Android JNI: reject invalid length at direct buffer entry points

[parasol-aser]

Summary

Fixes #3177.

The public Android JNI decoder wrapper (android_jni/avifandroidjni/src/main/jni/libavif_jni.cc) accepts a caller-supplied signed int length at every direct-ByteBuffer entry point and forwards it, unchecked, into the native size_t size boundary of avifDecoderSetIOMemory(). A negative length (e.g. -1) or a negative-as-int oversize (e.g. 0x80000000) sign-extends to a near-SIZE_MAX value; the parser then trusts the inflated advertised size and reads past the real direct-buffer allocation. ASan reports a heap-buffer-overflow in avifROStreamRead (see the stack in the issue).

This PR enforces the contract at the JNI boundary — where the signed int enters — so that every downstream caller of avifDecoderSetIOMemory() stays free to pass any size_t it likes.

Changes

• New ValidateDirectBuffer() helper in the libavif_jni.cc anonymous namespace that:
  • Rejects length < 0 before any cast.
  • Reads GetDirectBufferCapacity(encoded) once and rejects non-direct buffers (capacity < 0) and length > capacity (strict >).
  • Rejects a nullptr address from GetDirectBufferAddress().
  • Casts to size_t only after the checks pass; returns the validated (buffer, size_t size) pair.
• Hardened all four public JNI entry points that consume a direct ByteBuffer and an explicit length: isAvifImage, getInfo, decode, and createDecoder. Each now early-exits with its clean-failure value (false / 0) when validation fails.
• CreateDecoderAndParse() signature changed from int length to size_t length so the signed-to-unsigned sign-extension is gone from the internal helper too. All four call sites updated.
• New Android instrumentation regression test AvifDecoderLengthValidationTest covering:
  • The two issue-named boundary values (length = -1, length = Integer.MIN_VALUE).
  • length = capacity + 1 and length = Integer.MAX_VALUE (positive-but-oversized).
  • Non-direct (heap-backed) buffers.
  • Boundary cases that must still succeed: length == capacity, length == 0, and a valid image through AvifDecoder.create() / AvifDecoder.getInfo().
  • Every case verifies the clean-failure contract (false / null) instead of a crash.

Scope and non-goals

• src/read.c, src/stream.c — unchanged. The crash manifests there, but the parser is trusting an already-bad size handed to it. Fixing it lower down would silently paper over other callers of avifDecoderSetIOMemory() that are free to pass a large size_t deliberately.
• AvifDecoder.java public API surface — unchanged. The issue's secondary suggestion of deprecating the int length overloads in favor of encoded.remaining()-driven variants is deliberately not taken here; it would be a source-incompatible change, and the issue frames it as secondary hardening. Happy to do that as a follow-up if maintainers want it.
• No new linker deps, no new JNI reflection lookups, no C++ standard bump.

Test plan

• Repro confirmed pre-fix. Against b357f08 (current main), the 8-byte truncated-ftyp payload with length = -1 or length = 0x80000000 produces the ASan heap-buffer-overflow at avifROStreamRead → avifParseFileTypeBox → avifParse → avifDecoderParse (matches the issue).
• Repro cleared post-fix at the JNI boundary. With ValidateDirectBuffer() in place, the same inputs through AvifDecoder.getInfo / decode / isAvifImage / create return the clean-failure value (false / null) without entering the parser. Verified with a simulated-JNI-validation harness that mirrors the new helper before calling avifDecoderSetIOMemory().
• Parser contract unchanged. Calling avifDecoderSetIOMemory() directly with (size_t)-1 still crashes as before — expected; the fix is at the JNI boundary, not in the parser. This is intentionally not a behavior change for non-JNI callers.
• Android instrumentation suite. Run AvifDecoderLengthValidationTest on an Android emulator/device, e.g.:

  cd android_jni && ./gradlew :avifandroidjni:connectedDebugAndroidTest \
    --tests org.aomedia.avif.android.AvifDecoderLengthValidationTest

  Every test case must pass. I don't have an Android device/emulator in this environment to run the instrumentation suite myself; maintainers with CI access should confirm. The tests reuse the existing avif/fox.profile0.8bpc.yuv420.avif asset already shipped under androidTest/assets for the happy-path cases.
• Build hygiene. ./android_jni/gradlew :avifandroidjni:assembleDebug must continue to link cleanly after the CreateDecoderAndParse signature change. -Wsign-compare / -Wconversion remain clean across the edited block (the helper uses an explicit static_cast<jlong>(length) for the capacity comparison).

Boundary behavior verified by the test matrix

Entry point	length	Expected
getInfo	-1	false, no crash
getInfo	Integer.MIN_VALUE	false, no crash
getInfo	capacity + 1 (9)	false, no crash
getInfo	Integer.MAX_VALUE	false, no crash
getInfo	capacity (8, truncated ftyp)	false (parser — truncated data control)
getInfo	0 (empty direct buffer)	false (parser — not short-circuited at JNI)
getInfo	valid image, honest length	true (happy path guard)
decode	-1 / MIN_VALUE / cap+1 / MAX_VALUE	false, no crash
isAvifImage	truncated ftyp / heap-backed / empty	false, no crash
create	truncated / heap-backed / empty	null
create	valid image	non-null (happy path guard)

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