summaryrefslogtreecommitdiff
path: root/kernel/bpf/dispatcher.c
blob: 70fb82bf16370ed15c419cdb7a5b9ca0279660d5 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
// SPDX-License-Identifier: GPL-2.0-only
/* Copyright(c) 2019 Intel Corporation. */

#include <linux/hash.h>
#include <linux/bpf.h>
#include <linux/filter.h>
#include <linux/static_call.h>

/* The BPF dispatcher is a multiway branch code generator. The
 * dispatcher is a mechanism to avoid the performance penalty of an
 * indirect call, which is expensive when retpolines are enabled. A
 * dispatch client registers a BPF program into the dispatcher, and if
 * there is available room in the dispatcher a direct call to the BPF
 * program will be generated. All calls to the BPF programs called via
 * the dispatcher will then be a direct call, instead of an
 * indirect. The dispatcher hijacks a trampoline function it via the
 * __fentry__ of the trampoline. The trampoline function has the
 * following signature:
 *
 * unsigned int trampoline(const void *ctx, const struct bpf_insn *insnsi,
 *                         unsigned int (*bpf_func)(const void *,
 *                                                  const struct bpf_insn *));
 */

static struct bpf_dispatcher_prog *bpf_dispatcher_find_prog(
	struct bpf_dispatcher *d, struct bpf_prog *prog)
{
	int i;

	for (i = 0; i < BPF_DISPATCHER_MAX; i++) {
		if (prog == d->progs[i].prog)
			return &d->progs[i];
	}
	return NULL;
}

static struct bpf_dispatcher_prog *bpf_dispatcher_find_free(
	struct bpf_dispatcher *d)
{
	return bpf_dispatcher_find_prog(d, NULL);
}

static bool bpf_dispatcher_add_prog(struct bpf_dispatcher *d,
				    struct bpf_prog *prog)
{
	struct bpf_dispatcher_prog *entry;

	if (!prog)
		return false;

	entry = bpf_dispatcher_find_prog(d, prog);
	if (entry) {
		refcount_inc(&entry->users);
		return false;
	}

	entry = bpf_dispatcher_find_free(d);
	if (!entry)
		return false;

	bpf_prog_inc(prog);
	entry->prog = prog;
	refcount_set(&entry->users, 1);
	d->num_progs++;
	return true;
}

static bool bpf_dispatcher_remove_prog(struct bpf_dispatcher *d,
				       struct bpf_prog *prog)
{
	struct bpf_dispatcher_prog *entry;

	if (!prog)
		return false;

	entry = bpf_dispatcher_find_prog(d, prog);
	if (!entry)
		return false;

	if (refcount_dec_and_test(&entry->users)) {
		entry->prog = NULL;
		bpf_prog_put(prog);
		d->num_progs--;
		return true;
	}
	return false;
}

int __weak arch_prepare_bpf_dispatcher(void *image, void *buf, s64 *funcs, int num_funcs)
{
	return -ENOTSUPP;
}

static int bpf_dispatcher_prepare(struct bpf_dispatcher *d, void *image, void *buf)
{
	s64 ips[BPF_DISPATCHER_MAX] = {}, *ipsp = &ips[0];
	int i;

	for (i = 0; i < BPF_DISPATCHER_MAX; i++) {
		if (d->progs[i].prog)
			*ipsp++ = (s64)(uintptr_t)d->progs[i].prog->bpf_func;
	}
	return arch_prepare_bpf_dispatcher(image, buf, &ips[0], d->num_progs);
}

static void bpf_dispatcher_update(struct bpf_dispatcher *d, int prev_num_progs)
{
	void *new, *tmp;
	u32 noff = 0;

	if (prev_num_progs)
		noff = d->image_off ^ (PAGE_SIZE / 2);

	new = d->num_progs ? d->image + noff : NULL;
	tmp = d->num_progs ? d->rw_image + noff : NULL;
	if (new) {
		/* Prepare the dispatcher in d->rw_image. Then use
		 * bpf_arch_text_copy to update d->image, which is RO+X.
		 */
		if (bpf_dispatcher_prepare(d, new, tmp))
			return;
		if (IS_ERR(bpf_arch_text_copy(new, tmp, PAGE_SIZE / 2)))
			return;
	}

	__BPF_DISPATCHER_UPDATE(d, new ?: (void *)&bpf_dispatcher_nop_func);

	/* Make sure all the callers executing the previous/old half of the
	 * image leave it, so following update call can modify it safely.
	 */
	synchronize_rcu();

	if (new)
		d->image_off = noff;
}

void bpf_dispatcher_change_prog(struct bpf_dispatcher *d, struct bpf_prog *from,
				struct bpf_prog *to)
{
	bool changed = false;
	int prev_num_progs;

	if (from == to)
		return;

	mutex_lock(&d->mutex);
	if (!d->image) {
		d->image = bpf_prog_pack_alloc(PAGE_SIZE, bpf_jit_fill_hole_with_zero);
		if (!d->image)
			goto out;
		d->rw_image = bpf_jit_alloc_exec(PAGE_SIZE);
		if (!d->rw_image) {
			bpf_prog_pack_free(d->image, PAGE_SIZE);
			d->image = NULL;
			goto out;
		}
		bpf_image_ksym_add(d->image, PAGE_SIZE, &d->ksym);
	}

	prev_num_progs = d->num_progs;
	changed |= bpf_dispatcher_remove_prog(d, from);
	changed |= bpf_dispatcher_add_prog(d, to);

	if (!changed)
		goto out;

	bpf_dispatcher_update(d, prev_num_progs);
out:
	mutex_unlock(&d->mutex);
}