[PATCH bpf-next v5 3/8] bpf: Create links for BPF struct_ops maps.

Kui-Feng Lee
2023-03-08T00:51:12Z

BPF struct_ops maps are employed directly to register TCP Congestion
Control algorithms. Unlike other BPF programs that terminate when
their links gone. The link of a BPF struct_ops map provides a uniform
experience akin to other types of BPF programs.

bpf_links are responsible for registering their associated
struct_ops. You can only use a struct_ops that has the BPF_F_LINK flag
set to create a bpf_link, while a structs without this flag behaves in
the same manner as before and is registered upon updating its value.

The BPF_LINK_TYPE_STRUCT_OPS serves a dual purpose. Not only is it
used to craft the links for BPF struct_ops programs, but also to
create links for BPF struct_ops them-self. Since the links of BPF
struct_ops programs are only used to create trampolines internally,
they are never seen in other contexts. Thus, they can be reused for
struct_ops themself.

To maintain a reference to the map supporting this link, we add
bpf_struct_ops_link as an additional type. The pointer of the map is
RCU and won't be necessary until later in the patchset.

Signed-off-by: Kui-Feng Lee <kuifeng@meta.com>
---
include/linux/bpf.h | 11 +++
include/uapi/linux/bpf.h | 12 +++-
kernel/bpf/bpf_struct_ops.c | 124 +++++++++++++++++++++++++++++++--
kernel/bpf/syscall.c | 23 +++---
tools/include/uapi/linux/bpf.h | 12 +++-
5 files changed, 168 insertions(+), 14 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 00b6e1a2edaf..afca6c526fe4 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1548,6 +1548,7 @@ static inline void bpf_module_put(const void *data, struct module *owner)
else
module_put(owner);
}
+int bpf_struct_ops_link_create(union bpf_attr *attr);

#ifdef CONFIG_NET
/* Define it here to avoid the use of forward declaration */
@@ -1588,6 +1589,11 @@ static inline int bpf_struct_ops_map_sys_lookup_elem(struct bpf_map *map,
{
return -EINVAL;
}
+static inline int bpf_struct_ops_link_create(union bpf_attr *attr)
+{
+ return -EOPNOTSUPP;
+}
+
#endif

#if defined(CONFIG_CGROUP_BPF) && defined(CONFIG_BPF_LSM)
@@ -2379,6 +2385,11 @@ static inline void bpf_link_put(struct bpf_link *link)
{
}

+static inline int bpf_struct_ops_link_create(union bpf_attr *attr)
+{
+ return -EOPNOTSUPP;
+}
+
static inline int bpf_obj_get_user(const char __user *pathname, int flags)
{
return -EOPNOTSUPP;
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 976b194eb775..f9fc7b8af3c4 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -1033,6 +1033,7 @@ enum bpf_attach_type {
BPF_PERF_EVENT,
BPF_TRACE_KPROBE_MULTI,
BPF_LSM_CGROUP,
+ BPF_STRUCT_OPS,
__MAX_BPF_ATTACH_TYPE
};

@@ -1266,6 +1267,9 @@ enum {

/* Create a map that is suitable to be an inner map with dynamic max entries */
BPF_F_INNER_MAP = (1U << 12),
+
+/* Create a map that will be registered/unregesitered by the backed bpf_link */
+ BPF_F_LINK = (1U << 13),
};

/* Flags for BPF_PROG_QUERY. */
@@ -1507,7 +1511,10 @@ union bpf_attr {
} task_fd_query;

struct { /* struct used by BPF_LINK_CREATE command */
- __u32 prog_fd; /* eBPF program to attach */
+ union {
+ __u32 prog_fd; /* eBPF program to attach */
+ __u32 map_fd; /* struct_ops to attach */
+ };
union {
__u32 target_fd; /* object to attach to */
__u32 target_ifindex; /* target ifindex */
@@ -6379,6 +6386,9 @@ struct bpf_link_info {
struct {
__u32 ifindex;
} xdp;
+ struct {
+ __u32 map_id;
+ } struct_ops;
};
} __attribute__((aligned(8)));

diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
index 9e097fcc9cf4..5a7e86cf67b5 100644
--- a/kernel/bpf/bpf_struct_ops.c
+++ b/kernel/bpf/bpf_struct_ops.c
@@ -16,6 +16,7 @@ enum bpf_struct_ops_state {
BPF_STRUCT_OPS_STATE_INIT,
BPF_STRUCT_OPS_STATE_INUSE,
BPF_STRUCT_OPS_STATE_TOBEFREE,
+ BPF_STRUCT_OPS_STATE_READY,
};

#define BPF_STRUCT_OPS_COMMON_VALUE \
@@ -58,6 +59,11 @@ struct bpf_struct_ops_map {
struct bpf_struct_ops_value kvalue;
};

+struct bpf_struct_ops_link {
+ struct bpf_link link;
+ struct bpf_map __rcu *map;
+};
+
static DEFINE_MUTEX(update_mutex);

#define VALUE_PREFIX "bpf_struct_ops_"
@@ -496,11 +502,24 @@ static int bpf_struct_ops_map_update_elem(struct bpf_map *map, void *key,
*(unsigned long *)(udata + moff) = prog->aux->id;
}

- bpf_map_inc(map);
-
set_memory_rox((long)st_map->image, 1);
+ if (st_map->map.map_flags & BPF_F_LINK) {
+ if (st_ops->validate) {
+ err = st_ops->validate(kdata);
+ if (err)
+ goto unlock;
+ }
+ /* Let bpf_link handle registration & unregistration.
+ *
+ * Pair with smp_load_acquire() during lookup_elem().
+ */
+ smp_store_release(&kvalue->state, BPF_STRUCT_OPS_STATE_READY);
+ goto unlock;
+ }
+
err = st_ops->reg(kdata);
if (likely(!err)) {
+ bpf_map_inc(map);
/* Pair with smp_load_acquire() during lookup_elem().
* It ensures the above udata updates (e.g. prog->aux->id)
* can be seen once BPF_STRUCT_OPS_STATE_INUSE is set.
@@ -516,7 +535,6 @@ static int bpf_struct_ops_map_update_elem(struct bpf_map *map, void *key,
*/
set_memory_nx((long)st_map->image, 1);
set_memory_rw((long)st_map->image, 1);
- bpf_map_put(map);

reset_unlock:
bpf_struct_ops_map_put_progs(st_map);
@@ -534,6 +552,9 @@ static int bpf_struct_ops_map_delete_elem(struct bpf_map *map, void *key)
struct bpf_struct_ops_map *st_map;

st_map = (struct bpf_struct_ops_map *)map;
+ if (st_map->map.map_flags & BPF_F_LINK)
+ return -EOPNOTSUPP;
+
prev_state = cmpxchg(&st_map->kvalue.state,
BPF_STRUCT_OPS_STATE_INUSE,
BPF_STRUCT_OPS_STATE_TOBEFREE);
@@ -601,7 +622,7 @@ static void bpf_struct_ops_map_free(struct bpf_map *map)
static int bpf_struct_ops_map_alloc_check(union bpf_attr *attr)
{
if (attr->key_size != sizeof(unsigned int) || attr->max_entries != 1 ||
- attr->map_flags || !attr->btf_vmlinux_value_type_id)
+ (attr->map_flags & ~BPF_F_LINK) || !attr->btf_vmlinux_value_type_id)
return -EINVAL;
return 0;
}
@@ -712,3 +733,98 @@ void bpf_struct_ops_put(const void *kdata)

bpf_map_put(&st_map->map);
}
+
+static void bpf_struct_ops_map_link_dealloc(struct bpf_link *link)
+{
+ struct bpf_struct_ops_link *st_link;
+ struct bpf_struct_ops_map *st_map;
+
+ st_link = container_of(link, struct bpf_struct_ops_link, link);
+ st_map = (struct bpf_struct_ops_map *)st_link->map;
+ st_map->st_ops->unreg(&st_map->kvalue.data);
+ bpf_map_put(st_link->map);
+ kfree(st_link);
+}
+
+static void bpf_struct_ops_map_link_show_fdinfo(const struct bpf_link *link,
+ struct seq_file *seq)
+{
+ struct bpf_struct_ops_link *st_link;
+ struct bpf_map *map;
+
+ st_link = container_of(link, struct bpf_struct_ops_link, link);
+ rcu_read_lock();
+ map = rcu_dereference(st_link->map);
+ if (map)
+ seq_printf(seq, "map_id:\t%d\n", map->id);
+ rcu_read_unlock();
+}
+
+static int bpf_struct_ops_map_link_fill_link_info(const struct bpf_link *link,
+ struct bpf_link_info *info)
+{
+ struct bpf_struct_ops_link *st_link;
+ struct bpf_map *map;
+
+ st_link = container_of(link, struct bpf_struct_ops_link, link);
+ rcu_read_lock();
+ map = rcu_dereference(st_link->map);
+ if (map)
+ info->struct_ops.map_id = map->id;
+ rcu_read_unlock();
+ return 0;
+}
+
+static const struct bpf_link_ops bpf_struct_ops_map_lops = {
+ .dealloc = bpf_struct_ops_map_link_dealloc,
+ .show_fdinfo = bpf_struct_ops_map_link_show_fdinfo,
+ .fill_link_info = bpf_struct_ops_map_link_fill_link_info,
+};
+
+int bpf_struct_ops_link_create(union bpf_attr *attr)
+{
+ struct bpf_struct_ops_link *link = NULL;
+ struct bpf_link_primer link_primer;
+ struct bpf_struct_ops_map *st_map;
+ struct bpf_map *map;
+ int err;
+
+ map = bpf_map_get(attr->link_create.map_fd);
+ if (!map)
+ return -EINVAL;
+
+ st_map = (struct bpf_struct_ops_map *)map;
+
+ if (map->map_type != BPF_MAP_TYPE_STRUCT_OPS || !(map->map_flags & BPF_F_LINK) ||
+ /* Pair with smp_store_release() during map_update */
+ smp_load_acquire(&st_map->kvalue.state) != BPF_STRUCT_OPS_STATE_READY) {
+ err = -EINVAL;
+ goto err_out;
+ }
+
+ link = kzalloc(sizeof(*link), GFP_USER);
+ if (!link) {
+ err = -ENOMEM;
+ goto err_out;
+ }
+ bpf_link_init(&link->link, BPF_LINK_TYPE_STRUCT_OPS, &bpf_struct_ops_map_lops, NULL);
+ RCU_INIT_POINTER(link->map, map);
+
+ err = bpf_link_prime(&link->link, &link_primer);
+ if (err)
+ goto err_out;
+
+ err = st_map->st_ops->reg(st_map->kvalue.data);
+ if (err) {
+ bpf_link_cleanup(&link_primer);
+ goto err_out;
+ }
+
+ return bpf_link_settle(&link_primer);
+
+err_out:
+ bpf_map_put(map);
+ kfree(link);
+ return err;
+}
+
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 03273cddd6bd..3a4503987a48 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -2806,16 +2806,19 @@ static void bpf_link_show_fdinfo(struct seq_file *m, struct file *filp)
const struct bpf_prog *prog = link->prog;
char prog_tag[sizeof(prog->tag) * 2 + 1] = { };

- bin2hex(prog_tag, prog->tag, sizeof(prog->tag));
seq_printf(m,
"link_type:\t%s\n"
- "link_id:\t%u\n"
- "prog_tag:\t%s\n"
- "prog_id:\t%u\n",
+ "link_id:\t%u\n",
bpf_link_type_strs[link->type],
- link->id,
- prog_tag,
- prog->aux->id);
+ link->id);
+ if (prog) {
+ bin2hex(prog_tag, prog->tag, sizeof(prog->tag));
+ seq_printf(m,
+ "prog_tag:\t%s\n"
+ "prog_id:\t%u\n",
+ prog_tag,
+ prog->aux->id);
+ }
if (link->ops->show_fdinfo)
link->ops->show_fdinfo(link, m);
}
@@ -4290,7 +4293,8 @@ static int bpf_link_get_info_by_fd(struct file *file,

info.type = link->type;
info.id = link->id;
- info.prog_id = link->prog->aux->id;
+ if (link->prog)
+ info.prog_id = link->prog->aux->id;

if (link->ops->fill_link_info) {
err = link->ops->fill_link_info(link, &info);
@@ -4553,6 +4557,9 @@ static int link_create(union bpf_attr *attr, bpfptr_t uattr)
if (CHECK_ATTR(BPF_LINK_CREATE))
return -EINVAL;

+ if (attr->link_create.attach_type == BPF_STRUCT_OPS)
+ return bpf_struct_ops_link_create(attr);
+
prog = bpf_prog_get(attr->link_create.prog_fd);
if (IS_ERR(prog))
return PTR_ERR(prog);
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 976b194eb775..051b85525302 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -1033,6 +1033,7 @@ enum bpf_attach_type {
BPF_PERF_EVENT,
BPF_TRACE_KPROBE_MULTI,
BPF_LSM_CGROUP,
+ BPF_STRUCT_OPS,
__MAX_BPF_ATTACH_TYPE
};

@@ -1266,6 +1267,9 @@ enum {

/* Create a map that is suitable to be an inner map with dynamic max entries */
BPF_F_INNER_MAP = (1U << 12),
+
+/* Create a map that will be registered/unregesitered by the backed bpf_link */
+ BPF_F_LINK = (1U << 13),
};

/* Flags for BPF_PROG_QUERY. */
@@ -1507,7 +1511,10 @@ union bpf_attr {
} task_fd_query;

struct { /* struct used by BPF_LINK_CREATE command */
- __u32 prog_fd; /* eBPF program to attach */
+ union {
+ __u32 prog_fd; /* eBPF program to attach */
+ __u32 map_fd; /* eBPF struct_ops to attach */
+ };
union {
__u32 target_fd; /* object to attach to */
__u32 target_ifindex; /* target ifindex */
@@ -6379,6 +6386,9 @@ struct bpf_link_info {
struct {
__u32 ifindex;
} xdp;
+ struct {
+ __u32 map_id;
+ } struct_ops;
};
} __attribute__((aligned(8)));

--
2.34.1