1// SPDX-License-Identifier: GPL-2.0-only
2/* Copyright(c) 2019 Intel Corporation. */
3
4#include <linux/hash.h>
5#include <linux/bpf.h>
6#include <linux/filter.h>
7#include <linux/static_call.h>
8
9/* The BPF dispatcher is a multiway branch code generator. The
10 * dispatcher is a mechanism to avoid the performance penalty of an
11 * indirect call, which is expensive when retpolines are enabled. A
12 * dispatch client registers a BPF program into the dispatcher, and if
13 * there is available room in the dispatcher a direct call to the BPF
14 * program will be generated. All calls to the BPF programs called via
15 * the dispatcher will then be a direct call, instead of an
16 * indirect. The dispatcher hijacks a trampoline function it via the
17 * __fentry__ of the trampoline. The trampoline function has the
18 * following signature:
19 *
20 * unsigned int trampoline(const void *ctx, const struct bpf_insn *insnsi,
21 *                         unsigned int (*bpf_func)(const void *,
22 *                                                  const struct bpf_insn *));
23 */
24
25static struct bpf_dispatcher_prog *bpf_dispatcher_find_prog(
26	struct bpf_dispatcher *d, struct bpf_prog *prog)
27{
28	int i;
29
30	for (i = 0; i < BPF_DISPATCHER_MAX; i++) {
31		if (prog == d->progs[i].prog)
32			return &d->progs[i];
33	}
34	return NULL;
35}
36
37static struct bpf_dispatcher_prog *bpf_dispatcher_find_free(
38	struct bpf_dispatcher *d)
39{
40	return bpf_dispatcher_find_prog(d, NULL);
41}
42
43static bool bpf_dispatcher_add_prog(struct bpf_dispatcher *d,
44				    struct bpf_prog *prog)
45{
46	struct bpf_dispatcher_prog *entry;
47
48	if (!prog)
49		return false;
50
51	entry = bpf_dispatcher_find_prog(d, prog);
52	if (entry) {
53		refcount_inc(&entry->users);
54		return false;
55	}
56
57	entry = bpf_dispatcher_find_free(d);
58	if (!entry)
59		return false;
60
61	bpf_prog_inc(prog);
62	entry->prog = prog;
63	refcount_set(&entry->users, 1);
64	d->num_progs++;
65	return true;
66}
67
68static bool bpf_dispatcher_remove_prog(struct bpf_dispatcher *d,
69				       struct bpf_prog *prog)
70{
71	struct bpf_dispatcher_prog *entry;
72
73	if (!prog)
74		return false;
75
76	entry = bpf_dispatcher_find_prog(d, prog);
77	if (!entry)
78		return false;
79
80	if (refcount_dec_and_test(&entry->users)) {
81		entry->prog = NULL;
82		bpf_prog_put(prog);
83		d->num_progs--;
84		return true;
85	}
86	return false;
87}
88
89int __weak arch_prepare_bpf_dispatcher(void *image, void *buf, s64 *funcs, int num_funcs)
90{
91	return -ENOTSUPP;
92}
93
94static int bpf_dispatcher_prepare(struct bpf_dispatcher *d, void *image, void *buf)
95{
96	s64 ips[BPF_DISPATCHER_MAX] = {}, *ipsp = &ips[0];
97	int i;
98
99	for (i = 0; i < BPF_DISPATCHER_MAX; i++) {
100		if (d->progs[i].prog)
101			*ipsp++ = (s64)(uintptr_t)d->progs[i].prog->bpf_func;
102	}
103	return arch_prepare_bpf_dispatcher(image, buf, &ips[0], d->num_progs);
104}
105
106static void bpf_dispatcher_update(struct bpf_dispatcher *d, int prev_num_progs)
107{
108	void *new, *tmp;
109	u32 noff = 0;
110
111	if (prev_num_progs)
112		noff = d->image_off ^ (PAGE_SIZE / 2);
113
114	new = d->num_progs ? d->image + noff : NULL;
115	tmp = d->num_progs ? d->rw_image + noff : NULL;
116	if (new) {
117		/* Prepare the dispatcher in d->rw_image. Then use
118		 * bpf_arch_text_copy to update d->image, which is RO+X.
119		 */
120		if (bpf_dispatcher_prepare(d, new, tmp))
121			return;
122		if (IS_ERR(bpf_arch_text_copy(new, tmp, PAGE_SIZE / 2)))
123			return;
124	}
125
126	__BPF_DISPATCHER_UPDATE(d, new ?: (void *)&bpf_dispatcher_nop_func);
127
128	/* Make sure all the callers executing the previous/old half of the
129	 * image leave it, so following update call can modify it safely.
130	 */
131	synchronize_rcu();
132
133	if (new)
134		d->image_off = noff;
135}
136
137void bpf_dispatcher_change_prog(struct bpf_dispatcher *d, struct bpf_prog *from,
138				struct bpf_prog *to)
139{
140	bool changed = false;
141	int prev_num_progs;
142
143	if (from == to)
144		return;
145
146	mutex_lock(&d->mutex);
147	if (!d->image) {
148		d->image = bpf_prog_pack_alloc(PAGE_SIZE, bpf_jit_fill_hole_with_zero);
149		if (!d->image)
150			goto out;
151		d->rw_image = bpf_jit_alloc_exec(PAGE_SIZE);
152		if (!d->rw_image) {
153			bpf_prog_pack_free(d->image, PAGE_SIZE);
154			d->image = NULL;
155			goto out;
156		}
157		bpf_image_ksym_add(d->image, PAGE_SIZE, &d->ksym);
158	}
159
160	prev_num_progs = d->num_progs;
161	changed |= bpf_dispatcher_remove_prog(d, from);
162	changed |= bpf_dispatcher_add_prog(d, to);
163
164	if (!changed)
165		goto out;
166
167	bpf_dispatcher_update(d, prev_num_progs);
168out:
169	mutex_unlock(&d->mutex);
170}
171