1# Fuzzer for libstagefright_amrnbdec decoder
2
3## Plugin Design Considerations
4The fuzzer plugin for AMR-NB is designed based on the understanding of the
5codec and tries to achieve the following:
6
7##### Maximize code coverage
8The configuration parameters are not hardcoded, but instead selected based on
9incoming data. This ensures more code paths are reached by the fuzzer.
10
11AMR-NB supports the following parameters:
121. Stream format (parameter name: `input_format`)
132. 3GPP frame type (parameter name: `frame_type`)
14
15| Parameter| Valid Values| Configured Value|
16|------------- |-------------| ----- |
17| `input_format` | 0. `MIME_IETF` 1. `IF2` | Bit 0 (LSB) of 1st byte of data. |
18| `frame_type`   | 0. `AMR_475` 1. `AMR_515` 2. `AMR_59` 3. `AMR_67`  4. `AMR_74` 5. `AMR_795` 6. `AMR_102` 7. `AMR_122`  | Bits 3, 4 and 5 of 1st byte of data. |
19
20
21This also ensures that the plugin is always deterministic for any given input.
22
23##### Maximize utilization of input data
24The plugin feeds the entire input data to the codec using a loop.
25If the decode operation was successful, the input is advanced by the frame size
26which is based on `input_format` and `frame_type` selected.
27If the decode operation was un-successful, the input is still advanced by frame size so
28that the fuzzer can proceed to feed the next frame.
29
30This ensures that the plugin tolerates any kind of input (empty, huge,
31malformed, etc) and doesnt `exit()` on any input and thereby increasing the
32chance of identifying vulnerabilities.
33
34## Build
35
36This describes steps to build amrnb_dec_fuzzer binary.
37
38### Android
39
40#### Steps to build
41Build the fuzzer
42```
43  $ mm -j$(nproc) amrnb_dec_fuzzer
44```
45
46#### Steps to run
47Create a directory CORPUS_DIR and copy some amrnb files to that folder
48Push this directory to device.
49
50To run on device
51```
52  $ adb sync data
53  $ adb shell /data/fuzz/arm64/amrnb_dec_fuzzer/amrnb_dec_fuzzer CORPUS_DIR
54```
55To run on host
56```
57  $ $ANDROID_HOST_OUT/fuzz/x86_64/amrnb_dec_fuzzer/amrnb_dec_fuzzer CORPUS_DIR
58```
59
60## References:
61 * http://llvm.org/docs/LibFuzzer.html
62 * https://github.com/google/oss-fuzz
63