mysqld.cc:

  BUG #5731 key_buffer_size not properly restricted to 4GB; use UINT_MAX32 for clarity.

sql/mysqld.cc

@@ -4144,7 +4144,7 @@ replicating a LOAD DATA INFILE command",
   {"key_buffer_size", OPT_KEY_BUFFER_SIZE,
    "The size of the buffer used for index blocks. Increase this to get better index handling (for all reads and multiple writes) to as much as you can afford; 64M on a 256M machine that mainly runs MySQL is quite common.",
    (gptr*) &keybuff_size, (gptr*) &keybuff_size, 0, GET_ULL,
-   REQUIRED_ARG, KEY_CACHE_SIZE, MALLOC_OVERHEAD, (uint32) ~0, MALLOC_OVERHEAD,
+   REQUIRED_ARG, KEY_CACHE_SIZE, MALLOC_OVERHEAD, UINT_MAX32, MALLOC_OVERHEAD,
    IO_SIZE, 0},
   {"long_query_time", OPT_LONG_QUERY_TIME,
    "Log all queries that have taken more than long_query_time seconds to execute to file.",