src/chage.c: Unify long overflow checks in print_day_as_date()

The conversion from day to seconds can be done in print_date
(renamed to print_day_as_date for clarification).  This has the nice
benefit that DAY multiplication and long to time_t conversion are done
at just one place.

Co-developed-by: Tobias Stoeckmann <tobias@stoeckmann.org>
Co-developed-by: Alejandro Colomar <alx@kernel.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
Cherry-picked-from: 20100e4b22 ("src/chage.c: Unify long overflow checks in print_day_as_date()")
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Link: <https://github.com/shadow-maint/shadow/pull/876>
[alx: This is a pre-requisite for 674409e226 ("lib/: Saturate addition to avoid overflow")]
Signed-off-by: Alejandro Colomar <alx@kernel.org>

src/chage.c

@@ -72,7 +72,7 @@ static long expdate;
 /* local function prototypes */
 NORETURN static void usage (int status);
 static int new_fields (void);
-static void print_date (time_t date);
+static void print_day_as_date (long day);
 static void list_fields (void);
 static void process_flags (int argc, char **argv);
 static void check_flags (int argc, int opt_index);
@@ -227,10 +227,22 @@ static int new_fields (void)
 	return 1;
 }
 
-static void print_date (time_t date)
+
+static void
+print_day_as_date(long day)
 {
-	struct tm *tp;
 	char       buf[80];
+	time_t     date;
+	struct tm  *tp;
+
+	if (day < 0) {
+		puts(_("never"));
+		return;
+	}
+	if (__builtin_mul_overflow(day, DAY, &date)) {
+		puts(_("future"));
+		return;
+	}
 
 	tp = gmtime (&date);
 	if (NULL == tp) {
@@ -241,6 +253,7 @@ static void print_date (time_t date)
 	}
 }
 
+
 /*
  * list_fields - display the current values of the expiration fields
  *
@@ -250,21 +263,15 @@ static void print_date (time_t date)
  */
 static void list_fields (void)
 {
-	long changed = 0;
-	long expires;
-
 	/*
 	 * The "last change" date is either "never" or the date the password
 	 * was last modified. The date is the number of days since 1/1/1970.
 	 */
 	(void) fputs (_("Last password change\t\t\t\t\t: "), stdout);
-	if (lstchgdate < 0 || lstchgdate > LONG_MAX / DAY) {
+	if (lstchgdate == 0) {
-		(void) puts (_("never"));
-	} else if (lstchgdate == 0) {
 		(void) puts (_("password must be changed"));
 	} else {
-		changed = lstchgdate * DAY;
+		print_day_as_date(lstchgdate);
-		print_date (changed);
 	}
 
 	/*
@@ -277,11 +284,11 @@ static void list_fields (void)
 	} else if (   (lstchgdate < 0)
 	           || (maxdays >= 10000)
 	           || (maxdays < 0)
-	           || ((LONG_MAX - changed) / DAY < maxdays)) {
+	           || (LONG_MAX - lstchgdate < maxdays))
+	{
 		(void) puts (_("never"));
 	} else {
-		expires = changed + maxdays * DAY;
+		print_day_as_date(lstchgdate + maxdays);
-		print_date (expires);
 	}
 
 	/*
@@ -297,12 +304,12 @@ static void list_fields (void)
 	           || (inactdays < 0)
 	           || (maxdays >= 10000)
 	           || (maxdays < 0)
-	           || (maxdays > LONG_MAX - inactdays)
+	           || (LONG_MAX - inactdays < maxdays)
-	           || ((LONG_MAX - changed) / DAY < maxdays + inactdays)) {
+	           || (LONG_MAX - lstchgdate < maxdays + inactdays))
+	{
 		(void) puts (_("never"));
 	} else {
-		expires = changed + (maxdays + inactdays) * DAY;
+		print_day_as_date(lstchgdate + maxdays + inactdays);
-		print_date (expires);
 	}
 
 	/*
@@ -310,12 +317,7 @@ static void list_fields (void)
 	 * password expiring or not.
 	 */
 	(void) fputs (_("Account expires\t\t\t\t\t\t: "), stdout);
-	if (expdate < 0 || LONG_MAX / DAY < expdate) {
+	print_day_as_date(expdate);
-		(void) puts (_("never"));
-	} else {
-		expires = expdate * DAY;
-		print_date (expires);
-	}
 
 	/*
 	 * Start with the easy numbers - the number of days before the