bignum.c (estimate_initial_sqrt): prevent integer overflow

`Integer.sqrt(0xffff_ffff_ffff_ffff ** 2)` caused assertion failure
because of integer overflow.  [ruby-core:95453] [Bug #16269]

1 files changed, 9 insertions, 1 deletions

diff --git a/bignum.c b/bignum.c
index f379b10489..05392325b3 100644
--- a/bignum.c
+++ b/bignum.c
@@ -6888,7 +6888,15 @@ estimate_initial_sqrt(VALUE *xp, const size_t xn, const BDIGIT *nds, size_t len)
     rshift /= 2;
     rshift += (2-(len&1))*BITSPERDIG/2;
     if (rshift >= 0) {
-	d <<= rshift;
+        if (nlz((BDIGIT)d) + rshift >= BITSPERDIG) {
+            /* (d << rshift) does cause overflow.
+             * example: Integer.sqrt(0xffff_ffff_ffff_ffff ** 2)
+             */
+            d = ~(BDIGIT_DBL)0;
+        }
+        else {
+            d <<= rshift;
+        }
     }
     BDIGITS_ZERO(xds, xn-2);
     bdigitdbl2bary(&xds[xn-2], 2, d);