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Limit instruction nesting depth in the wast parser #2815

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Limit instruction nesting depth in the wast parser #2815

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dab5a82
Limit instruction nesting depth in the wast parser
Nishuuzz Aug 8, 2026
505d3fa
Lower the nesting limit to 128 and address review
Nishuuzz Aug 9, 2026
656e0f5
Add the expected output back to the nesting test
Nishuuzz Aug 9, 2026
b3d8822
Raise the nesting limit to 512 after measuring real modules
Nishuuzz Aug 9, 2026
8746583
Give the tools an 8MB stack on msvc
Nishuuzz Aug 9, 2026
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24 changes: 24 additions & 0 deletions include/wabt/wast-parser.h
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Original file line number Diff line number Diff line change
Expand Up @@ -315,11 +315,35 @@ class WastParser {
bool HasError() const;
bool CheckRefType(Type::Enum type);
void VarToType(const Var& var, Type* type);
Result CheckNestingDepth();

// Unlike the binary reader, which tracks nesting with an explicit label
// stack, this parser is recursive descent: every level of nested
// instructions costs several stack frames. The limit is therefore much
// lower than BinaryReaderIR's kMaxNestingDepth, and is chosen to stay
// within the smallest default stack we build against (1MB on MSVC).
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static constexpr int kMaxNestingDepth = 1000;

// Increments the parser's nesting depth for as long as it is in scope.
class NestingGuard {
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public:
explicit NestingGuard(WastParser* parser) : parser_(parser) {
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++parser_->nesting_depth_;
}
~NestingGuard() { --parser_->nesting_depth_; }
NestingGuard(const NestingGuard&) = delete;
NestingGuard& operator=(const NestingGuard&) = delete;

private:
WastParser* parser_;
};

WastLexer* lexer_;
Index last_module_index_ = kInvalidIndex;
Errors* errors_;
WastParseOptions* options_;
int nesting_depth_ = 0;
bool nesting_limit_hit_ = false;

// Reference types can have names or indicies. For example (ref $foo)
// represents a type which name is $foo, and (ref 5) represents
Expand Down
25 changes: 25 additions & 0 deletions src/wast-parser.cc
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Expand Up @@ -2280,15 +2280,35 @@ Result WastParser::ParseResultList(TypeVector* result_types,
return ParseUnboundValueTypeList(TokenType::Result, result_types, type_vars);
}

Result WastParser::CheckNestingDepth() {
if (nesting_depth_ > kMaxNestingDepth) {
// Exceeding the limit is not recoverable: the error is reported once and
// the flag stops the callers below from resynchronizing and walking back
// into the same too-deep input, which would report it once per level.
if (!nesting_limit_hit_) {
nesting_limit_hit_ = true;
Error(GetLocation(), "instruction nesting depth exceeds max of %d",
kMaxNestingDepth);
}
return Result::Error;
}
return Result::Ok;
}

Result WastParser::ParseInstrList(ExprList* exprs) {
WABT_TRACE(ParseInstrList);
NestingGuard nesting_guard(this);
CHECK_RESULT(CheckNestingDepth());
ExprList new_exprs;
while (true) {
auto pair = PeekPair();
if (IsInstr(pair)) {
if (Succeeded(ParseInstr(&new_exprs))) {
exprs->splice(exprs->end(), new_exprs);
} else {
if (nesting_limit_hit_) {
return Result::Error;
}
CHECK_RESULT(Synchronize(IsInstr));
}
} else if (IsLparAnn(pair)) {
Expand Down Expand Up @@ -3432,11 +3452,16 @@ Result WastParser::ParseBlock(Block* block) {

Result WastParser::ParseExprList(ExprList* exprs) {
WABT_TRACE(ParseExprList);
NestingGuard nesting_guard(this);
CHECK_RESULT(CheckNestingDepth());
ExprList new_exprs;
while (PeekMatchExpr()) {
if (Succeeded(ParseExpr(&new_exprs))) {
exprs->splice(exprs->end(), new_exprs);
} else {
if (nesting_limit_hit_) {
return Result::Error;
}
CHECK_RESULT(Synchronize(IsExpr));
}
}
Expand Down
112 changes: 112 additions & 0 deletions test/parse/expr/bad-nesting-depth.txt
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Original file line number Diff line number Diff line change
@@ -0,0 +1,112 @@
;;; TOOL: wat2wasm
;;; ERROR: 1
;; Deeply nested blocks used to overflow the stack while parsing;
;; the parser now reports the nesting limit instead. See #2377.
(module (func
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
(block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block (block
nop
))))))))))))))))))))
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))
(;; STDERR ;;;
out/test/parse/expr/bad-nesting-depth.txt:56:1: error: instruction nesting depth exceeds max of 1000
nop
^^^
;;; STDERR ;;)
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