1），特殊版本标识，CUSTOM_ROOT_VERSION=yes，编译版本时需要export下该环境变量

       2），添加flag，路径build/core/soong_config.mk

--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -58,6 +58,9 @@
 
 $(call add_json_bool, Debuggable,                        $(filter userdebug eng,$(TARGET_BUILD_VARIANT)))
 $(call add_json_bool, Eng,                               $(filter eng,$(TARGET_BUILD_VARIANT)))
+$(call add_json_bool, ROOTVersion,                         $(filter yes,$(CUSTOM_ROOT_VERSION)))
 $(call add_json_str,  DeviceName,                        $(TARGET_DEVICE))

 3),添加root版本的数据声明，设置root相关的flag需要

--- a/android/variable.go
+++ b/android/variable.go
@@ -151,6 +151,14 @@
 			}
 		}
 
+		ROOTVersion struct {
+			Cflags          []string
+			Cppflags        []string
+			Init_rc         []string
+		}
 
 		Pdk struct {
 			Enabled *bool `android:"arch_variant"`
 		} `android:"arch_variant"`
@@ -315,6 +323,9 @@
 	UseRBED8                         *bool    `json:",omitempty"`
 	Debuggable                       *bool    `json:",omitempty"`
 	Eng                              *bool    `json:",omitempty"`
+	ROOTVersion                        *bool    `json:",omitempty"`
 	Treble_linker_namespaces         *bool    `json:",omitempty"`

4),添加su 和 remount 模块，在产品的mk文件中添加

PRODUCT_PACKAGES +=remount

PRODUCT_PACKAGES +=su

5)，在文件系统模块fs_mgr中添加ROOTVersion相应的flag，property_service中设置ro.secure和ro.debuggable的值

--- a/fs_mgr/Android.bp
+++ b/fs_mgr/Android.bp
@@ -118,6 +118,14 @@
                 "-DALLOW_ADBD_DISABLE_VERITY=1",
             ],
         },
+        ROOTVersion: {
+            cppflags: [
+                "-UALLOW_ADBD_DISABLE_VERITY",
+                "-DALLOW_ADBD_DISABLE_VERITY=1",
+            ],
+        },
     },
     header_libs: [
         "libfiemap_headers",
@@ -248,6 +256,17 @@
                 "clean_scratch_files.rc",
             ],
         },
+        ROOTVersion: {
+            cppflags: [
+                "-UALLOW_ADBD_DISABLE_VERITY",
+                "-DALLOW_ADBD_DISABLE_VERITY=1",
+            ],
+            init_rc: [
+                "clean_scratch_files.rc",
+            ],
+        },
     },
     symlinks: [
         "clean_scratch_files",
 
--- a/init/Android.bp
+++ b/init/Android.bp
@@ -149,6 +149,13 @@
                 "-DSHUTDOWN_ZERO_TIMEOUT=1",
             ],
         },
+        ROOTVersion: {
+            cppflags: [
+                "-DROOT_VERSION",
+            ],
+        },
         uml: {
             cppflags: ["-DUSER_MODE_LINUX"],
         },
 
--- a/init/property_service.cpp
+++ b/init/property_service.cpp
@@ -1328,6 +1328,19 @@
         }
     }
 
+    bool adbAuthorized = false;
+#ifdef ROOT_VERSION
+    adbAuthorized = true;
+#endif
+    if (adbAuthorized) {
+        InitPropertySet("ro.adb.secure", "0");
+        InitPropertySet("ro.debuggable", "1");
+    }
+
     for (const auto& [name, value] : properties) {

6）adb模块添加权限的判断，和5）中设置属性的值有重复，此处是为了确保生效。有时间的同学可以验证下去掉这一步看是否生效。

should_drop_privileges()函数最后添加

#ifdef CUSTON_ROOT_VERSION
   return false;
#endif

drop_privileges()函数最后添加

#ifdef CUSTON_ROOT_VERSION
   auth_required=false;
#endif

7),最重要的一步，更换sepolicy文件为debug版本的

(1)添加sepolicy，src文件是debug版本的，修改路径system/sepolicy/android.bp

--- a/Android.bp
+++ b/Android.bp
     },
 }
 
+se_policy_cil {
+    name: "userdebug_plat_sepolicy_root.cil",
+    src: ":userdebug_plat_sepolicy.conf",
+    additional_cil_files: [":sepolicy_technical_debt{.plat_private}"],
+    dist: {
+        targets: ["droidcore"],
+    },
+}
+
 // A copy of the userdebug_plat_policy in GSI.

(2),同路径下mk文件加入编译

--- a/Android.mk
+++ b/Android.mk
 
 LOCAL_REQUIRED_MODULES += \
     userdebug_plat_sepolicy.cil \
 
+ifeq ($(strip $(CUSTOM_ROOT_VERSION)),yes)
+LOCAL_REQUIRED_MODULES += \
+    userdebug_plat_sepolicy_root.cil
+endif

(3)代码中加载sepolicy地方GetUserdebugPlatformPolicyFile()也使用上面生成的sepolicy文件，代码路径system/core/init/selinux.cpp

 std::optional<const char*> GetUserdebugPlatformPolicyFile() {
+#ifdef DF_VERSION
+    return "/system/etc/selinux/userdebug_plat_sepolicy_root.cil";
+#endif

至此，Android U版本user+root+remount方案修改完成。