frameobject: undefined 32-bit left shifts in stack_swap() #161666

Crash report

stack_swap() in Objects/frameobject.c shifted MASK and top as
32-bit values and converted the result to int64_t, while the value-stack
model is a 63-bit int64_t and shift = BITS_PER_BLOCK*(n-1) can reach 60.
The shifts overflow / drop bits (undefined behavior) and corrupt the stack
model used by frame_setlineno() to validate line jumps.

import dis
import sys

fors = " ".join(f"for v{i} in range(1)" for i in range(11))
ns = {}
exec(f"def f():\n    return [0 {fors}]\n", ns)
f = ns["f"]

triggered = False
def tracer(frame, event, arg):
    global triggered
    if event == "line" and frame.f_code is f.__code__ and not triggered:
        triggered = True
        frame.f_lineno = frame.f_lineno  # mark_stacks -> stack_swap
    return tracer

sys.settrace(tracer)
try:
    f()
finally:
    sys.settrace(None)

UBSan output:

Objects/frameobject.c:1323:44: runtime error: shift exponent 33 is too large for 32-bit type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior Objects/frameobject.c:1323:44 in 
Objects/frameobject.c:1323:62: runtime error: shift exponent 33 is too large for 32-bit type 'unsigned int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior Objects/frameobject.c:1323:62 in 

Found by Linux Verification Center (linuxtesting.org) with SVACE

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Output from running 'python -VV' on the command line:

No response

Linked PRs

• gh-161666: Fix undefined left shifts in stack_swap() #161667