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Coredump: add local/stack value typing #199

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Feb 16, 2023
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Coredump: add local/stack value typing #199

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28 changes: 21 additions & 7 deletions Coredump.md
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Expand Up @@ -95,16 +95,29 @@ For each thread a [Custom section], called `corestack`, is used to store the
debugging information.

```
corestack ::= customsec(thread-info vec(frame))
corestack ::= customsec(thread-info vec(frame))
thread-info ::= 0x0 thread-name:name
frame ::= codeoffset:u32
locals:vec(u32)
stack:vec(u32)
locals:vec(value)
stack:vec(value)
reserved:u32
```

The `reserved` byte is decoded as an empty vector and reserved for future use.

Local and stack values are encoded using one byte for the type (similar to
Wasm's [Number Types]) followed by bytes representing the actual value:
```
value ::= 0x01 => ∅
| 0x7F n:i32 => n
| 0x7E n:i64 => n
| 0x7D n:f32 => n
| 0x7C n:f64 => n
```

The special byte `0x01` is used to represent a missing value, usually because it
was optimized out by the WebAssembly engine.

## Memory

The process memory is captured in the [Data Section], either entirely as one
Expand All @@ -131,17 +144,18 @@ tooling: [demo].
[ELF coredump]: https://www.gabriel.urdhr.fr/2015/05/29/core-file/
[Core dump on Wikipedia]: https://en.wikipedia.org/wiki/Core_dump
[gdb]: https://linux.die.net/man/1/gdb
[wasm-edit coredump]: https://github.com/xtuc/wasm-edit/blob/main/src/coredump.rs
[wasm-edit]: https://github.com/xtuc/wasm-edit
[wasmgdb]: https://github.com/xtuc/wasmgdb
[wasm-edit coredump]: https://github.com/xtuc/wasm-coredump/blob/main/bin/rewriter/src/rewriter.rs
[wasm-edit]: https://github.com/xtuc/wasm-coredump/tree/main/bin/rewriter
[wasmgdb]: https://github.com/xtuc/wasm-coredump/tree/main/bin/wasmgdb
[DWARF]: https://yurydelendik.github.io/webassembly-dwarf
[Wasmer FrameInfo]: https://docs.rs/wasmer/latest/wasmer/struct.FrameInfo.html
[Wasm u32]: https://webassembly.github.io/spec/core/binary/values.html#binary-int
[demo]: https://github.com/xtuc/wasmgdb/wiki/Demo
[demo]: https://github.com/xtuc/wasm-coredump/blob/main/bin/wasmgdb/demo.md
[multi-memory]: https://github.com/WebAssembly/multi-memory
[Wasm binary format]: https://webassembly.github.io/spec/core/binary/index.html
[Data Section]: https://webassembly.github.io/spec/core/binary/modules.html#data-section
[Custom section]: https://webassembly.github.io/spec/core/binary/modules.html#binary-customsec
[Memory Section]: https://webassembly.github.io/spec/core/binary/modules.html#binary-memsec
[instantiated]: https://webassembly.github.io/spec/core/exec/modules.html#instantiation
[Global Section]: https://webassembly.github.io/spec/core/binary/modules.html#binary-globalsec
[Number Types]: https://webassembly.github.io/spec/core/binary/types.html#binary-numtype