samples/seccomp/bpf-helper.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * Seccomp BPF helper functions
   4  *
   5  * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
   6  * Author: Will Drewry <wad@chromium.org>
   7  *
   8  * The code may be used by anyone for any purpose,
   9  * and can serve as a starting point for developing
  10  * applications using prctl(PR_ATTACH_SECCOMP_FILTER).
  11  */
  12
  13 #include <stdio.h>
  14 #include <stdlib.h>
  15 #include <string.h>
  16
  17 #include "bpf-helper.h"
  18
  19 int bpf_resolve_jumps(struct bpf_labels *labels,
  20                       struct sock_filter *filter, size_t count)
  21 {
  22         size_t i;
  23
  24         if (count < 1 || count > BPF_MAXINSNS)
  25                 return -1;
  26         /*
  27         * Walk it once, backwards, to build the label table and do fixups.
  28         * Since backward jumps are disallowed by BPF, this is easy.
  29         */
  30         for (i = 0; i < count; ++i) {
  31                 size_t offset = count - i - 1;
  32                 struct sock_filter *instr = &filter[offset];
  33                 if (instr->code != (BPF_JMP+BPF_JA))
  34                         continue;
  35                 switch ((instr->jt<<8)|instr->jf) {
  36                 case (JUMP_JT<<8)|JUMP_JF:
  37                         if (labels->labels[instr->k].location == 0xffffffff) {
  38                                 fprintf(stderr, "Unresolved label: '%s'\n",
  39                                         labels->labels[instr->k].label);
  40                                 return 1;
  41                         }
  42                         instr->k = labels->labels[instr->k].location -
  43                                     (offset + 1);
  44                         instr->jt = 0;
  45                         instr->jf = 0;
  46                         continue;
  47                 case (LABEL_JT<<8)|LABEL_JF:
  48                         if (labels->labels[instr->k].location != 0xffffffff) {
  49                                 fprintf(stderr, "Duplicate label use: '%s'\n",
  50                                         labels->labels[instr->k].label);
  51                                 return 1;
  52                         }
  53                         labels->labels[instr->k].location = offset;
  54                         instr->k = 0; /* fall through */
  55                         instr->jt = 0;
  56                         instr->jf = 0;
  57                         continue;
  58                 }
  59         }
  60         return 0;
  61 }
  62
  63 /* Simple lookup table for labels. */
  64 __u32 seccomp_bpf_label(struct bpf_labels *labels, const char *label)
  65 {
  66         struct __bpf_label *begin = labels->labels, *end;
  67         int id;
  68
  69         if (labels->count == BPF_LABELS_MAX) {
  70                 fprintf(stderr, "Too many labels\n");
  71                 exit(1);
  72         }
  73         if (labels->count == 0) {
  74                 begin->label = label;
  75                 begin->location = 0xffffffff;
  76                 labels->count++;
  77                 return 0;
  78         }
  79         end = begin + labels->count;
  80         for (id = 0; begin < end; ++begin, ++id) {
  81                 if (!strcmp(label, begin->label))
  82                         return id;
  83         }
  84         begin->label = label;
  85         begin->location = 0xffffffff;
  86         labels->count++;
  87         return id;
  88 }
  89
  90 void seccomp_bpf_print(struct sock_filter *filter, size_t count)
  91 {
  92         struct sock_filter *end = filter + count;
  93         for ( ; filter < end; ++filter)
  94                 printf("{ code=%u,jt=%u,jf=%u,k=%u },\n",
  95                         filter->code, filter->jt, filter->jf, filter->k);
  96 }