reverse-skills-max: initial public release (restored, redacted, hardened)
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# Extension verification — differential hardening for Dex-VMP
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Covers the extension pass that added `references/vmp-differential-analysis.md` and
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`scripts/vmp_diff_harness.py`. Strength labels as defined in this directory's README:
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**observed** (exact command + output here), **inferred** (follows from observed facts,
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step not executed), **unverified** (assumed or externally reported, not independently
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confirmed).
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Reference environment: Windows 11 host, Python 3.14.0, JDK 17.0.4.1
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(`C:\Program Files\Java\jdk-17.0.4.1\bin\javac.exe`), Android build-tools 2.19 / D8
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8.2.2-dev off-PATH. **No device and no network were involved in this pass** — unlike the
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unpacking and rootdump passes, this one is deliberately machine-local, because the thing
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worth measuring here is the comparison logic, not the platform.
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**Headline: the local half of the route is measured and closes exactly; the
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platform half was not exercised at all, and the cost table says why it cannot be.**
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---
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## 1. Environment discovery — observed, and one correction to `tools/_work/ENV.md`
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`ENV.md` records `E:\tools\android-14\` as a complete build-tools directory. At pass time
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it held **three** entries:
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```
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$ Get-ChildItem 'E:\tools\android-14\' -Name
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apksigner.bat
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lib
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zipalign.exe
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```
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`aapt2.exe`, `d8.bat` and `dexdump.exe` were absent, so the `build` subcommand had no
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toolchain. The complete build-tools tree was sitting unextracted in `E:\tools\bt.zip`:
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```
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$ (bt.zip entries) | Where-Object { $_.Name -match '^(aapt2|d8|dexdump|split-select)' }
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android-14/aapt2.exe 4566296
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android-14/d8.bat 3156
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android-14/dexdump.exe 986392
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android-14/split-select.exe 1589016
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android-14/lib/d8.jar 14929869
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```
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After `Expand-Archive -Path 'E:\tools\bt.zip' -DestinationPath 'E:\tools\' -Force`:
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```
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$ E:\tools\android-14\aapt2.exe version
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Android Asset Packaging Tool (aapt) 2.19-10229193
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$ E:\tools\android-14\d8.bat --version
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D8 8.2.2-dev (build facedf41bbd28b563d1e9e09c5f72d7c5ca598d8 from go/r8bot (luci-r8-custom-ci-focal-14-zjz7))
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```
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Two further tool facts worth recording, both **observed**:
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- **The apktool jar the wrapper points at does not exist**, so `<tools>\bin\apktool.bat` (which is
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`java -jar "<tools>\agb_apktool.jar" %*`) resolves to nothing. apktool is unavailable on this
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machine, and a bare `apktool d` fails with a jar-not-found error rather than a
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recognisable "tool missing" message.
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- `javac` is on PATH only through the Oracle `javapath` forwarder; the real JDK is at
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`C:\Program Files\Java\jdk-17.0.4.1`. The harness takes `--javac` explicitly for this
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reason.
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## 2. `build` — observed
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```
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$ python skills/apk-reverse/scripts/vmp_diff_harness.py build \
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--build-tools E:\tools\android-14 --out tools/_work/t5-vmp/fixture \
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--javac "C:\Program Files\Java\jdk-17.0.4.1\bin\javac.exe" --apk
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javac: C:\Program Files\Java\jdk-17.0.4.1\bin\javac.exe
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d8: E:\tools\android-14\d8.bat
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aapt2: E:\tools\android-14\aapt2.exe
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generated: ...\src\OpCoverBig.java (37196 B)
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$ ... javac -g --release 11 -d ...\classes ...\OpCoverProbe.java ...\OpCoverBig.java
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javac ok: 9 class files
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$ E:\tools\android-14\d8.bat --min-api 26 --no-desugaring --output ...\dex <9 .class paths>
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d8 ok: ...\dex\classes.dex (35028 B)
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== ...\classes.dex (35028 bytes) ==
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classes=9 methods_with_code=75 instructions=3065 desynced=0
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opcode coverage: 218/224 (97.3%)
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not emitted by this fixture (6):
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0x09 move-object/16 move-object/16 (32x) needs both registers >= 256; the object
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frame reaches 256 on the source side only, which d8 encodes as 0x08
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0x1b const-string/jumbo needs a string_ids index > 65535, i.e. a fixture carrying
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65536+ string constants
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0x2a goto/32 needs a >32767-code-unit backward jump; javac refuses the method
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first with 'code too large' (its own 64 KB per-method bytecode cap)
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0xfd invoke-custom/range d8 emitted the 35c form for every lambda shape tried, including
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a six-parameter one
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0xfe const-method-handle no Java-language literal; reachable only by hand-written smali
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or direct dex construction
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0xff const-method-type same as 0xFE
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$ E:\tools\android-14\aapt2.exe link --manifest ...\apk\AndroidManifest.xml -o ...\apk\base.apk
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aapt2 ok: ...\probe.apk (20482 B, unsigned)
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build rc=0
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```
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Structural facts behind those numbers: **0 desynced methods** (every `decode_all` walk
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ended exactly on `insns_off + insns_size*2`, i.e. the format table and the emitted stream
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agree), 3,065 instructions across 75 method bodies, 9 classes.
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### 2a. The two toolchain traps this measured — observed
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Both are encoded in the script so they are not re-derived:
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- **`d8` refuses a non-existent `--output` directory.** First run:
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`Error: Invalid output: ...\dex / Output must be a .zip or .jar archive or an existing
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directory / Compilation failed` and rc=1. Same class of trap as `frida-dexdump -o`.
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`build` creates the directory first.
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- **`aapt2 link` needs `android.jar` for every `android:` attribute.** A manifest carrying
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`android:versionCode` / `android:versionName` / `android:minSdkVersion` produced five
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`error: attribute android:... not found` lines and rc=1. A manifest using only the
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un-namespaced `package` attribute links cleanly with no `-I` flag at all — which is what
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the harness ships, at the cost of an APK with no version attributes. That is acceptable
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for a dex carrier and is stated in the output.
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### 2b. `goto/32` is unreachable from Java — observed
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Driving the generator to 33,000 loop-body statements to push the backward jump past the
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20t range:
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```
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$ python tools/_work/t5-vmp/gen_big_probe.py 33000
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generated ...\OpCoverBig.java: 34442 lines, 766468 bytes
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javac rc=1
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OpCoverBig.java:1427: error: code too large
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public static int gotoFar(int x) {
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1 error
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```
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The JVM's own 64 KB per-method bytecode limit fires before the dex branch-distance
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threshold can be crossed. `goto/32` therefore cannot be covered by any javac-built
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fixture, at any generator size — this is a structural ceiling, not a tuning problem.
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## 3. Closed loop — observed, and this is the load-bearing result
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`simulate` relabels every opcode through a bijection whose ground truth it writes to disk.
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`compare` is then given only the two dex files and must re-derive that table.
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```
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$ python .../vmp_diff_harness.py simulate .../fixture/dex/classes.dex \
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--out tools/_work/t5-vmp/sim.dex --seed 7 --table tools/_work/t5-vmp/truth.json
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simulated: ...classes.dex -> ...sim.dex
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mode=relabel, 3065 method body(ies)/instruction(s) touched (seed 7)
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header recomputed: checksum_ok=True signature_ok=True
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ground-truth table written to ...truth.json (compare must re-derive it)
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$ python .../vmp_diff_harness.py compare .../fixture/dex/classes.dex tools/_work/t5-vmp/sim.dex
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aligned=75 resized=0 stripped=0 missing=0 added=0
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aligned slots compared: 3065
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verdict: USABLE
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(map: 218 rows, every one verdict=high)
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compare rc=0
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```
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Scoring the derivation against the ground truth:
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```
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ground-truth bijection size : 256
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opcodes the fixture emits : 218
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compare rows : 218
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compare high-confidence rows : 218
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compare undetermined : 6
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compare non-injective : 0
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compare run verdict : usable
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derived entries that are WRONG : 0 (none)
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derived entries with no truth : 0 []
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emitted opcodes NOT derived : 0 []
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EXACT RECOVERY: 218/218 emitted opcodes (100.0%)
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CLOSED-LOOP VERDICT: IDENTICAL
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```
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Why the denominator is 218 and not 256: the ground-truth bijection covers all 256 byte
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values, but 32 of those are unallocated dex slots and 6 are opcodes this fixture never
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emits — and an opcode that never appears leaves no known plaintext. The six are reported
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as `undetermined` with their reasons, never guessed. **Every entry the run did produce was
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correct, and every opcode that could be read was read.**
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## 4. Negative shapes — observed
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The decision table's failure branches were driven on purpose, because a detector that has
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never been seen to fire is not a detector.
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**Stub shape** (`--mode stub`: every body replaced by an equal-length `return-void` fill —
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the extraction shape):
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```
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$ python .../vmp_diff_harness.py compare .../classes.dex tools/_work/t5-vmp/stub.dex --quiet
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aligned=75 resized=0 stripped=0 missing=0 added=0
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aligned slots compared: 3065
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verdict: NOT-USABLE
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! 1 private byte(s) are claimed by up to 218 original opcodes each. No injective
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relabelling can do that: it is the signature of method bodies replaced by one
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common stub, and any substitution read off such a pair describes the stub, not a
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private opcode space.
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rc=1
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```
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This is the failure mode worth reading twice. In the per-opcode direction the reading is
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**perfect** — 218 rows, all `high`, zero conflicts — because every original opcode maps to
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the same private byte. Only the reverse (`private byte → set of originals`) check exposes
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it. An implementation without that check would have returned a confident, wholly
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meaningless table, and the first version of this one did exactly that: measured before the
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fix, the same input returned `verdict: PARTIALLY-USABLE` alongside a note reading
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"0 of 218 opcodes saw more than one candidate byte" — a warning that contradicted itself.
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**Length-changing shape** (`--mode shrink`: `insns_size` rewritten without moving bytes, a
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deliberately inconsistent image that exists only to drive the detector):
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```
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$ python .../vmp_diff_harness.py compare .../classes.dex tools/_work/t5-vmp/shrink.dex --quiet
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aligned=0 resized=75 stripped=0 missing=0 added=0
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aligned slots compared: 0
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verdict: NOT-APPLICABLE
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! 75 method(s) kept a body but changed its length. Boundaries cannot be projected
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across a length change; a private opcode stream with its own length table is not
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reachable this way.
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rc=1
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```
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Regression after both fixes: the relabel loop still reports `USABLE` and the closed-loop
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check still reports `IDENTICAL` — recorded above, re-run after the verdict-logic change.
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**Not driven:** the `stripped` shape (`code_off == 0`) needs a class-data rewrite rather
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than a byte edit, so its detector path is **inferred** from the shape counters, not
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observed firing. The counters themselves (`stripped=0` in every run above) are observed.
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## 5. `emit-smali` — observed
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```
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$ python .../vmp_diff_harness.py emit-smali tools/_work/t5-vmp/sim.dex \
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--table tools/_work/t5-vmp/cmp.json --class LOpCoverProbe --method arithInt
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; instructions restored: 3065 ; unmapped-opcode stops: 0
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; THIS IS A READING SKELETON, NOT ASSEMBLABLE SMALI.
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...
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.class public LOpCoverProbe;
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.super Ljava/lang/Object;
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.source "OpCoverProbe.java"
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# ---- arithInt(II)I registers=4 insns_size=29
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.method public static arithInt(II)I
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.registers 4
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# 0x0000 90000203 add-int
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# 0x0004 d000e803 add-int/lit16
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# 0x0008 d8000007 add-int/lit8
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# 0x000c b130 sub-int/2addr
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...
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# 0x0038 0f00 return
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.end method
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```
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Two checks that the restore is real rather than plausible: the stream decodes to the
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**original fixture's** `arithInt` body (compare it against `OpCoverProbe.java`'s 17
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operations), and the payload-bearing `switchPacked` method also restores cleanly —
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`packed-switch` followed by its payload block, stepped over by its own layout rather than
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mistaken for opcodes. `unmapped-opcode stops: 0` is the honest coverage measure, and it is
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zero only because the table is complete for this fixture.
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Three defects were found and fixed by this run, each one a silent-wrong-answer shape:
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- `--class LOpCoverProbe;` matched **zero** classes because the filter accepted only one of
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the three spellings (`LOpCoverProbe;` / `LOpCoverProbe` / `OpCoverProbe`) — output read
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exactly like a class that is not in the file.
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- `.source` printed `"<string_idx 34748 out of range>"`: `source_file_idx` sits at
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`class_def_item + 16`, not `+ 20` (that is `annotations_off`).
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- Methods rendered without access flags, so the skeleton was not readable as smali.
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## 6. What was not done
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- **No hardening platform was contacted.** The entire submission link of the chain is
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`unverified`, and its cost estimate in the reference file is reasoning about how those
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services are distributed (accounts, rate limits, download-page delivery, possible refusal
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of a synthetic fixture), not a measured upload. This is the pass's largest gap and it is
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not closeable from this machine.
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- **No real VMP sample was used.** `tools/_work/bench/repos/DexPatcher/assets/ezAndroid.apk`
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is recorded in `ENV.md` as a real Dex-VMP challenge sample and was **not** opened in this
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pass; the differential route needs a *matching hardened counterpart of the same build*,
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which no available artifact provides.
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- **The engine-behaviour assumption is inferred**: that a real VMP substitutes per-opcode
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at stable instruction length. `simulate` implements the weakest form of that assumption
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deliberately — it is the only form this harness can check itself against.
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- **Table stability across inputs is unverified.** Whether one fixture's table applies to
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another build of the same app, or survives a vendor version change, was not tested and is
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the reason the reference file says to re-derive per target.
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- **The `stripped` detector path did not fire** (see §4).
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- `--mode shrink` writes a structurally inconsistent dex by design; `dexutil.Dex.check()`
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will not reject it because it does not range-check `insns` against the file. It is a
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detector probe, is labelled as such in `--help`, and must not be treated as a build.
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