linux/samples/seccomp/bpf-helper.c
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   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
  17int 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
  83void 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}
  90
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