config: use unsigned_mult_overflows to check for overflows

parse_unit_factor() checks if a K, M or G is present after a number and
multiplies it by 2^10, 2^20 or 2^30, respectively.  One of its callers
checks if the result is smaller than the number alone to detect
overflows.  The other one passes 1 as the number and does multiplication
and overflow check itself in a similar manner.

This works, but is inconsistent, and it would break if we added support
for a bigger unit factor.  E.g. 16777217T is 2^64 + 2^40, i.e. too big
for a 64-bit number.  Modulo 2^64 we get 2^40 == 1TB, which is bigger
than the raw number 16777217 == 2^24 + 1, so the overflow would go
undetected by that method.

Let both callers pass 1 and handle overflow check and multiplication
themselves.  Do the check before the multiplication, using
unsigned_mult_overflows, which is simpler and can deal with larger unit
factors.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -870,8 +870,8 @@ static int git_parse_signed(const char *value, intmax_t *ret, intmax_t max)
 			return 0;
 		}
 		uval = val < 0 ? -val : val;
-		uval *= factor;
-		if (uval > max || (val < 0 ? -val : val) > uval) {
+		if (unsigned_mult_overflows(factor, uval) ||
+		    factor * uval > max) {
 			errno = ERANGE;
 			return 0;
 		}
@@ -888,21 +888,22 @@ static int git_parse_unsigned(const char *value, uintmax_t *ret, uintmax_t max)
 	if (value && *value) {
 		char *end;
 		uintmax_t val;
-		uintmax_t oldval;
+		uintmax_t factor = 1;
 
 		errno = 0;
 		val = strtoumax(value, &end, 0);
 		if (errno == ERANGE)
 			return 0;
-		oldval = val;
-		if (!parse_unit_factor(end, &val)) {
+		if (!parse_unit_factor(end, &factor)) {
 			errno = EINVAL;
 			return 0;
 		}
-		if (val > max || oldval > val) {
+		if (unsigned_mult_overflows(factor, val) ||
+		    factor * val > max) {
 			errno = ERANGE;
 			return 0;
 		}
+		val *= factor;
 		*ret = val;
 		return 1;
 	}