8000 py/compile: Fix potential Py-stack overflow in try-finally with return. by dpgeorge · Pull Request #13569 · micropython/micropython · GitHub
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py/compile: Fix potential Py-stack overflow in try-finally with return. #13569

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Merged
merged 1 commit into from
Jan 31, 2024
Merged

py/compile: Fix potential Py-stack overflow in try-finally with return. #13569

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11 changes: 11 additions & 0 deletions py/compile.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1650,9 +1650,11 @@ STATIC void compile_try_except(compiler_t *comp, mp_parse_node_t pn_body, int n_
if (qstr_exception_local != 0) {
EMIT_ARG(load_const_tok, MP_TOKEN_KW_NONE);
EMIT_ARG(label_assign, l3);
EMIT_ARG(adjust_stack_size, 1); // stack adjust for possible return value
EMIT_ARG(load_const_tok, MP_TOKEN_KW_NONE);
compile_store_id(comp, qstr_exception_local);
compile_delete_id(comp, qstr_exception_local);
EMIT_ARG(adjust_stack_size, -1);
compile_decrease_except_level(comp);
}

Expand Down Expand Up @@ -1682,9 +1684,18 @@ STATIC void compile_try_finally(compiler_t *comp, mp_parse_node_t pn_body, int n
} else {
compile_try_except(comp, pn_body, n_except, pn_except, pn_else);
}

// If the code reaches this point then the try part of the try-finally exited normally.
// This is indicated to the runtime by None sitting on the stack.
EMIT_ARG(load_const_tok, MP_TOKEN_KW_NONE);

// Compile the finally block.
// The stack needs to be adjusted by 1 to account for the possibility that the finally is
// being executed as part of a return, and the return value is on the top of the stack.
EMIT_ARG(label_assign, l_finally_block);
EMIT_ARG(adjust_stack_size, 1);
compile_node(comp, pn_finally);
EMIT_ARG(adjust_stack_size, -1);

compile_decrease_except_level(comp);
}
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10 changes: 10 additions & 0 deletions tests/basics/try_finally_return.py
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Expand Up @@ -70,3 +70,13 @@ def f():
finally:
print('finally 1')
print(f())

# the finally block uses a lot of Python stack and then a local is accessed
# (tests that the use of the stack doesn't clobber the local)
def f(x):
try:
return x
finally:
print(2, 3, 4, 5, 6)
print(x)
print(f(1))
0