Re: out of range struct tm fields in strftime

[Szabolcs Nagy <nsz@port70.net>]

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* Szabolcs Nagy <nsz@port70.net> [2015-09-20 18:44:35 +0200]:
> * Rich Felker <dalias@libc.org> [2015-09-20 12:36:29 -0400]:
> > On Sun, Sep 20, 2015 at 02:44:50PM +0200, Szabolcs Nagy wrote:
> > > 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
> > 
> > Wouldn't it be less invasive to just make some small changes like
> > putting a U on some of the constants so that the arithmetic happens as
> > unsigned?
> > 
> 
> that might be simpler (and probably generates better
> code for div,mod by const)
> 
> but for the nl_langinfo item computation the range
> has to be limited properly (tm_wday and tm_mon are
> affected).

implemented this approach with wday%7U and using some 0U+

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From e8891b06e0c698c0334f08e996f3b0d733f8ede7 Mon Sep 17 00:00:00 2001
From: Szabolcs Nagy <nsz@port70.net>
Date: Sun, 20 Sep 2015 19:41:23 +0000
Subject: [PATCH] fix strftime to handle out of range tm fields without UB

strftime returns unspecifed result with out of range tm fields, but
it should not invoke undefined behaviour.

tm_wday, tm_yday, tm_mon and tm_year fields were used in signed int
arithmetics that could overflow.
---
 src/time/strftime.c | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/src/time/strftime.c b/src/time/strftime.c
index e945bb7..a1db37c 100644
--- a/src/time/strftime.c
+++ b/src/time/strftime.c
@@ -21,24 +21,24 @@ static int is_leap(int y)
 
 static int week_num(const struct tm *tm)
 {
-	int val = (tm->tm_yday + 7 - (tm->tm_wday+6)%7) / 7;
+	int val = (tm->tm_yday + 7U - (tm->tm_wday+6U)%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 ((0U + tm->tm_wday - tm->tm_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;
+		int dec31 = (0U + tm->tm_wday - tm->tm_yday - 1 + 7) % 7;
 		if (dec31 == 4 || (dec31 == 5 && is_leap(tm->tm_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;
+		int jan1 = (0U + tm->tm_wday - tm->tm_yday + 371) % 7;
 		if (jan1 != 4 && (jan1 != 3 || !is_leap(tm->tm_year)))
 			val = 1;
 	}
@@ -57,17 +57,17 @@ const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *
 
 	switch (f) {
 	case 'a':
-		item = ABDAY_1 + tm->tm_wday;
+		item = ABDAY_1 + tm->tm_wday%7U;
 		goto nl_strcat;
 	case 'A':
-		item = DAY_1 + tm->tm_wday;
+		item = DAY_1 + tm->tm_wday%7U;
 		goto nl_strcat;
 	case 'h':
 	case 'b':
-		item = ABMON_1 + tm->tm_mon;
+		item = ABMON_1 + tm->tm_mon%12U;
 		goto nl_strcat;
 	case 'B':
-		item = MON_1 + tm->tm_mon;
+		item = MON_1 + tm->tm_mon%12U;
 		goto nl_strcat;
 	case 'c':
 		item = D_T_FMT;
@@ -143,10 +143,10 @@ const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *
 		width = 1;
 		goto number;
 	case 'U':
-		val = (tm->tm_yday + 7 - tm->tm_wday) / 7;
+		val = (tm->tm_yday + 7U - tm->tm_wday) / 7;
 		goto number;
 	case 'W':
-		val = (tm->tm_yday + 7 - (tm->tm_wday+6)%7) / 7;
+		val = (tm->tm_yday + 7U - (tm->tm_wday+6U)%7) / 7;
 		goto number;
 	case 'V':
 		val = week_num(tm);
@@ -165,7 +165,7 @@ 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