Zygote: Additional whitelisting for legacy devices.

On M and below, we provide a blanket whitelist for all files under
"/vendor/zygote_whitelist". This path is whitelisted purely to allow
this patch to be applied easily on legacy devices and configurations.

Note that this does not amount to a loosening of our security policy
because whitelisted files are reopened anyway.

Bug: 32691930
Test: manual

(cherry-picked from commit 5e2f7c6229d7191183888d685b57a7d0a2835fce)

Change-Id: I12a3f0d84e3b7454e77f917b71960cd81e2309e3

Author: narayank

vm/native/fd_utils-inl.h

@@ -244,6 +244,10 @@ class FileDescriptorInfo {
     is_sock(false) {
   }
 
+  static bool StartsWith(const std::string& str, const std::string& prefix) {
+    return str.compare(0, prefix.size(), prefix) == 0;
+  }
+
   // Returns true iff. a given path is whitelisted. A path is whitelisted
   // if it belongs to the whitelist (see kPathWhitelist) or if it's a path
   // under /system/framework that ends with ".jar".
@@ -256,10 +260,18 @@ class FileDescriptorInfo {
 
     static const std::string kFrameworksPrefix = "/system/framework/";
     static const std::string kJarSuffix = ".jar";
-    if (path.compare(0, kFrameworksPrefix.size(), kFrameworksPrefix) == 0 &&
+    if (StartsWith(path, kFrameworksPrefix) &&
         path.compare(path.size() - kJarSuffix.size(), kJarSuffix.size(), kJarSuffix) == 0) {
       return true;
     }
+
+    // All regular files that are placed under this path are whitelisted
+    // automatically.
+    static const std::string kZygoteWhitelistPath = "/vendor/zygote_whitelist/";
+    if (StartsWith(path, kZygoteWhitelistPath) && path.find("/../") == std::string::npos) {
+      return true;
+    }
+
     return false;
   }