mruby-string-ext: fix String#prepend with self-referencing arguments

String#prepend(s, s) read RSTRING_LEN(argv[i]) in the copy loop after
mrb_str_resize had already updated the receiver's length, causing the
memcpy to write past the allocated buffer.

Detect self-references with mrb_obj_eq() and read from the memmoved
original data at p + total_prepend_len using the captured self_len.
This also handles mixed cases like s.prepend("X", s) where earlier
writes would otherwise corrupt the source of later reads.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-string-ext/src/string.c

@@ -2201,15 +2201,27 @@ str_prepend(mrb_state *mrb, mrb_value self)
 
   char *p = RSTRING_PTR(self);
 
-  /* Move original content to the end */
+  /* Move original content to the end. The original self data now lives
+     at p + total_prepend_len, which we use as the source for any
+     self-referencing arguments (e.g., s.prepend(s, s)) to avoid reading
+     data that has already been overwritten by earlier copies. */
   memmove(p + total_prepend_len, p, self_len);
 
   /* Copy prepended strings in order */
   mrb_int offset = 0;
   for (mrb_int i = 0; i < argc; i++) {
-    mrb_int arg_len = RSTRING_LEN(argv[i]);
+    const char *src;
+    mrb_int arg_len;
+    if (mrb_obj_eq(mrb, self, argv[i])) {
+      src = p + total_prepend_len;
+      arg_len = self_len;
+    }
+    else {
+      src = RSTRING_PTR(argv[i]);
+      arg_len = RSTRING_LEN(argv[i]);
+    }
     if (arg_len > 0) {
-      memcpy(p + offset, RSTRING_PTR(argv[i]), arg_len);
+      memcpy(p + offset, src, arg_len);
       offset += arg_len;
     }
   }

mrbgems/mruby-string-ext/test/string.rb

@@ -492,6 +492,21 @@ assert('String#prepend') do
   g = "world"
   assert_equal "hello world", g.prepend("", "hello ", "")
   assert_equal "hello world", g
+
+  # Self-referencing arguments (GHSA-3hgj-g76g-878c)
+  h = "A" * 100
+  h.prepend(h, h)
+  assert_equal 300, h.length
+  assert_equal "A" * 300, h
+
+  # Mixed self-reference and literal
+  i = "AB"
+  i.prepend("XYZ", i)
+  assert_equal "XYZABAB", i
+
+  j = "AB"
+  j.prepend(j, "X", j)
+  assert_equal "ABXABAB", j
 end
 
 assert('String#ljust') do