samples/seccomp/bpf-helper.c

   1 /*
   2  * Seccomp BPF helper functions
   3  *
   4  * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
   5  * Author: Will Drewry <wad@chromium.org>
   6  *
   7  * The code may be used by anyone for any purpose,
   8  * and can serve as a starting point for developing
   9  * applications using prctl(PR_ATTACH_SECCOMP_FILTER).
  10  */
  11
  12 #include <stdio.h>
  13 #include <string.h>
  14
  15 #include "bpf-helper.h"
  16
  17 int bpf_resolve_jumps(struct bpf_labels *labels,
  18                       struct sock_filter *filter, size_t count)
  19 {
  20         struct sock_filter *begin = filter;
  21         __u8 insn = count - 1;
  22
  23         if (count < 1)
  24                 return -1;
  25         /*
  26         * Walk it once, backwards, to build the label table and do fixups.
  27         * Since backward jumps are disallowed by BPF, this is easy.
  28         */
  29         filter += insn;
  30         for (; filter >= begin; --insn, --filter) {
  31                 if (filter->code != (BPF_JMP+BPF_JA))
  32                         continue;
  33                 switch ((filter->jt<<8)|filter->jf) {
  34                 case (JUMP_JT<<8)|JUMP_JF:
  35                         if (labels->labels[filter->k].location == 0xffffffff) {
  36                                 fprintf(stderr, "Unresolved label: '%s'\n",
  37                                         labels->labels[filter->k].label);
  38                                 return 1;
  39                         }
  40                         filter->k = labels->labels[filter->k].location -
  41                                     (insn + 1);
  42                         filter->jt = 0;
  43                         filter->jf = 0;
  44                         continue;
  45                 case (LABEL_JT<<8)|LABEL_JF:
  46                         if (labels->labels[filter->k].location != 0xffffffff) {
  47                                 fprintf(stderr, "Duplicate label use: '%s'\n",
  48                                         labels->labels[filter->k].label);
  49                                 return 1;
  50                         }
  51                         labels->labels[filter->k].location = insn;
  52                         filter->k = 0; /* fall through */
  53                         filter->jt = 0;
  54                         filter->jf = 0;
  55                         continue;
  56                 }
  57         }
  58         return 0;
  59 }
  60
  61 /* Simple lookup table for labels. */
  62 __u32 seccomp_bpf_label(struct bpf_labels *labels, const char *label)
  63 {
  64         struct __bpf_label *begin = labels->labels, *end;
  65         int id;
  66         if (labels->count == 0) {
  67                 begin->label = label;
  68                 begin->location = 0xffffffff;
  69                 labels->count++;
  70                 return 0;
  71         }
  72         end = begin + labels->count;
  73         for (id = 0; begin < end; ++begin, ++id) {
  74                 if (!strcmp(label, begin->label))
  75                         return id;
  76         }
  77         begin->label = label;
  78         begin->location = 0xffffffff;
  79         labels->count++;
  80         return id;
  81 }
  82
  83 void seccomp_bpf_print(struct sock_filter *filter, size_t count)
  84 {
  85         struct sock_filter *end = filter + count;
  86         for ( ; filter < end; ++filter)
  87                 printf("{ code=%u,jt=%u,jf=%u,k=%u },\n",
  88                         filter->code, filter->jt, filter->jf, filter->k);
  89 }