[PATCH bpf-next v5 1/8] bpf: Retire the struct_ops map kvalue->refcnt.

Kui-Feng Lee
2023-03-08T00:51:16Z

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. Moreover, since 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, struct_ops is
safeguarded 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>
---
include/linux/bpf.h | 2 ++
kernel/bpf/bpf_struct_ops.c | 60 ++++++++++++++++++-------------------
kernel/bpf/syscall.c | 2 +-
3 files changed, 32 insertions(+), 32 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 6792a7940e1e..50cfc2388cbc 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -78,6 +78,7 @@ struct bpf_map_ops {
struct bpf_map *(*map_alloc)(union bpf_attr *attr);
void (*map_release)(struct bpf_map *map, struct file *map_file);
void (*map_free)(struct bpf_map *map);
+ void (*map_free_rcu)(struct bpf_map *map);
int (*map_get_next_key)(struct bpf_map *map, void *key, void *next_key);
void (*map_release_uref)(struct bpf_map *map);
void *(*map_lookup_elem_sys_only)(struct bpf_map *map, void *key);
@@ -1934,6 +1935,7 @@ 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);
diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
index 38903fb52f98..9e097fcc9cf4 100644
--- a/kernel/bpf/bpf_struct_ops.c
+++ b/kernel/bpf/bpf_struct_ops.c
@@ -58,6 +58,8 @@ struct bpf_struct_ops_map {
struct bpf_struct_ops_value kvalue;
};

+static DEFINE_MUTEX(update_mutex);
+
#define VALUE_PREFIX "bpf_struct_ops_"
#define VALUE_PREFIX_LEN (sizeof(VALUE_PREFIX) - 1)

@@ -249,6 +251,7 @@ 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 +270,9 @@ 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));
+
+ refcnt = atomic64_read(&map->refcnt) - atomic64_read(&map->usercnt);
+ refcount_set(&uvalue->refcnt, max_t(s64, refcnt, 0));

return 0;
}
@@ -491,7 +496,6 @@ static int 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 +540,7 @@ static int 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;
@@ -574,6 +577,19 @@ static void bpf_struct_ops_map_free(struct bpf_map *map)
{
struct bpf_struct_ops_map *st_map = (struct bpf_struct_ops_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.
+ *
+ * Thus, a rcu grace period is needed here.
+ */
+ synchronize_rcu();
+
if (st_map->links)
bpf_struct_ops_map_put_progs(st_map);
bpf_map_area_free(st_map->links);
@@ -676,41 +692,23 @@ 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);
- /* 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);
- }
+ st_map = container_of(kvalue, struct bpf_struct_ops_map, kvalue);
+
+ bpf_map_put(&st_map->map);
}
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index f406dfa13792..03273cddd6bd 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -1288,7 +1288,7 @@ struct bpf_map *bpf_map_get_with_uref(u32 ufd)
}

/* map_idr_lock should have been held */
-static struct bpf_map *__bpf_map_inc_not_zero(struct bpf_map *map, bool uref)
+struct bpf_map *__bpf_map_inc_not_zero(struct bpf_map *map, bool uref)
{
int refold;

--
2.34.1