LXR linux/net/netfilter/xt

   1/*
   2 * Today's hack: quantum tunneling in structs
   3 *
   4 * 'entries' and 'term' are never anywhere referenced by word in code. In fact,
   5 * they serve as the hanging-off data accessed through repl.data[].
   6 */
   7
   8/* tbl has the following structure equivalent, but is C99 compliant:
   9 * struct {
  10 *      struct type##_replace repl;
  11 *      struct type##_standard entries[nhooks];
  12 *      struct type##_error term;
  13 * } *tbl;
  14 */
  15
  16#define xt_alloc_initial_table(type, typ2) ({ \
  17        unsigned int hook_mask = info->valid_hooks; \
  18        unsigned int nhooks = hweight32(hook_mask); \
  19        unsigned int bytes = 0, hooknum = 0, i = 0; \
  20        struct { \
  21                struct type##_replace repl; \
  22                struct type##_standard entries[]; \
  23        } *tbl; \
  24        struct type##_error *term; \
  25        size_t term_offset = (offsetof(typeof(*tbl), entries[nhooks]) + \
  26                __alignof__(*term) - 1) & ~(__alignof__(*term) - 1); \
  27        tbl = kzalloc(term_offset + sizeof(*term), GFP_KERNEL); \
  28        if (tbl == NULL) \
  29                return NULL; \
  30        term = (struct type##_error *)&(((char *)tbl)[term_offset]); \
  31        strncpy(tbl->repl.name, info->name, sizeof(tbl->repl.name)); \
  32        *term = (struct type##_error)typ2##_ERROR_INIT;  \
  33        tbl->repl.valid_hooks = hook_mask; \
  34        tbl->repl.num_entries = nhooks + 1; \
  35        tbl->repl.size = nhooks * sizeof(struct type##_standard) + \
  36                         sizeof(struct type##_error); \
  37        for (; hook_mask != 0; hook_mask >>= 1, ++hooknum) { \
  38                if (!(hook_mask & 1)) \
  39                        continue; \
  40                tbl->repl.hook_entry[hooknum] = bytes; \
  41                tbl->repl.underflow[hooknum]  = bytes; \
  42                tbl->entries[i++] = (struct type##_standard) \
  43                        typ2##_STANDARD_INIT(NF_ACCEPT); \
  44                bytes += sizeof(struct type##_standard); \
  45        } \
  46        tbl; \
  47})
  48