From f6d39c31d2177549a986d170e192d8351bd571e2 Mon Sep 17 00:00:00 2001
From: Bram Moolenaar <Bram@vim.org>
Date: Tue, 16 Aug 2022 17:50:38 +0100
Subject: [PATCH] patch 9.0.0220: invalid memory access with for loop over NULL
 string

Problem:    Invalid memory access with for loop over NULL string.
Solution:   Make sure mb_ptr2len() consistently returns zero for NUL.
---
 src/globals.h                   |  3 ++-
 src/mbyte.c                     | 21 +++++++++++++--------
 src/testdir/test_eval_stuff.vim | 12 ++++++++++++
 3 files changed, 27 insertions(+), 9 deletions(-)

diff --git a/src/globals.h b/src/globals.h
index 888f6e9..9b40be4 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1033,7 +1033,8 @@ EXTERN vimconv_T output_conv;			// type of output conversion
  * (DBCS).
  * The value is set in mb_init();
  */
-// length of char in bytes, including following composing chars
+// Length of char in bytes, including any following composing chars.
+// NUL has length zero.
 EXTERN int (*mb_ptr2len)(char_u *p) INIT(= latin_ptr2len);
 
 // idem, with limit on string length
diff --git a/src/mbyte.c b/src/mbyte.c
index 3656880..782a7ad 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -1077,24 +1077,28 @@ dbcs_char2bytes(int c, char_u *buf)
 }
 
 /*
- * mb_ptr2len() function pointer.
- * Get byte length of character at "*p" but stop at a NUL.
- * For UTF-8 this includes following composing characters.
- * Returns 0 when *p is NUL.
+ * Get byte length of character at "*p".  Returns zero when "*p" is NUL.
+ * Used for mb_ptr2len() when 'encoding' latin.
  */
     int
 latin_ptr2len(char_u *p)
 {
- return MB_BYTE2LEN(*p);
+    return *p == NUL ? 0 : 1;
 }
 
+/*
+ * Get byte length of character at "*p".  Returns zero when "*p" is NUL.
+ * Used for mb_ptr2len() when 'encoding' DBCS.
+ */
     static int
-dbcs_ptr2len(
-    char_u	*p)
+dbcs_ptr2len(char_u *p)
 {
     int		len;
 
-    // Check if second byte is not missing.
+    if (*p == NUL)
+	return 0;
+
+    // if the second byte is missing the length is 1
     len = MB_BYTE2LEN(*p);
     if (len == 2 && p[1] == NUL)
 	len = 1;
@@ -2105,6 +2109,7 @@ utf_ptr2len_len(char_u *p, int size)
 /*
  * Return the number of bytes the UTF-8 encoding of the character at "p" takes.
  * This includes following composing characters.
+ * Returns zero for NUL.
  */
     int
 utfc_ptr2len(char_u *p)
diff --git a/src/testdir/test_eval_stuff.vim b/src/testdir/test_eval_stuff.vim
index c63082e..313d791 100644
--- a/src/testdir/test_eval_stuff.vim
+++ b/src/testdir/test_eval_stuff.vim
@@ -75,6 +75,18 @@ func Test_for_invalid()
   redraw
 endfunc
 
+func Test_for_over_null_string()
+  let save_enc = &enc
+  set enc=iso8859
+  let cnt = 0
+  for c in test_null_string()
+    let cnt += 1
+  endfor
+  call assert_equal(0, cnt)
+
+  let &enc = save_enc
+endfunc
+
 func Test_readfile_binary()
   new
   call setline(1, ['one', 'two', 'three'])
-- 
2.36.1