bpf-helper.c

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/*
 * Seccomp BPF helper functions
 *
 * Copyright (c) 2012 The Chromium OS Authors <[email protected]>
 * Author: Will Drewry <[email protected]>
 *
 * The code may be used by anyone for any purpose,
 * and can serve as a starting point for developing
 * applications using prctl(PR_ATTACH_SECCOMP_FILTER).
 */

#include <stdio.h>
#include <string.h>

#include "bpf-helper.h"

int bpf_resolve_jumps(struct bpf_labels *labels,
              struct sock_filter *filter, size_t count)
{
    struct sock_filter *begin = filter;
    __u8 insn = count - 1;

    if (count < 1)
        return -1;
    /*
    * Walk it once, backwards, to build the label table and do fixups.
    * Since backward jumps are disallowed by BPF, this is easy.
    */
    filter += insn;
    for (; filter >= begin; --insn, --filter) {
        if (filter->code != (BPF_JMP+BPF_JA))
            continue;
        switch ((filter->jt<<8)|filter->jf) {
        case (JUMP_JT<<8)|JUMP_JF:
            if (labels->labels[filter->k].location == 0xffffffff) {
                fprintf(stderr, "Unresolved label: '%s'\n",
                    labels->labels[filter->k].label);
                return 1;
            }
            filter->k = labels->labels[filter->k].location -
                    (insn + 1);
            filter->jt = 0;
            filter->jf = 0;
            continue;
        case (LABEL_JT<<8)|LABEL_JF:
            if (labels->labels[filter->k].location != 0xffffffff) {
                fprintf(stderr, "Duplicate label use: '%s'\n",
                    labels->labels[filter->k].label);
                return 1;
            }
            labels->labels[filter->k].location = insn;
            filter->k = 0; /* fall through */
            filter->jt = 0;
            filter->jf = 0;
            continue;
        }
    }
    return 0;
}

/* Simple lookup table for labels. */
__u32 seccomp_bpf_label(struct bpf_labels *labels, const char *label)
{
    struct __bpf_label *begin = labels->labels, *end;
    int id;
    if (labels->count == 0) {
        begin->label = label;
        begin->location = 0xffffffff;
        labels->count++;
        return 0;
    }
    end = begin + labels->count;
    for (id = 0; begin < end; ++begin, ++id) {
        if (!strcmp(label, begin->label))
            return id;
    }
    begin->label = label;
    begin->location = 0xffffffff;
    labels->count++;
    return id;
}

void seccomp_bpf_print(struct sock_filter *filter, size_t count)
{
    struct sock_filter *end = filter + count;
    for ( ; filter < end; ++filter)
        printf("{ code=%u,jt=%u,jf=%u,k=%u },\n",
            filter->code, filter->jt, filter->jf, filter->k);
}