dispatcher.c
3.63 KB
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// SPDX-License-Identifier: GPL-2.0-only
/* Copyright(c) 2019 Intel Corporation. */
#include <linux/hash.h>
#include <linux/bpf.h>
#include <linux/filter.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, s64 *funcs, int num_funcs)
{
return -ENOTSUPP;
}
static int bpf_dispatcher_prepare(struct bpf_dispatcher *d, void *image)
{
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, &ips[0], d->num_progs);
}
static void bpf_dispatcher_update(struct bpf_dispatcher *d, int prev_num_progs)
{
void *old, *new;
u32 noff;
int err;
if (!prev_num_progs) {
old = NULL;
noff = 0;
} else {
old = d->image + d->image_off;
noff = d->image_off ^ (PAGE_SIZE / 2);
}
new = d->num_progs ? d->image + noff : NULL;
if (new) {
if (bpf_dispatcher_prepare(d, new))
return;
}
err = bpf_arch_text_poke(d->func, BPF_MOD_JUMP, old, new);
if (err || !new)
return;
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_jit_alloc_exec_page();
if (!d->image)
goto out;
bpf_image_ksym_add(d->image, &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);
}