https://git.ruby-lang.org/ruby.git/commit/?id=f36859869f

From f36859869f31a4cf10805cc620ee6338b66558d1 Mon Sep 17 00:00:00 2001
From: Peter Zhu <peter@p...>
Date: Wed, 6 Jul 2022 17:30:36 -0400
Subject: Improve error message for segv in read_barrier_handler

If the page_body is a null pointer, then read_barrier_handler will
crash with an unrelated message. This commit improves the error message.

Before:

test.rb:1: [BUG] Couldn't unprotect page 0x0000000000000000, errno: Cannot allocate memory

After:

test.rb:1: [BUG] read_barrier_handler: segmentation fault at 0x14
---
 gc.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/gc.c b/gc.c
index 1ef9cab171..7197e81061 100644
--- a/gc.c
+++ b/gc.c
@@ -5210,18 +5210,27 @@ static void invalidate_moved_page(rb_objspace_t *objspace, struct heap_page *pag https://github.com/ruby/ruby/blob/trunk/gc.c#L5210
 
 #if GC_CAN_COMPILE_COMPACTION
 static void
-read_barrier_handler(uintptr_t address)
+read_barrier_handler(uintptr_t original_address)
 {
     VALUE obj;
     rb_objspace_t * objspace = &rb_objspace;
 
-    address -= address % BASE_SLOT_SIZE;
+    /* Calculate address aligned to slots. */
+    uintptr_t address = original_address - (original_address % BASE_SLOT_SIZE);
 
     obj = (VALUE)address;
 
+    struct heap_page_body *page_body = GET_PAGE_BODY(obj);
+
+    /* If the page_body is NULL, then mprotect cannot handle it and will crash
+     * with "Cannot allocate memory". */
+    if (page_body == NULL) {
+        rb_bug("read_barrier_handler: segmentation fault at 0x%lx", original_address);
+    }
+
     RB_VM_LOCK_ENTER();
     {
-        unlock_page_body(objspace, GET_PAGE_BODY(obj));
+        unlock_page_body(objspace, page_body);
 
         objspace->profile.read_barrier_faults++;
 
-- 
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