[PATCH 5/5] date: avoid overflow in approxidate relative-time multiplication

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

The `approxidate_alpha()` function multiplies a time unit length (e.g.,
604800 for weeks) by a user-supplied number to compute a time offset in
seconds. Both operands are `int`, and the product is passed to
`update_tm()` as `time_t`. For inputs like "99999 weeks ago", the int
multiplication overflows (604800 * 99999 is roughly 60 billion, far
beyond `INT_MAX`), producing a wrapped value that results in a wrong
date.

Use the checked `time_t` multiplication so oversized products do not
overflow or wrap around.

On platforms where `time_t` is unsigned (and where the pre-processor
allows us to determine this fact), reject counts that are too large and
report a parse error. Narrowing them first could otherwise produce
negative or truncated values that become bogus offsets. Leave signed
`time_t` count parsing unchanged.

Pointed out by Coverity.

Assisted-by: Claude Opus 4.6 and GPT-6.1 Sol
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 date.c | 26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/date.c b/date.c
index 014065b419..0d65804d2f 100644
--- a/date.c
+++ b/date.c
@@ -1298,7 +1298,18 @@ static const char *approxidate_alpha(const char *date, struct tm *tm, struct tm
 	while (tl->type) {
 		size_t len = strlen(tl->type);
 		if (match_string(date, tl->type) >= len-1) {
-			update_tm(tm, now, tl->length * *num);
+			time_t length = tl->length, num_t = *num;
+			time_t seconds;
+#ifndef is_unsigned_type
+			seconds = length * num_t;
+#else
+			time_t max_val = is_unsigned_type(length)
+				? maximum_unsigned_value_of_type(length)
+				: maximum_signed_value_of_type(length);
+			seconds = mult_overflows(length, num_t)
+				? max_val : length * num_t;
+#endif
+			update_tm(tm, now, seconds);
 			*num = 0;
 			*touched = 1;
 			return end;
@@ -1368,8 +1379,15 @@ static const char *approxidate_digit(const char *date, struct tm *tm, int *num,
 	}
 
 	/* Accept zero-padding only for small numbers ("Dec 02", never "Dec 0002") */
-	if (date[0] != '0' || end - date <= 2)
+	if (date[0] != '0' || end - date <= 2) {
+#ifdef is_unsigned_type
+		if (is_unsigned_type((time_t)0) &&
+		    (number > INT_MAX ||
+		     number > maximum_unsigned_value_of_type((time_t)0)))
+			return NULL;
+#endif
 		*num = number;
+	}
 	return end;
 }
 
@@ -1398,6 +1416,10 @@ static timestamp_t approxidate_str(const char *date,
 		if (isdigit(c)) {
 			pending_number(&tm, &number);
 			date = approxidate_digit(date-1, &tm, &number, time_sec);
+			if (!date) {
+				*error_ret = 1;
+				return 0;
+			}
 			touched = 1;
 			continue;
 		}
-- 
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