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ref: 1c8ba5d798e7676ca4240e73db7fc36cbcfecdbb
parent: fe02e90431559e9440f10983457eb01eece016c0
author: Naveen Narayanan <zerous@simple-cc.org>
date: Tue Sep 29 13:43:17 EDT 2020

libc: Fix bug in mktime.c

norm() decrements tm_min(tm_hour, tm_mday, tm_year) if there is an
overshoot of tm_sec(tm_min,tm_hour,tm_mon) instead of respectively
incrementing tm_min(tm_hour, tm_mday, tm_year).
norm() increments tm_min(tm_hour, tm_mday, tm_year) if there is an
undershoot of tm_sec(tm_min,tm_hour,tm_mon) instead of respectively
decrementing tm_min(tm_hour, tm_mday, tm_year).

normalize() doesn't set the day of the month appropriately in the case
of day < 1 as it doesn't decrement mon prior to that. This happens
when there is an undershoot of tm_sec through tm_hour.

You can reproduce the bug by setting struct tm to the following and
calling mktime:

tim.tm_hour = -2;
tim.tm_mday = 1;
tim.tm_mon = 2;
tim.tm_year = 120;
tim.tm_isdst = 0;

t = mktime(&tim);
assert(t != -1);
assert(tim.tm_hour == 22);
assert(tim.tm_mday == 29);
assert(tim.tm_mon == 1);

--- a/src/libc/time/mktime.c

+++ b/src/libc/time/mktime.c

@@ -14,7 +14,7 @@

 		v += d * qty;

 		if (n > INT_MAX - d)

 			return 0;

-		n += d;

+		n -= d;

 	}

 	if (v >= qty) {

 		d = v / qty;

@@ -21,7 +21,7 @@

 		v -= d * qty;

 		if (n < INT_MIN + d)

 			return 0;

-		n -= d;

+		n += d;

 	}

 	*val = v;

@@ -48,7 +48,6 @@

 	_daysmon[FEB] = FEBDAYS(year);

 	for (mon = tm->tm_mon; day < 1; --mon) {

-		day += _daysmon[mon];

 		if (mon == JAN) {

 			if (year == EPOCH)

 				return 0;

@@ -56,6 +55,7 @@

 			_daysmon[FEB] = FEBDAYS(year);

 			mon = DEC+1;

 		}

+		day += _daysmon[mon-1];

 	}

 	for (; day > _daysmon[mon]; ++mon) {