malloc flags: even more strict 'S'

[Otto Moerbeek]

Hi,

at the cost of some speed, reduce the malloc cache size to 0 with
flag 'S'.  This means that pages that become free will be unmapped asap.
This detects more use-after-free bugs. The slowdown is because of more
unmap/mmap calls. 

ok?

        -Otto

Index: malloc.c
===================================================================
RCS file: /cvs/src/lib/libc/stdlib/malloc.c,v
retrieving revision 1.138
diff -u -p -r1.138 malloc.c
--- malloc.c    20 Jun 2011 18:04:06 -0000      1.138
+++ malloc.c    12 Jul 2011 11:18:41 -0000
@@ -68,6 +68,8 @@
 #define MALLOC_MAXCACHE                256
 #define MALLOC_DELAYED_CHUNKS  15      /* max of getrnibble() */
 #define MALLOC_INITIAL_REGIONS 512
+#define MALLOC_DEFAULT_CACHE   64
+
 /*
  * When the P option is active, we move allocations between half a page
  * and a whole page towards the end, subject to alignment constraints.
@@ -461,7 +463,7 @@ omalloc_init(struct dir_info **dp)
         */
        mopts.malloc_abort = 1;
        mopts.malloc_move = 1;
-       mopts.malloc_cache = 64;
+       mopts.malloc_cache = MALLOC_DEFAULT_CACHE;
 
        for (i = 0; i < 3; i++) {
                switch (i) {
@@ -551,10 +553,12 @@ omalloc_init(struct dir_info **dp)
                        case 's':
                                mopts.malloc_freeprot = mopts.malloc_junk = 0;
                                mopts.malloc_guard = 0;
+                               mopts.malloc_cache = MALLOC_DEFAULT_CACHE;
                                break;
                        case 'S':
                                mopts.malloc_freeprot = mopts.malloc_junk = 1;
                                mopts.malloc_guard = MALLOC_PAGESIZE;
+                               mopts.malloc_cache = 0;
                                break;
                        case 'x':
                                mopts.malloc_xmalloc = 0;