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There are a couple of valid use cases for a minimal low-level bpf asm like tool, for example, using/linking to libpcap is not an option, the required BPF filters use Linux extensions that are not supported by libpcap's compiler, a filter might be more complex and not cleanly implementable with libpcap's compiler, particular filter codes should be optimized differently than libpcap's internal BPF compiler does, or for security audits of emitted BPF JIT code for prepared set of BPF instructions resp. BPF JIT compiler development in general. Then, in such cases writing such a filter in low-level syntax can be an good alternative, for example, xt_bpf and cls_bpf users might have requirements that could result in more complex filter code, or one that cannot be expressed with libpcap (e.g. different return codes in cls_bpf for flowids on various BPF code paths). Moreover, BPF JIT implementors may wish to manually write test cases in order to verify the resulting JIT image, and thus need low-level access to BPF code generation as well. Therefore, complete the available toolchain for BPF with this small bpf_asm helper tool for the tools/net/ directory. These 3 complementary minimal helper tools round up and facilitate BPF development. Signed-off-by: Daniel Borkmann <dborkman@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
53 lines
1.2 KiB
C
53 lines
1.2 KiB
C
/*
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* Minimal BPF assembler
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*
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* Instead of libpcap high-level filter expressions, it can be quite
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* useful to define filters in low-level BPF assembler (that is kept
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* close to Steven McCanne and Van Jacobson's original BPF paper).
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* In particular for BPF JIT implementors, JIT security auditors, or
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* just for defining BPF expressions that contain extensions which are
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* not supported by compilers.
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*
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* How to get into it:
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*
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* 1) read Documentation/networking/filter.txt
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* 2) Run `bpf_asm [-c] <filter-prog file>` to translate into binary
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* blob that is loadable with xt_bpf, cls_bpf et al. Note: -c will
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* pretty print a C-like construct.
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*
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* Copyright 2013 Daniel Borkmann <borkmann@redhat.com>
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* Licensed under the GNU General Public License, version 2.0 (GPLv2)
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*/
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#include <stdbool.h>
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#include <stdio.h>
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#include <string.h>
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extern void bpf_asm_compile(FILE *fp, bool cstyle);
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int main(int argc, char **argv)
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{
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FILE *fp = stdin;
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bool cstyle = false;
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int i;
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for (i = 1; i < argc; i++) {
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if (!strncmp("-c", argv[i], 2)) {
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cstyle = true;
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continue;
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}
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fp = fopen(argv[i], "r");
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if (!fp) {
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fp = stdin;
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continue;
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}
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break;
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}
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bpf_asm_compile(fp, cstyle);
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return 0;
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}
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