Drop caches after installing key policy to avoid cache clashes

Note that this is an ugly workaround for a kernel bug.

Bug: 28373400
Change-Id: Iec1ae53f4e18f06e41e8cf1fcc3ab03fc9848632

Ext4Crypt.cpp

@@ -176,6 +176,14 @@ static bool install_key(const std::string& key, std::string* raw_ref) {
     }
     LOG(DEBUG) << "Added key " << key_id << " (" << ref << ") to keyring " << device_keyring
                << " in process " << getpid();
+
+    // *TODO* Remove this code when kernel is fixed - see b/28373400
+    // Kernel preserves caches across a key insertion with ext4ice, which leads
+    // to contradictory dirents
+    if (!android::base::WriteStringToFile("3", "/proc/sys/vm/drop_caches")) {
+        PLOG(ERROR) << "Failed to drop_caches";
+    }
+
     return true;
 }