From b573ef3813baf14b9f4bec403b941b5f5d3e69e8 Mon Sep 17 00:00:00 2001
From: Konstantin Sharlaimov <konstantin.sharlaimov@gmail.com>
Date: Mon, 13 Jul 2026 20:57:04 +0200
Subject: [PATCH] fix(gc): correct old size calculation in block realloc.

Using blocksPerStateByte instead of bytesPerBlock caused the old size to be underestimated for multi-block allocations, leading to data truncation on growing reallocations.
---
 src/runtime/gc_blocks.go | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/runtime/gc_blocks.go b/src/runtime/gc_blocks.go
index 2aadaeaf8..78387cb31 100644
--- a/src/runtime/gc_blocks.go
+++ b/src/runtime/gc_blocks.go
@@ -510,7 +510,7 @@ func realloc(ptr unsafe.Pointer, size uintptr) unsafe.Pointer {
 	lastBlock := firstBlock.findHead()
 
 	// Calculate the size of the original allocation body.
-	oldSize := uintptr(lastBlock-firstBlock)*blocksPerStateByte + (bytesPerBlock - unsafe.Sizeof(objHeader{}))
+	oldSize := uintptr(lastBlock-firstBlock)*bytesPerBlock + (bytesPerBlock - unsafe.Sizeof(objHeader{}))
 
 	if size <= oldSize {
 		// The requested size is less than the old size.