Add nowrap u64 multiplication function

Follows #30920

Signed-off-by: José M. Guisado <guigom@riseup.net>

2 files changed, 16 insertions, 0 deletions

diff --git a/src/lib/intmath/muldiv.c b/src/lib/intmath/muldiv.c
index 6a292db7ba..3330a4c569 100644
--- a/src/lib/intmath/muldiv.c
+++ b/src/lib/intmath/muldiv.c
@@ -69,6 +69,20 @@ gcd64(uint64_t a, uint64_t b)
   return a;
 }
 
+/** Return the unsigned integer product of <b>a</b> and <b>b</b>, if overflow
+ * is detected return UINT64_MAX instead. */
+uint64_t
+tor_mul_u64_nowrap(uint64_t a, uint64_t b)
+{
+  if (a == 0 || b == 0) {
+    return 0;
+  } else if (PREDICT_UNLIKELY(UINT64_MAX / a < b)) {
+    return UINT64_MAX;
+  } else {
+    return a*b;
+  }
+}
+
 /* Given a fraction *<b>numer</b> / *<b>denom</b>, simplify it.
  * Requires that the denominator is greater than 0. */
 void
diff --git a/src/lib/intmath/muldiv.h b/src/lib/intmath/muldiv.h
index 64500b6dce..7aa0f9b235 100644
--- a/src/lib/intmath/muldiv.h
+++ b/src/lib/intmath/muldiv.h
@@ -18,6 +18,8 @@ unsigned round_to_next_multiple_of(unsigned number, unsigned divisor);
 uint32_t round_uint32_to_next_multiple_of(uint32_t number, uint32_t divisor);
 uint64_t round_uint64_to_next_multiple_of(uint64_t number, uint64_t divisor);
 
+uint64_t tor_mul_u64_nowrap(uint64_t a, uint64_t b);
+
 void simplify_fraction64(uint64_t *numer, uint64_t *denom);
 
 /* Compute the CEIL of <b>a</b> divided by <b>b</b>, for nonnegative <b>a</b>