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Resource Modes

Introduced in Apktool v2.9.0 added configuration for how Apktool decides to handle unresolved resources during disassembly.

Background​

Given an application you could leverage aapt (aapt d resources app.apk) to get a view of the resources in play.

➜ aapt d resources bugged.apk
Package Groups (1)
Package Group 0 id=0x7f packageCount=1 name=com.ibotpeaches.arsctest
  Package 0 id=0x7f name=com.ibotpeaches.arsctest
    type 0 configCount=1 entryCount=10
      spec resource 0x7f010000 com.ibotpeaches.arsctest:color/black: flags=0x00000000
      spec resource 0x7f010001 com.ibotpeaches.arsctest:color/purple_200: flags=0x00000000
      spec resource 0x7f010002 com.ibotpeaches.arsctest:color/purple_500: flags=0x00000000
      spec resource 0x7f010003 com.ibotpeaches.arsctest:color/purple_700: flags=0x00000000
      spec resource 0x7f010004 com.ibotpeaches.arsctest:color/teal_200: flags=0x00000000
      spec resource 0x7f010005 com.ibotpeaches.arsctest:color/teal_700: flags=0x00000000
      spec resource 0x7f010006 com.ibotpeaches.arsctest:color/white: flags=0x00000000
      INVALID TYPE CONFIG FOR RESOURCE 0x7f010007
      INVALID TYPE CONFIG FOR RESOURCE 0x7f010008
      spec resource 0x7f010009 com.ibotpeaches.arsctest:color/bugged: flags=0x00000000

In this example you can see the following two resources had issues being resolved:

  • 0x7f010007
  • 0x7f010008

The problem being though that the message exposed to end user is just a wrapper of the underlying issue, which could be one of the following:

  • No package found for resource.
  • No type found for resource.
  • Invalid identifier for resource name.
  • Invalid UTF-8 index during unpack of strings.
  • Invalid UTF-16 index during unpack of strings.
  • Entry not found for resource.

So basically it is difficult to understand the exact reason this occurs.

History​

Apktool prior to being able to keep resource ids static - would intentionally make dummy resources for any of these issues.

These would be resources like:

<item type="color" name="APKTOOL_DUMMY_7f010007" />
<item type="color" name="APKTOOL_DUMMY_7f010008" />

So when the resource table was built - the ids would not shift as dummy resources filled all locations in which resources did not disassemble properly.

Over time, this became unneeded because resource ids were kept static at the cost of turning them public. Something that continues to bite us in the increasing security of Android to this day.

Additionally, this became a bit of a problem because Apktool would unintentionally fingerprint applications with dummy resources. In 2019 this was noticed when a large company used Apktool in order to rename their package and republish to the store. This post can be found here.

So unintentionally dummy resources just broke during the support of sparse resources and people didn't really notice. This was a good thing, but it led to some odd issues that were previously handled.

Given an example like follows:

<attr name="actionBarSize" format="dimension">
    <enum name="@null" value="0" />
</attr>

This is how Apktool disassembled an unknown resource during the v2.8.x release cycle. This was improper as an unresolved resource name cannot be @null.

However, would replacing this with a dummy resource be better?

<attr name="actionBarSize" formats="dimension">
    <enum name="APKTOOL_DUMMY_0x7f090459" value="0" />
</attr>

You could argue it was better, but chances are if you are disassembling a resource with no references to this attribute - it would seem odd to invent a dummy resource to fulfill it.

So it seemed another alternative was to just remove unresolved resources.

<attr name="actionBarSize" formats="dimension"/>

The challenge here that every single use-case might prefer a different option.

Modes​

During disassembly run -resm <mode> to set the mode for how Apktool should handle unresolved resources.

Remove (default)​

  • remove | delete

Apktool will remove any unresolved resource. This is the default behavior.

<attr name="actionBarSize" formats="dimension"/>

Dummy​

  • dummy | dummies

Apktool will populate unresolved resources with dummy values.

<attr name="actionBarSize" formats="dimension">
    <enum name="APKTOOL_DUMMY_0x7f090459" value="0" />
</attr>

Keep​

  • keep | preserve

Apktool will leave resources, but properly make up valid resource names if the name was stripped.

<attr name="actionBarSize" format="dimension">
    <enum name="resUnk0x7f090459" value="0" />
</attr>