out of range struct tm fields in strftime

[Szabolcs Nagy <nsz@port70.net>]

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out of range tm fields should not be treated as ub
as noted in the thread
http://sourceware.org/ml/libc-alpha/2015-09/msg00546.html

i have a patch but there might be simpler approaches

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From 042305a745991d2de8e7fd0111f158086d283f35 Mon Sep 17 00:00:00 2001
From: Szabolcs Nagy <nsz@port70.net>
Date: Sun, 20 Sep 2015 12:02:21 +0000
Subject: [PATCH] fix strftime to handle out of range tm fields without UB

Calling strftime with out of range tm fields is not undefined
behaviour, it should return a result (though in this case the stored
string is unspecified).

tm_wday, tm_yday, tm_mon and tm_year fields are used in signed int
arithmetics that may overflow. For tm_year long long arithmetics is
used and the rest is fixed by limiting the input.
---
 src/time/strftime.c | 46 +++++++++++++++++++++++++---------------------
 1 file changed, 25 insertions(+), 21 deletions(-)

diff --git a/src/time/strftime.c b/src/time/strftime.c
index e945bb7..464b546 100644
--- a/src/time/strftime.c
+++ b/src/time/strftime.c
@@ -19,27 +19,27 @@ static int is_leap(int y)
 	return !(y%4) && ((y%100) || !(y%400));
 }
 
-static int week_num(const struct tm *tm)
+static int week_num(int wday, int yday, int year)
 {
-	int val = (tm->tm_yday + 7 - (tm->tm_wday+6)%7) / 7;
+	int val = (yday + 7 - (wday+6)%7) / 7;
 	/* If 1 Jan is just 1-3 days past Monday,
 	 * the previous week is also in this year. */
-	if ((tm->tm_wday - tm->tm_yday - 2 + 371) % 7 <= 2)
+	if ((wday - yday - 2 + 371) % 7 <= 2)
 		val++;
 	if (!val) {
 		val = 52;
 		/* If 31 December of prev year a Thursday,
 		 * or Friday of a leap year, then the
 		 * prev year has 53 weeks. */
-		int dec31 = (tm->tm_wday - tm->tm_yday - 1 + 7) % 7;
-		if (dec31 == 4 || (dec31 == 5 && is_leap(tm->tm_year%400-1)))
+		int dec31 = (wday - yday - 1 + 7) % 7;
+		if (dec31 == 4 || (dec31 == 5 && is_leap(year%400-1)))
 			val++;
 	} else if (val == 53) {
 		/* If 1 January is not a Thursday, and not
 		 * a Wednesday of a leap year, then this
 		 * year has only 52 weeks. */
-		int jan1 = (tm->tm_wday - tm->tm_yday + 371) % 7;
-		if (jan1 != 4 && (jan1 != 3 || !is_leap(tm->tm_year)))
+		int jan1 = (wday - yday + 371) % 7;
+		if (jan1 != 4 && (jan1 != 3 || !is_leap(year)))
 			val = 1;
 	}
 	return val;
@@ -55,19 +55,23 @@ const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *
 	const char *fmt;
 	int width = 2;
 
+	int wday = tm->tm_wday < 7U ? tm->tm_wday : 0;
+	int yday = tm->tm_yday < 366U ? tm->tm_yday : 0;
+	int mon = tm->tm_mon < 12U ? tm->tm_mon : 0;
+
 	switch (f) {
 	case 'a':
-		item = ABDAY_1 + tm->tm_wday;
+		item = ABDAY_1 + wday;
 		goto nl_strcat;
 	case 'A':
-		item = DAY_1 + tm->tm_wday;
+		item = DAY_1 + wday;
 		goto nl_strcat;
 	case 'h':
 	case 'b':
-		item = ABMON_1 + tm->tm_mon;
+		item = ABMON_1 + mon;
 		goto nl_strcat;
 	case 'B':
-		item = MON_1 + tm->tm_mon;
+		item = MON_1 + mon;
 		goto nl_strcat;
 	case 'c':
 		item = D_T_FMT;
@@ -90,8 +94,8 @@ const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *
 	case 'g':
 	case 'G':
 		val = tm->tm_year + 1900LL;
-		if (tm->tm_yday < 3 && week_num(tm) != 1) val--;
-		else if (tm->tm_yday > 360 && week_num(tm) == 1) val++;
+		if (yday < 3 && week_num(wday, yday, tm->tm_year) != 1) val--;
+		else if (yday > 360 && week_num(wday, yday, tm->tm_year) == 1) val++;
 		if (f=='g') val %= 100;
 		else width = 4;
 		goto number;
@@ -104,11 +108,11 @@ const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *
 		else if (val > 12) val -= 12;
 		goto number;
 	case 'j':
-		val = tm->tm_yday+1;
+		val = yday+1;
 		width = 3;
 		goto number;
 	case 'm':
-		val = tm->tm_mon+1;
+		val = mon+1;
 		goto number;
 	case 'M':
 		val = tm->tm_min;
@@ -139,20 +143,20 @@ const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *
 		fmt = "%H:%M:%S";
 		goto recu_strftime;
 	case 'u':
-		val = tm->tm_wday ? tm->tm_wday : 7;
+		val = wday ? wday : 7;
 		width = 1;
 		goto number;
 	case 'U':
-		val = (tm->tm_yday + 7 - tm->tm_wday) / 7;
+		val = (yday + 7 - wday) / 7;
 		goto number;
 	case 'W':
-		val = (tm->tm_yday + 7 - (tm->tm_wday+6)%7) / 7;
+		val = (yday + 7 - (wday+6)%7) / 7;
 		goto number;
 	case 'V':
-		val = week_num(tm);
+		val = week_num(wday, yday, tm->tm_year);
 		goto number;
 	case 'w':
-		val = tm->tm_wday;
+		val = wday;
 		width = 1;
 		goto number;
 	case 'x':
@@ -165,7 +169,7 @@ const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *
 		val = tm->tm_year % 100;
 		goto number;
 	case 'Y':
-		val = tm->tm_year + 1900;
+		val = tm->tm_year + 1900LL;
 		if (val >= 10000) {
 			*l = snprintf(*s, sizeof *s, "+%lld", val);
 			return *s;
-- 
2.4.1