[Custom Descriptors] Use exact imports in wasm-split#8043
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Validate that exact function imports require custom descriptors. An alternative approach (that we may switch to in the future if necessary) would be to always allow exact imports, but then strip them out in the binary writer when custom descriptors are not enabled. For now take the more explicit what-you-see-is-what-you-get approach.
When custom descriptors are enabled, import primary functions into secondary modules using exact imports. This allows references to those functions in the secondary module to remain exact, just like they are before splitting. Remove the exact casts we previously inserted to fix this problem. Do not use exact imports when custom descriptors are not enabled. In principle this could cause validation errors because we allow e.g. exact locals even when custom descriptors are not enabled. This could be worked around in the future either by running a pass to remove exactness before splitting or by always using and allowing exact imports, but then emitting them as inexact imports when custom descriptors are disabled.
kripken
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Nov 12, 2025
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lgtm if the fuzzer somehow doesn't trip up on this, but I think it will without some workaround..?
Can test locally by running the Split fuzzer with higher priority to verify.
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Yep, will fuzz heavily before landing. I think the fuzzer should be ok here, though, because wasm-split does not do any optimization like refining the types of locals between reading the input and splitting. And if custom descriptors is not allowed, the input file will also not have any exact locals or other exact locations that could cause validation problems. |
aheejin
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We are currently validating inputs but not outputs. This PR makes wasm-split validate output modules too unless `--no-validation` is given. I had to add `--no-validation` to three existing tests to make them pass. They fail because of this reason, from WebAssembly#8043's description: > Do not use exact imports when custom descriptors are not enabled. In > principle this could cause validation errors because we allow e.g. > exact locals even when custom descriptors are not enabled. This could > be worked around in the future either by running a pass to remove > exactness before splitting or by always using and allowing exact > imports, but then emitting them as inexact imports when custom > descriptors are disabled.
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When custom descriptors are enabled, import primary functions into
secondary modules using exact imports. This allows references to those
functions in the secondary module to remain exact, just like they are
before splitting. Remove the exact casts we previously inserted to fix
this problem.
Do not use exact imports when custom descriptors are not enabled. In
principle this could cause validation errors because we allow e.g. exact
locals even when custom descriptors are not enabled. This could be
worked around in the future either by running a pass to remove exactness
before splitting or by always using and allowing exact imports, but then
emitting them as inexact imports when custom descriptors are disabled.