xref: /openbmc/linux/samples/seccomp/bpf-helper.c (revision 6c33a6f4)
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 }
97