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bpf: Retire the struct_ops map kvalue->refcnt.

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We have replaced kvalue-refcnt with synchronize_rcu() to wait for an
RCU grace period.

Maintenance of kvalue->refcnt was a complicated task, as we had to
simultaneously keep track of two reference counts: one for the
reference count of bpf_map. When the kvalue->refcnt reaches zero, we
also have to reduce the reference count on bpf_map - yet these steps
are not performed in an atomic manner and require us to be vigilant
when managing them. By eliminating kvalue->refcnt, we can make our
maintenance more straightforward as the refcount of bpf_map is now
solely managed!

To prevent the trampoline image of a struct_ops from being released
while it is still in use, we wait for an RCU grace period. The
setsockopt(TCP_CONGESTION, "...") command allows you to change your
socket's congestion control algorithm and can result in releasing the
old struct_ops implementation. It is fine. However, this function is
exposed through bpf_setsockopt(), it may be accessed by BPF programs
as well. To ensure that the trampoline image belonging to struct_op
can be safely called while its method is in use, the trampoline
safeguarde the BPF program with rcu_read_lock(). Doing so prevents any
destruction of the associated images before returning from a
trampoline and requires us to wait for an RCU grace period.

Signed-off-by: Kui-Feng Lee <kuifeng@meta.com>
Link: https://lore.kernel.org/r/20230323032405.3735486-2-kuifeng@meta.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
This commit is contained in:
Kui-Feng Lee 2023-03-22 20:23:58 -07:00 committed by Martin KaFai Lau
parent b63cbc490e
commit b671c2067a
3 changed files with 48 additions and 34 deletions
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1
include/linux/bpf.h
View File

@ -1945,6 +1945,7 @@ struct bpf_map *bpf_map_get_with_uref(u32 ufd);
struct bpf_map *__bpf_map_get(struct fd f);
void bpf_map_inc(struct bpf_map *map);
void bpf_map_inc_with_uref(struct bpf_map *map);
struct bpf_map *__bpf_map_inc_not_zero(struct bpf_map *map, bool uref);
struct bpf_map * __must_check bpf_map_inc_not_zero(struct bpf_map *map);
void bpf_map_put_with_uref(struct bpf_map *map);
void bpf_map_put(struct bpf_map *map);

75
kernel/bpf/bpf_struct_ops.c
View File

@ -11,6 +11,7 @@
#include <linux/refcount.h>
#include <linux/mutex.h>
#include <linux/btf_ids.h>
#include <linux/rcupdate_wait.h>
enum bpf_struct_ops_state {
BPF_STRUCT_OPS_STATE_INIT,
@ -249,6 +250,7 @@ int bpf_struct_ops_map_sys_lookup_elem(struct bpf_map *map, void *key,
struct bpf_struct_ops_map *st_map = (struct bpf_struct_ops_map *)map;
struct bpf_struct_ops_value *uvalue, *kvalue;
enum bpf_struct_ops_state state;
s64 refcnt;
if (unlikely(*(u32 *)key != 0))
return -ENOENT;
@ -267,7 +269,14 @@ int bpf_struct_ops_map_sys_lookup_elem(struct bpf_map *map, void *key,
uvalue = value;
memcpy(uvalue, st_map->uvalue, map->value_size);
uvalue->state = state;
refcount_set(&uvalue->refcnt, refcount_read(&kvalue->refcnt));
/* This value offers the user space a general estimate of how
* many sockets are still utilizing this struct_ops for TCP
* congestion control. The number might not be exact, but it
* should sufficiently meet our present goals.
*/
refcnt = atomic64_read(&map->refcnt) - atomic64_read(&map->usercnt);
refcount_set(&uvalue->refcnt, max_t(s64, refcnt, 0));
return 0;
}
@ -491,7 +500,6 @@ static long bpf_struct_ops_map_update_elem(struct bpf_map *map, void *key,
*(unsigned long *)(udata + moff) = prog->aux->id;
}
refcount_set(&kvalue->refcnt, 1);
bpf_map_inc(map);
set_memory_rox((long)st_map->image, 1);
@ -536,8 +544,7 @@ static long bpf_struct_ops_map_delete_elem(struct bpf_map *map, void *key)
switch (prev_state) {
case BPF_STRUCT_OPS_STATE_INUSE:
st_map->st_ops->unreg(&st_map->kvalue.data);
if (refcount_dec_and_test(&st_map->kvalue.refcnt))
bpf_map_put(map);
bpf_map_put(map);
return 0;
case BPF_STRUCT_OPS_STATE_TOBEFREE:
return -EINPROGRESS;
@ -570,7 +577,7 @@ static void bpf_struct_ops_map_seq_show_elem(struct bpf_map *map, void *key,
kfree(value);
}
static void bpf_struct_ops_map_free(struct bpf_map *map)
static void __bpf_struct_ops_map_free(struct bpf_map *map)
{
struct bpf_struct_ops_map *st_map = (struct bpf_struct_ops_map *)map;
@ -582,6 +589,28 @@ static void bpf_struct_ops_map_free(struct bpf_map *map)
bpf_map_area_free(st_map);
}
static void bpf_struct_ops_map_free(struct bpf_map *map)
{
/* The struct_ops's function may switch to another struct_ops.
*
* For example, bpf_tcp_cc_x->init() may switch to
* another tcp_cc_y by calling
* setsockopt(TCP_CONGESTION, "tcp_cc_y").
* During the switch, bpf_struct_ops_put(tcp_cc_x) is called
* and its refcount may reach 0 which then free its
* trampoline image while tcp_cc_x is still running.
*
* A vanilla rcu gp is to wait for all bpf-tcp-cc prog
* to finish. bpf-tcp-cc prog is non sleepable.
* A rcu_tasks gp is to wait for the last few insn
* in the tramopline image to finish before releasing
* the trampoline image.
*/
synchronize_rcu_mult(call_rcu, call_rcu_tasks);
__bpf_struct_ops_map_free(map);
}
static int bpf_struct_ops_map_alloc_check(union bpf_attr *attr)
{
if (attr->key_size != sizeof(unsigned int) || attr->max_entries != 1 ||
@ -630,7 +659,7 @@ static struct bpf_map *bpf_struct_ops_map_alloc(union bpf_attr *attr)
NUMA_NO_NODE);
st_map->image = bpf_jit_alloc_exec(PAGE_SIZE);
if (!st_map->uvalue || !st_map->links || !st_map->image) {
bpf_struct_ops_map_free(map);
__bpf_struct_ops_map_free(map);
return ERR_PTR(-ENOMEM);
}
@ -676,41 +705,23 @@ const struct bpf_map_ops bpf_struct_ops_map_ops = {
bool bpf_struct_ops_get(const void *kdata)
{
struct bpf_struct_ops_value *kvalue;
struct bpf_struct_ops_map *st_map;
struct bpf_map *map;
kvalue = container_of(kdata, struct bpf_struct_ops_value, data);
st_map = container_of(kvalue, struct bpf_struct_ops_map, kvalue);
return refcount_inc_not_zero(&kvalue->refcnt);
}
static void bpf_struct_ops_put_rcu(struct rcu_head *head)
{
struct bpf_struct_ops_map *st_map;
st_map = container_of(head, struct bpf_struct_ops_map, rcu);
bpf_map_put(&st_map->map);
map = __bpf_map_inc_not_zero(&st_map->map, false);
return !IS_ERR(map);
}
void bpf_struct_ops_put(const void *kdata)
{
struct bpf_struct_ops_value *kvalue;
struct bpf_struct_ops_map *st_map;
kvalue = container_of(kdata, struct bpf_struct_ops_value, data);
if (refcount_dec_and_test(&kvalue->refcnt)) {
struct bpf_struct_ops_map *st_map;
st_map = container_of(kvalue, struct bpf_struct_ops_map, kvalue);
st_map = container_of(kvalue, struct bpf_struct_ops_map,
kvalue);
/* The struct_ops's function may switch to another struct_ops.
*
* For example, bpf_tcp_cc_x->init() may switch to
* another tcp_cc_y by calling
* setsockopt(TCP_CONGESTION, "tcp_cc_y").
* During the switch, bpf_struct_ops_put(tcp_cc_x) is called
* and its map->refcnt may reach 0 which then free its
* trampoline image while tcp_cc_x is still running.
*
* Thus, a rcu grace period is needed here.
*/
call_rcu(&st_map->rcu, bpf_struct_ops_put_rcu);
}
bpf_map_put(&st_map->map);
}

6
kernel/bpf/syscall.c
View File

@ -1303,8 +1303,10 @@ struct bpf_map *bpf_map_get_with_uref(u32 ufd)
return map;
}
/* map_idr_lock should have been held */
static struct bpf_map *__bpf_map_inc_not_zero(struct bpf_map *map, bool uref)
/* map_idr_lock should have been held or the map should have been
* protected by rcu read lock.
*/
struct bpf_map *__bpf_map_inc_not_zero(struct bpf_map *map, bool uref)
{
int refold;