Avoid passing NULL to 'memcpy' and 'memcmp'.

Reported by Jeffrey Walton <noloader@gmail.com> in
<https://lists.gnu.org/archive/html/guile-devel/2019-03/msg00001.html>.

Note that C11 section 7.1.4 (Use of library functions) states that:
"unless explicitly stated otherwise in the detailed descriptions [of
library functions] that follow: If an argument to a function has an
invalid value (such as ... a null pointer ...) ..., the behavior is
undefined."  Note that 'strxfrm' is an example of a standard C function
that explicitly states otherwise, allowing NULL to be passed in the
first argument if the size argument is zero, but no similar allowance is
specified for 'memcpy' or 'memcmp'.

* libguile/bytevectors.c (scm_uniform_array_to_bytevector): Call memcpy
only if 'byte_len' is non-zero.
* libguile/srfi-14.c (charsets_equal): Call memcmp only if the number of
ranges is non-zero.
* libguile/stime.c (setzone): Pass 1-character buffer to
'scm_to_locale_stringbuf', instead of NULL.
* libguile/strings.c (scm_to_locale_stringbuf): Call memcpy only if the
number of bytes to copy is non-zero.

1 files changed, 8 insertions, 3 deletions

diff --git a/libguile/strings.c b/libguile/strings.c
index a0a1555f5..b366f5b37 100644
--- a/libguile/strings.c
+++ b/libguile/strings.c
@@ -1,4 +1,4 @@
-/* Copyright 1995-1996,1998,2000-2001,2004,2006,2008-2016,2018
+/* Copyright 1995-1996,1998,2000-2001,2004,2006,2008-2016,2018-2019
      Free Software Foundation, Inc.
 
    This file is part of Guile.
@@ -2288,13 +2288,18 @@ scm_to_stringn (SCM str, size_t *lenp, const char *encoding,
 size_t
 scm_to_locale_stringbuf (SCM str, char *buf, size_t max_len)
 {
-  size_t len;
+  size_t len, copy_len;
   char *result = NULL;
   if (!scm_is_string (str))
     scm_wrong_type_arg_msg (NULL, 0, str, "string");
   result = scm_to_locale_stringn (str, &len);
 
-  memcpy (buf, result, (len > max_len) ? max_len : len);
+  copy_len = (len > max_len) ? max_len : len;
+  if (copy_len != 0)
+    /* Some users of 'scm_to_locale_stringbuf' may pass NULL for buf
+       when max_len is zero, and yet we must avoid passing NULL to
+       memcpy to avoid undefined behavior. */
+    memcpy (buf, result, copy_len);
   free (result);
 
   scm_remember_upto_here_1 (str);