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[3.10] gh-97786: Fix compiler warnings in pytime.c (GH-101826) #102150

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Merged
merged 3 commits into from
Feb 26, 2023
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[3.10] gh-97786: Fix compiler warnings in pytime.c (GH-101826) #102150

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3d56ece
[3.10] gh-97786: Fix compiler warnings in pytime.c (GH-101826)
mdickinson Feb 20, 2023
a54b679
Merge branch '3.10' into backport-b1b375e-3.10
gpshead Feb 23, 2023
7e97e91
Add comment about the casts
mdickinson Feb 26, 2023
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2023-02-11-13-23-29.gh-issue-97786.QjvQ1B.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix potential undefined behaviour in corner cases of floating-point-to-time
conversions.
45 changes: 41 additions & 4 deletions Python/pytime.c
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Original file line number Diff line number Diff line change
Expand Up @@ -34,6 +34,25 @@
#define NS_TO_MS (1000 * 1000)
#define NS_TO_US (1000)

#if SIZEOF_TIME_T == SIZEOF_LONG_LONG
# define PY_TIME_T_MAX LLONG_MAX
# define PY_TIME_T_MIN LLONG_MIN
#elif SIZEOF_TIME_T == SIZEOF_LONG
# define PY_TIME_T_MAX LONG_MAX
# define PY_TIME_T_MIN LONG_MIN
#else
# error "unsupported time_t size"
#endif

#if PY_TIME_T_MAX + PY_TIME_T_MIN != -1
# error "time_t is not a two's complement integer type"
#endif

#if _PyTime_MIN + _PyTime_MAX != -1
# error "_PyTime_t is not a two's complement integer type"
#endif


static void
error_time_t_overflow(void)
{
Expand Down Expand Up @@ -157,7 +176,21 @@ _PyTime_DoubleToDenominator(double d, time_t *sec, long *numerator,
}
assert(0.0 <= floatpart && floatpart < denominator);

if (!_Py_InIntegralTypeRange(time_t, intpart)) {
/*
Conversion of an out-of-range value to time_t gives undefined behaviour
(C99 §6.3.1.4p1), so we must guard against it. However, checking that
`intpart` is in range is delicate: the obvious expression `intpart <=
PY_TIME_T_MAX` will first convert the value `PY_TIME_T_MAX` to a double,
potentially changing its value and leading to us failing to catch some
UB-inducing values. The code below works correctly under the mild
assumption that time_t is a two's complement integer type with no trap
representation, and that `PY_TIME_T_MIN` is within the representable
range of a C double.

Note: we want the `if` condition below to be true for NaNs; therefore,
resist any temptation to simplify by applying De Morgan's laws.
*/
if (!((double)PY_TIME_T_MIN <= intpart && intpart < -(double)PY_TIME_T_MIN)) {
error_time_t_overflow();
return -1;
}
Expand Down Expand Up @@ -210,7 +243,8 @@ _PyTime_ObjectToTime_t(PyObject *obj, time_t *sec, _PyTime_round_t round)
d = _PyTime_Round(d, round);
(void)modf(d, &intpart);

if (!_Py_InIntegralTypeRange(time_t, intpart)) {
/* See comments in _PyTime_DoubleToDenominator */
if (!((double)PY_TIME_T_MIN <= intpart && intpart < -(double)PY_TIME_T_MIN)) {
error_time_t_overflow();
return -1;
}
Expand Down Expand Up @@ -395,7 +429,8 @@ _PyTime_FromDouble(_PyTime_t *t, double value, _PyTime_round_t round,
d *= (double)unit_to_ns;
d = _PyTime_Round(d, round);

if (!_Py_InIntegralTypeRange(_PyTime_t, d)) {
/* See comments in _PyTime_DoubleToDenominator */
if (!((double)_PyTime_MIN <= d && d < -(double)_PyTime_MIN)) {
_PyTime_overflow();
return -1;
}
Expand Down Expand Up @@ -722,7 +757,9 @@ py_get_system_clock(_PyTime_t *tp, _Py_clock_info_t *info, int raise)
info->monotonic = 0;
info->adjustable = 1;
if (clock_getres(CLOCK_REALTIME, &res) == 0) {
info->resolution = res.tv_sec + res.tv_nsec * 1e-9;
/* the explicit (double) casts silence loss-of-precision warnings
on some platforms */
info->resolution = (double)res.tv_sec + (double)res.tv_nsec * 1e-9;
}
else {
info->resolution = 1e-9;
Expand Down