Clean up format_hex_ex()

Fix a potential null-pointer dereference, and make the code a bit more
readable while doing so.

The NULL dereference could not be triggered, because the current code
never called format_hex_ex() with maxouput == 0 and separator == NULL.
But it's nicer to not depend on that.

Our use of int vs size_t for lengths needs some attention too, but I'm
not pulling that into this patch.  Instead I decided to just make the
(previously existing) assumption that INT_MAX <= SIZE_MAX explicit by
adding a static_assert().

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1480343200-25908-1-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13259.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

diff --git a/src/openvpn/buffer.c b/src/openvpn/buffer.c
index 52c6ab92f8d6db093b38861719b31103fa6bb058..5341d35e25f5c226153aeb62388ba7ce740a4b54 100644 (file)
--- a/src/openvpn/buffer.c
+++ b/src/openvpn/buffer.c
@@ -438,13 +438,16 @@ format_hex_ex (const uint8_t *data, int size, int maxoutput,
               unsigned int space_break_flags, const char* separator,
               struct gc_arena *gc)
 {
-  struct buffer out = alloc_buf_gc (maxoutput ? maxoutput :
-                                   ((size * 2) + (size / (space_break_flags & FHE_SPACE_BREAK_MASK)) * (int) strlen (separator) + 2),
-                                   gc);
-  int i;
-  for (i = 0; i < size; ++i)
+  const size_t bytes_per_hexblock = space_break_flags & FHE_SPACE_BREAK_MASK;
+  const size_t separator_len = separator ? strlen (separator) : 0;
+  static_assert (INT_MAX <= SIZE_MAX, "Code assumes INT_MAX <= SIZE_MAX");
+  const size_t out_len = maxoutput > 0 ? maxoutput :
+           ((size * 2) + ((size / bytes_per_hexblock) * separator_len) + 2);
+
+  struct buffer out = alloc_buf_gc (out_len, gc);
+  for (int i = 0; i < size; ++i)
     {
-      if (separator && i && !(i % (space_break_flags & FHE_SPACE_BREAK_MASK)))
+      if (separator && i && !(i % bytes_per_hexblock))
        buf_printf (&out, "%s", separator);
       if (space_break_flags & FHE_CAPS)
        buf_printf (&out, "%02X", data[i]);