net/netfilter/xt_repldata.h

   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 })