Prevent potential buffer overrun in onigmo

A code pattern `p + enclen(enc, p, pend)` may lead to a buffer overrun
if incomplete bytes of a UTF-8 character is placed at the end of a
string. Because this pattern is used in several places in onigmo,
this change fixes the issue in the side of `enclen`: the function should
not return a number that is larger than `pend - p`.

Co-Authored-By: Nobuyoshi Nakada <nobu@ruby-lang.org>

include/ruby/onigmo.h

@@ -356,9 +356,9 @@ int onigenc_ascii_only_case_map(OnigCaseFoldType* flagP, const OnigUChar** pp, c
 #define ONIGENC_PRECISE_MBC_ENC_LEN(enc,p,e)   (enc)->precise_mbc_enc_len(p,e,enc)
 
 ONIG_EXTERN
-int onigenc_mbclen_approximate(const OnigUChar* p,const OnigUChar* e, const struct OnigEncodingTypeST* enc);
+int onigenc_mbclen(const OnigUChar* p,const OnigUChar* e, const struct OnigEncodingTypeST* enc);
 
-#define ONIGENC_MBC_ENC_LEN(enc,p,e)           onigenc_mbclen_approximate(p,e,enc)
+#define ONIGENC_MBC_ENC_LEN(enc,p,e)           onigenc_mbclen(p,e,enc)
 #define ONIGENC_MBC_MAXLEN(enc)               ((enc)->max_enc_len)
 #define ONIGENC_MBC_MAXLEN_DIST(enc)           ONIGENC_MBC_MAXLEN(enc)
 #define ONIGENC_MBC_MINLEN(enc)               ((enc)->min_enc_len)

regenc.c

@@ -51,6 +51,21 @@ onigenc_set_default_encoding(OnigEncoding enc)
   return 0;
 }
 
+extern int
+onigenc_mbclen(const OnigUChar* p,const OnigUChar* e, OnigEncoding enc)
+{
+  int ret = ONIGENC_PRECISE_MBC_ENC_LEN(enc, p, e);
+  if (ONIGENC_MBCLEN_CHARFOUND_P(ret)) {
+    ret = ONIGENC_MBCLEN_CHARFOUND_LEN(ret);
+    if (ret > (int)(e - p)) ret = (int)(e - p); // just for case
+    return ret;
+  }
+  else if (ONIGENC_MBCLEN_NEEDMORE_P(ret)) {
+    return (int)(e - p);
+  }
+  return p < e ? 1 : 0;
+}
+
 extern int
 onigenc_mbclen_approximate(const OnigUChar* p,const OnigUChar* e, OnigEncoding enc)
 {

regparse.c

@@ -3799,7 +3799,6 @@ fetch_token(OnigToken* tok, UChar** src, UChar* end, ScanEnv* env)
 	}
 	else { /* string */
 	  p = tok->backp + enclen(enc, tok->backp, end);
-	  if (p > end) return ONIGERR_END_PATTERN_AT_ESCAPE;
 	}
       }
       break;