Implement validation of custom sections - #2668
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alexcrichton merged 2 commits intoSep 17, 2026
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Custom sections aren't validated by default in WebAssembly, but upstream spec tests have an `assert_invalid_custom` directive for still indicating whether a custom section is intended to be invalid or not. Additionally for custom sections like the `name` section the upstream specification still specifies validity criteria. Previously `wasm-tools wast` did not implement the `assert_invalid_custom` directive, simply succeeding on it, and there was no validation of any custom sections. This commit addresses this by implementing the `assert_invalid_custom` directive, adding a `--validate-custom` flag to `wasm-tools validate`, and otherwise implementing validation for custom sections. This is done in such a way to avoid impacting main module validation and is additionally something embedders must opt-in to executing, if even. My hunch is that most embedders won't bother with this and this'll be a niche-useful command line flag but that's pretty much it.
pchickey
approved these changes
Sep 17, 2026
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Custom sections aren't validated by default in WebAssembly, but upstream spec tests have an
assert_invalid_customdirective for still indicating whether a custom section is intended to be invalid or not. Additionally for custom sections like thenamesection the upstream specification still specifies validity criteria. Previouslywasm-tools wastdid not implement theassert_invalid_customdirective, simply succeeding on it, and there was no validation of any custom sections.This commit addresses this by implementing the
assert_invalid_customdirective, adding a--validate-customflag towasm-tools validate, and otherwise implementing validation for custom sections. This is done in such a way to avoid impacting main module validation and is additionally something embedders must opt-in to executing, if even. My hunch is that most embedders won't bother with this and this'll be a niche-useful command line flag but that's pretty much it.