Give users and devices control over sdcardfs.

Instead of relying only on kernel support for sdcardfs, give each
device the ability to quickly toggle between sdcardfs and FUSE.  Also
add the ability to users to explicitly enable/disable the behavior
for testing and debugging purposes.

Bug: 27991427
Change-Id: Ie188cb044be2ad87166f2d43c32a1f6b97660de0

sdcard/sdcard.c

@@ -43,6 +43,7 @@
 #include <cutils/hashmap.h>
 #include <cutils/log.h>
 #include <cutils/multiuser.h>
+#include <cutils/properties.h>
 #include <packagelistparser/packagelistparser.h>
 
 #include <private/android_filesystem_config.h>
@@ -89,6 +90,9 @@
 
 #define ERROR(x...) ALOGE(x)
 
+#define PROP_SDCARDFS_DEVICE "ro.sys.sdcardfs"
+#define PROP_SDCARDFS_USER "persist.sys.sdcardfs"
+
 #define FUSE_UNKNOWN_INO 0xffffffff
 
 /* Maximum number of bytes to write in one request. */
@@ -1993,6 +1997,29 @@ static bool supports_sdcardfs(void) {
     return false;
 }
 
+static bool should_use_sdcardfs(void) {
+    char property[PROPERTY_VALUE_MAX];
+
+    // Allow user to have a strong opinion about state
+    property_get(PROP_SDCARDFS_USER, property, "");
+    if (!strcmp(property, "force_on")) {
+        ALOGW("User explicitly enabled sdcardfs");
+        return supports_sdcardfs();
+    } else if (!strcmp(property, "force_off")) {
+        ALOGW("User explicitly disabled sdcardfs");
+        return false;
+    }
+
+    // Fall back to device opinion about state
+    if (property_get_bool(PROP_SDCARDFS_DEVICE, false)) {
+        ALOGW("Device explicitly enabled sdcardfs");
+        return supports_sdcardfs();
+    } else {
+        ALOGW("Device explicitly disabled sdcardfs");
+        return false;
+    }
+}
+
 int main(int argc, char **argv) {
     const char *source_path = NULL;
     const char *label = NULL;
@@ -2065,7 +2092,7 @@ int main(int argc, char **argv) {
         sleep(1);
     }
 
-    if (supports_sdcardfs()) {
+    if (should_use_sdcardfs()) {
         run_sdcardfs(source_path, label, uid, gid, userid, multi_user, full_write);
     } else {
         run(source_path, label, uid, gid, userid, multi_user, full_write);