From 64a4bf0e28fdebda2e82652208d9eb3c9db728e3 Mon Sep 17 00:00:00 2001
From: "Yukihiro \"Matz\" Matsumoto" <matz@ruby.or.jp>
Date: Tue, 9 Jan 2024 10:19:37 +0900
Subject: [PATCH] string.c: avoid infinite loop; fix #6143

`mrb_utf8len` returns 0 for invalid characters.
---
 src/string.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/string.c b/src/string.c
index 47a6ebfe0..5c770ec11 100644
--- a/src/string.c
+++ b/src/string.c
@@ -409,7 +409,10 @@ str_index_str_by_char_search(mrb_state *mrb, const char *p, const char *pend, co
     if (pivot >= pend || pivot < p /* overflowed */) { return -1; }
 
     do {
-      p += mrb_utf8len(p, pend);
+      mrb_int len = mrb_utf8len(p, pend);
+
+      if (len == 0) p++;      /* invalid character */
+      else p += len;
       off++;
     } while (p < pivot);
   }