Document WAST test contributing

cmake/spectests_contributing.md

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+
+
+## Run fuzzer
+
+Run Fizzy vs WABT verification fuzzer, currently in `fuzzing` branch.
+Start with small inputs.
+
+```shell script
+bin/fizzy-fuzz-parser corpus -max_len=13
+```
+
+In case it finds a failure you will see output like
+```text
+INFO: Seed: 770491456
+INFO: Loaded 1 modules   (64068 inline 8-bit counters): 64068 [0xb8dc90, 0xb9d6d4), 
+INFO: Loaded 1 PC tables (64068 PCs): 64068 [0xb9d6d8,0xc97b18), 
+INFO:    10986 files found in corpus
+INFO: seed corpus: files: 10986 min: 1b max: 27419b total: 740862b rss: 40Mb
+#11160  INITED cov: 7782 ft: 9644 corp: 227/2677b exec/s: 0 rss: 186Mb
+#65536  pulse  cov: 7782 ft: 9644 corp: 227/2677b lim: 13 exec/s: 32768 rss: 215Mb
+#131072 pulse  cov: 7782 ft: 9644 corp: 227/2677b lim: 13 exec/s: 32768 rss: 248Mb
+#262144 pulse  cov: 7782 ft: 9644 corp: 227/2677b lim: 13 exec/s: 32768 rss: 314Mb
+  MALFORMED: invalid limits 4
+/home/chfast/Projects/wasmx/fizzy/test/fuzzer/parser_fuzzer.cpp:47:9: runtime error: execution reached an unreachable program point
+SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /home/chfast/Projects/wasmx/fizzy/test/fuzzer/parser_fuzzer.cpp:47:9 in 
+MS: 1 ChangeBit-; base unit: 846f4928f92649789961543e93544b4c2ed6fc44
+0x0,0x61,0x73,0x6d,0x1,0x0,0x0,0x0,0x5,0x3,0x1,0x4,0x2b,
+\x00asm\x01\x00\x00\x00\x05\x03\x01\x04+
+artifact_prefix='./'; Test unit written to ./crash-016934f781c41276bfacda9a90385cef85a88d5f
+Base64: AGFzbQEAAAAFAwEEKw==
+```
+
+In this example Fizzy reported the error `MALFORMED: invalid limits 4`
+while WABT considers the Wasm binary valid.
+The test unit (Wasm binary) is also printed in various forms and saved as
+`crash-016934f781c41276bfacda9a90385cef85a88d5f`.
+
+
+## Check WebAssembly spec interpreter
+
+In case of false negative validation result in WABT it is likely that
+such case is missing in the WAST spec test.
+
+Build the `wasm reference interpreter` following official instructions.
+
+Rename `crash-016934f781c41276bfacda9a90385cef85a88d5f` to `crash.wasm`
+as the interpreter need proper files extension to recognize file type.
+
+```shell script
+./wasm crash.wasm
+```
+
+Output:
+```text
+crash.wasm:0xb: decoding error: integer too large
+```
+
+Good news. This is effectively the same error reported as in Fizzy. In Wasm 1.0 `limits` kind
+can be 0 or 1. The the value 4 in the `crash.wasm` is larger than the max 1.
+
+
+## Inspect with WABT tools
+
+You can use `wasm2wat` or `wasm-validate` tools to confirm the bug in WABT.
+Make sure you use the same version as used for fuzzing.
+Disable all extensions except "mutable globals".
+
+In our example, `wasm2wat crash.wasm`:
+```webassembly
+(module
+  (memory (;0;) 43 i64))
+```
+This indicates that the issue is in the memory section.
+
+Unfortunately, we get different error as the data section also has invalid length.
+Let's fix that.
+
+
+## Prepare WAST test
+
+Dump the test unit (`xxd -p -c1000 crash.wasm`):
+
+```hexdump
+0061736d01000000050301042b
+```
+
+Here are created WAST tests, placed in `new.wast` file. In the original `crash.wasm` the invalid 4
+is followed by `2b` byte. We added 2 more variants of the test with byte `00` and without any 
+following byte (remember to change the section length if you actually modify the length).
+
+```wast
+(assert_malformed
+  (module binary
+    "\00asm" "\01\00\00\00"
+    "\05\03\01"                          ;; memory section
+    "\04\2b"                             ;; malformed memory limit 4
+  )
+  "integer too large"
+)
+
+(assert_malformed
+  (module binary
+    "\00asm" "\01\00\00\00"
+    "\05\03\01"                          ;; memory section
+    "\04\00"                             ;; malformed memory limit 4
+  )
+  "integer too large"
+)
+
+(assert_malformed
+  (module binary
+    "\00asm" "\01\00\00\00"
+    "\05\02\01"                          ;; memory section
+    "\04"                                ;; malformed memory limit 4
+  )
+  "integer too large"
+)
+```
+
+These tests can be rechecked with WABT.
+
+```shell script
+wast2json new.wast
+wasm-validate new.0.wasm
+wasm-validate new.1.wasm
+wasm-validate new.2.wasm
+```
+
+The last test case is actually failing in WABT for some other reason, but it can stay as a valid
+spectests addition.
+
+```text
+000000c: error: unable to read u32 leb128: memory initial page count
+```
+
+
+## Debug & fix WABT
+
+Debug `wasm-validate new.0.wasm` to find out why it passes validation.
+Binary loading code is in `binary-reader.cc`.
+
+The example bug is fixed by https://github.com/WebAssembly/wabt/pull/1547.
+
+
+## Send WAST tests upstream