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 <stdlib.h>
  14 #include <string.h>
  15
  16 #include "bpf-helper.h"
  17
  18 int bpf_resolve_jumps(struct bpf_labels *labels,
  19                       struct sock_filter *filter, size_t count)
  20 {
  21         size_t i;
  22
  23         if (count < 1 || count > BPF_MAXINSNS)
  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         for (i = 0; i < count; ++i) {
  30                 size_t offset = count - i - 1;
  31                 struct sock_filter *instr = &filter[offset];
  32                 if (instr->code != (BPF_JMP+BPF_JA))
  33                         continue;
  34                 switch ((instr->jt<<8)|instr->jf) {
  35                 case (JUMP_JT<<8)|JUMP_JF:
  36                         if (labels->labels[instr->k].location == 0xffffffff) {
  37                                 fprintf(stderr, "Unresolved label: '%s'\n",
  38                                         labels->labels[instr->k].label);
  39                                 return 1;
  40                         }
  41                         instr->k = labels->labels[instr->k].location -
  42                                     (offset + 1);
  43                         instr->jt = 0;
  44                         instr->jf = 0;
  45                         continue;
  46                 case (LABEL_JT<<8)|LABEL_JF:
  47                         if (labels->labels[instr->k].location != 0xffffffff) {
  48                                 fprintf(stderr, "Duplicate label use: '%s'\n",
  49                                         labels->labels[instr->k].label);
  50                                 return 1;
  51                         }
  52                         labels->labels[instr->k].location = offset;
  53                         instr->k = 0; /* fall through */
  54                         instr->jt = 0;
  55                         instr->jf = 0;
  56                         continue;
  57                 }
  58         }
  59         return 0;
  60 }
  61
  62 /* Simple lookup table for labels. */
  63 __u32 seccomp_bpf_label(struct bpf_labels *labels, const char *label)
  64 {
  65         struct __bpf_label *begin = labels->labels, *end;
  66         int id;
  67
  68         if (labels->count == BPF_LABELS_MAX) {
  69                 fprintf(stderr, "Too many labels\n");
  70                 exit(1);
  71         }
  72         if (labels->count == 0) {
  73                 begin->label = label;
  74                 begin->location = 0xffffffff;
  75                 labels->count++;
  76                 return 0;
  77         }
  78         end = begin + labels->count;
  79         for (id = 0; begin < end; ++begin, ++id) {
  80                 if (!strcmp(label, begin->label))
  81                         return id;
  82         }
  83         begin->label = label;
  84         begin->location = 0xffffffff;
  85         labels->count++;
  86         return id;
  87 }
  88
  89 void seccomp_bpf_print(struct sock_filter *filter, size_t count)
  90 {
  91         struct sock_filter *end = filter + count;
  92         for ( ; filter < end; ++filter)
  93                 printf("{ code=%u,jt=%u,jf=%u,k=%u },\n",
  94                         filter->code, filter->jt, filter->jf, filter->k);
  95 }