log: handle integer overflow in timestamps

If an ident line has a ridiculous date value like (2^64)+1,
we currently just pass ULONG_MAX along to the date code,
which can produce nonsensical dates.

On systems with a signed long time_t (e.g., 64-bit glibc
systems), this actually doesn't end up too bad. The
ULONG_MAX is converted to -1, we apply the timezone field to
that, and the result ends up somewhere between Dec 31, 1969
and Jan 1, 1970.

However, there is still a few good reasons to detect the
overflow explicitly:

  1. On systems where "unsigned long" is smaller than
     time_t, we get a nonsensical date in the future.

  2. Even where it would produce "Dec 31, 1969", it's easier
     to recognize "midnight Jan 1" as a consistent sentinel
     value for "we could not parse this".

  3.  Values which do not overflow strtoul but do overflow a
      signed time_t produce nonsensical values in the past.
      For example, on a 64-bit system with a signed long
      time_t, a timestamp of 18446744073000000000 produces a
      date in 1947.

We also recognize overflow in the timezone field, which
could produce nonsensical results. In this case we show the
parsed date, but in UTC.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

pretty.c

@@ -401,8 +401,14 @@ static const char *show_ident_date(const struct ident_split *ident,
 
 	if (ident->date_begin && ident->date_end)
 		date = strtoul(ident->date_begin, NULL, 10);
-	if (ident->tz_begin && ident->tz_end)
-		tz = strtol(ident->tz_begin, NULL, 10);
+	if (date_overflows(date))
+		date = 0;
+	else {
+		if (ident->tz_begin && ident->tz_end)
+			tz = strtol(ident->tz_begin, NULL, 10);
+		if (tz == LONG_MAX || tz == LONG_MIN)
+			tz = 0;
+	}
 	return show_date(date, tz, mode);
 }
 

t/t4212-log-corrupt.sh

@@ -60,4 +60,20 @@ test_expect_success 'unparsable dates produce sentinel value (%ad)' '
 	test_cmp expect actual
 '
 
+# date is 2^64 + 1
+test_expect_success 'date parser recognizes integer overflow' '
+	commit=$(munge_author_date HEAD 18446744073709551617) &&
+	echo "Thu Jan 1 00:00:00 1970 +0000" >expect &&
+	git log -1 --format=%ad $commit >actual &&
+	test_cmp expect actual
+'
+
+# date is 2^64 - 2
+test_expect_success 'date parser recognizes time_t overflow' '
+	commit=$(munge_author_date HEAD 18446744073709551614) &&
+	echo "Thu Jan 1 00:00:00 1970 +0000" >expect &&
+	git log -1 --format=%ad $commit >actual &&
+	test_cmp expect actual
+'
+
 test_done