https://git.ruby-lang.org/ruby.git/commit/?id=87784fdeb2

From 87784fdeb2340574d11887474f6e2d9b0d5d3bc3 Mon Sep 17 00:00:00 2001
From: Peter Zhu <peter@p...>
Date: Mon, 24 Jan 2022 13:38:15 -0500
Subject: Keep right operand within width when right shifting

NUM_IN_PAGE could return a value much larger than 64. According to the
C11 spec 6.5.7 paragraph 3 this is undefined behavior:

> If the value of the right operand is negative or is greater than or
> equal to the width of the promoted left operand, the behavior is
> undefined.

On most platforms, this is usually not a problem as the architecture
will mask off all out-of-range bits.
---
 gc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/gc.c b/gc.c
index 96a99cc81a2..d189c42cde6 100644
--- a/gc.c
+++ b/gc.c
@@ -4956,7 +4956,7 @@ try_move(rb_objspace_t *objspace, rb_heap_t *heap, struct heap_page *sweep_page, https://github.com/ruby/ruby/blob/trunk/gc.c#L4956
 
         bits_t bits = mark_bits[index] & ~pin_bits[index];
 
-        bits >>= NUM_IN_PAGE(p);
+        bits >>= NUM_IN_PAGE(p) % BITS_BITLENGTH;
         if (try_move_plane(objspace, heap, sweep_page, (uintptr_t)p, bits, dest)) return 1;
 
         if (index == 0) {
-- 
cgit v1.2.1


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