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bpo-41299: QueryPerformanceFrequency() cannot fail #28552

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Merged
merged 2 commits into from
Sep 24, 2021
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bpo-41299: QueryPerformanceFrequency() cannot fail #28552

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bpo-41299: QueryPerformanceFrequency() cannot fail
vstinner Sep 24, 2021
71a6609
Fix build error
vstinner Sep 24, 2021
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28 changes: 7 additions & 21 deletions Python/pytime.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1050,26 +1050,14 @@ py_win_perf_counter_frequency(LONGLONG *pfrequency, int raise)
LONGLONG frequency;

LARGE_INTEGER freq;
if (!QueryPerformanceFrequency(&freq)) {
if (raise) {
PyErr_SetFromWindowsErr(0);
}
return -1;
}
// Since Windows XP, the function cannot fail.
(void)QueryPerformanceFrequency(&freq);
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Why query this every time? It's fixed at boot, so it isn't going to change.

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This function must be called exactly once in a process: py_get_win_perf_counter() uses a static variable for that.

frequency = freq.QuadPart;

/* Sanity check: should never occur in practice */
if (frequency < 1) {
if (raise) {
PyErr_SetString(PyExc_RuntimeError,
"invalid QueryPerformanceFrequency");
}
return -1;
}

/* Check that frequency can be casted to _PyTime_t.
// Since Windows XP, frequency cannot be zero.
assert(frequency >= 1);

Make also sure that (ticks * SEC_TO_NS) cannot overflow in
/* Make also sure that (ticks * SEC_TO_NS) cannot overflow in
_PyTime_MulDiv(), with ticks < frequency.

Known QueryPerformanceFrequency() values:
Expand All @@ -1078,10 +1066,8 @@ py_win_perf_counter_frequency(LONGLONG *pfrequency, int raise)
* 3,579,545 Hz (3.6 MHz): 279 ns resolution

None of these frequencies can overflow with 64-bit _PyTime_t, but
check for overflow, just in case. */
if (frequency > _PyTime_MAX
|| frequency > (LONGLONG)_PyTime_MAX / (LONGLONG)SEC_TO_NS)
{
check for integer overflow just in case. */
if (frequency > _PyTime_MAX / SEC_TO_NS) {
if (raise) {
PyErr_SetString(PyExc_OverflowError,
"QueryPerformanceFrequency is too large");
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