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Fix segfault on cyclic or deeply-nested AST in compile() #7630

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Merged
youknowone merged 1 commit into from
Apr 19, 2026
Merged

Fix segfault on cyclic or deeply-nested AST in compile() #7630

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17 changes: 15 additions & 2 deletions crates/vm/src/stdlib/_ast/node.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -31,7 +31,14 @@ impl<T: Node> Node for Vec<T> {
source_file: &SourceFile,
object: PyObjectRef,
) -> PyResult<Self> {
vm.extract_elements_with(&object, |obj| Node::ast_from_object(vm, source_file, obj))
// Recursion guard for each element: prevents stack overflow when a
// sequence element transitively references the sequence itself
// (e.g. `l = ast.List(...); l.elts = [l]`). See issue #4862.
vm.extract_elements_with(&object, |obj| {
vm.with_recursion("while traversing AST node", || {
Node::ast_from_object(vm, source_file, obj)
})
})
}
}

Expand All @@ -45,7 +52,13 @@ impl<T: Node> Node for Box<T> {
source_file: &SourceFile,
object: PyObjectRef,
) -> PyResult<Self> {
T::ast_from_object(vm, source_file, object).map(Self::new)
// Recursion guard: every descent through a Box<AstNode> increments the
// VM's recursion depth so cyclic or pathologically deep ASTs raise
// RecursionError instead of overflowing the native stack.
// See issue #4862.
vm.with_recursion("while traversing AST node", || {
T::ast_from_object(vm, source_file, object).map(Self::new)
})
}

fn is_none(&self) -> bool {
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51 changes: 51 additions & 0 deletions extra_tests/snippets/stdlib_ast.py
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Original file line number Diff line number Diff line change
Expand Up @@ -37,3 +37,54 @@ def foo():
assert i.module is None
assert i.names[0].name == "a"
assert i.names[0].asname is None


# Regression test for issue #4862:
# A cyclic AST fed to compile() used to overflow the Rust stack and SIGSEGV.
# After the fix, the recursion guard in ast_from_object raises RecursionError,
# matching CPython's behavior. Covers both Box<T> descents (UnaryOp, BinOp,
# Call, Attribute) and Vec<T> descents (List, Tuple).
import warnings


def _cyclic_cases():
# Box<Expr> descents
u = ast.UnaryOp(op=ast.Not(), lineno=0, col_offset=0)
u.operand = u
yield "UnaryOp", u

b = ast.BinOp(
op=ast.Add(),
right=ast.Constant(value=0, lineno=0, col_offset=0),
lineno=0,
col_offset=0,
)
b.left = b
yield "BinOp", b

c = ast.Call(args=[], keywords=[], lineno=0, col_offset=0)
c.func = c
yield "Call", c

a = ast.Attribute(attr="x", ctx=ast.Load(), lineno=0, col_offset=0)
a.value = a
yield "Attribute", a

# Vec<Expr> descents
lst = ast.List(ctx=ast.Load(), lineno=0, col_offset=0)
lst.elts = [lst]
yield "List", lst

tup = ast.Tuple(ctx=ast.Load(), lineno=0, col_offset=0)
tup.elts = [tup]
yield "Tuple", tup


with warnings.catch_warnings():
warnings.simplefilter("ignore")
for name, node in _cyclic_cases():
try:
compile(ast.Expression(node), "<cyclic>", "eval")
raise AssertionError(f"cyclic {name} should raise RecursionError")
except RecursionError:
pass # expected; matches CPython
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