From 087e4ed6cc9da9cfca1a107058905446ff474bd1 Mon Sep 17 00:00:00 2001
From: nagachika <nagachika@ruby-lang.org>
Date: Fri, 18 Oct 2024 13:03:48 +0900
Subject: merge revision(s) 35e124832e29b65c84d4e0e4e434616859f9bdf5: [Backport
 #20755]

	[Bug #20755] Frozen string should not be writable via IO::Buffer
---
 io_buffer.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

(limited to 'io_buffer.c')

diff --git a/io_buffer.c b/io_buffer.c
index c276838a3b..87b51c0b8c 100644
--- a/io_buffer.c
+++ b/io_buffer.c
@@ -1340,7 +1340,8 @@ rb_io_buffer_get_bytes(VALUE self, void **base, size_t *size)
 static inline void
 io_buffer_get_bytes_for_writing(struct rb_io_buffer *buffer, void **base, size_t *size)
 {
-    if (buffer->flags & RB_IO_BUFFER_READONLY) {
+    if (buffer->flags & RB_IO_BUFFER_READONLY ||
+        (!NIL_P(buffer->source) && OBJ_FROZEN(buffer->source))) {
         rb_raise(rb_eIOBufferAccessError, "Buffer is not writable!");
     }
 
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