mathoms.c: Make safer a deprecated function

This function is deprecated because it is lacking a length parameter, so
malformed UTF-8 may cause it to read beyond the buffer.  This commit
causes it to not read beyond a NUL character, which makes it safe for
the common case that the input is a C string.

1 files changed, 3 insertions, 2 deletions

diff --git a/mathoms.c b/mathoms.c
index ed466c2c71..59d8aeacf7 100644
--- a/mathoms.c
+++ b/mathoms.c
@@ -1660,8 +1660,9 @@ Perl_is_utf8_char(const U8 *s)
 {
     PERL_ARGS_ASSERT_IS_UTF8_CHAR;
 
-    /* Assumes we have enough space, which is why this is deprecated */
-    return isUTF8_CHAR(s, s + UTF8SKIP(s));
+    /* Assumes we have enough space, which is why this is deprecated.  But the
+     * strnlen() makes it safe for the common case of NUL-terminated strings */
+    return isUTF8_CHAR(s, s + strnlen((char *) s, UTF8SKIP(s)));
 }
 
 /*