[PATCH v2 3/3] git_parse_signed(): avoid integer overflow

["Phillip Wood via GitGitGadget" <gitgitgadget@gmail.com>]

From: Phillip Wood <phillip.wood@dunelm.org.uk>

git_parse_signed() checks that the absolute value of the parsed string
is less than or equal to a caller supplied maximum value. When
calculating the absolute value there is a integer overflow if `val ==
INTMAX_MIN`. To fix this avoid negating `val` when it is negative by
having separate overflow checks for positive and negative values.

An alternative would be to special case INTMAX_MIN before negating `val`
as it is always out of range. That would enable us to keep the existing
code but I'm not sure that the current two-stage check is any clearer
than the new version.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
---
 config.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/config.c b/config.c
index b7fb68026d8..aad3e00341d 100644
--- a/config.c
+++ b/config.c
@@ -1160,8 +1160,10 @@ static int git_parse_signed(const char *value, intmax_t *ret, intmax_t max)
 	if (value && *value) {
 		char *end;
 		intmax_t val;
-		uintmax_t uval;
-		uintmax_t factor;
+		intmax_t factor;
+
+		if (max < 0)
+			BUG("max must be a positive integer");
 
 		errno = 0;
 		val = strtoimax(value, &end, 0);
@@ -1176,9 +1178,8 @@ static int git_parse_signed(const char *value, intmax_t *ret, intmax_t max)
 			errno = EINVAL;
 			return 0;
 		}
-		uval = val < 0 ? -val : val;
-		if (unsigned_mult_overflows(factor, uval) ||
-		    factor * uval > max) {
+		if ((val < 0 && -max / factor > val) ||
+		    (val > 0 && max / factor < val)) {
 			errno = ERANGE;
 			return 0;
 		}
-- 
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