Spec interpreter/implementation discrepancy - a local with an invalid type and 0 count

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还没有人认领这个 Issue。

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难度
5/5
预计耗时
一周以上
新手友好度
35/100
Issue 类型
缺陷
描述清晰度
需要澄清
活跃度
活跃
技术栈
wasm
领域
compilers

调研方向

Start with the linked code-section specification and the reference interpreter, then reproduce the behavior using foo.wasm and compare it with V8, SpiderMonkey, and Wasmtime. Determine whether a zero-count local's type should be validated or discarded, and document the agreed specification or implementation outcome.

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描述

My understanding of the definition of the code section in the online specification is that if an entry in locals has n=0 count that it's discarded and not represented in the AST of a module. This means that the corresponding local's type is parsed, but not validated. The reference interpreter implements this behavior and for this input:

(module binary
  "\00asm\01\00\00\00"

  "\01\04\01\60\00\00"       ;; Type section: 1 type
  "\03\02\01\00"             ;; Function section: 1 function

  "\0a\07"                 ;; code section, 7 bytes
  "\01"                    ;;   1 function body
  "\05"                    ;;   body size, 5 bytes
  "\01"                    ;;     1 locals entry
  "\00"                    ;;       count: 0
  "\63\01"                 ;;       type: (ref null 1) -- not a valid type index
  "\0b"                    ;;     end
)

the spec interpreter passes. This same module (foo.wasm) does not validate in V8, SpiderMonkey, or Wasmtime, however. These engines all validate the local's type and reject the module saying that it's out-of-bounds.

I wanted to raise this as an issue to see if others have thoughts about this. I don't have a preference for how exactly to go here myself.

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