bpf: Introduce BPF_MAP_TYPE_STRUCT_OPS

The patch introduces BPF_MAP_TYPE_STRUCT_OPS.  The map value
is a kernel struct with its func ptr implemented in bpf prog.
This new map is the interface to register/unregister/introspect
a bpf implemented kernel struct.

The kernel struct is actually embedded inside another new struct
(or called the "value" struct in the code).  For example,
"struct tcp_congestion_ops" is embbeded in:
struct bpf_struct_ops_tcp_congestion_ops {
	refcount_t refcnt;
	enum bpf_struct_ops_state state;
	struct tcp_congestion_ops data;  /* <-- kernel subsystem struct here */
}
The map value is "struct bpf_struct_ops_tcp_congestion_ops".
The "bpftool map dump" will then be able to show the
state ("inuse"/"tobefree") and the number of subsystem's refcnt (e.g.
number of tcp_sock in the tcp_congestion_ops case).  This "value" struct
is created automatically by a macro.  Having a separate "value" struct
will also make extending "struct bpf_struct_ops_XYZ" easier (e.g. adding
"void (*init)(void)" to "struct bpf_struct_ops_XYZ" to do some
initialization works before registering the struct_ops to the kernel
subsystem).  The libbpf will take care of finding and populating the
"struct bpf_struct_ops_XYZ" from "struct XYZ".

Register a struct_ops to a kernel subsystem:
1. Load all needed BPF_PROG_TYPE_STRUCT_OPS prog(s)
2. Create a BPF_MAP_TYPE_STRUCT_OPS with attr->btf_vmlinux_value_type_id
   set to the btf id "struct bpf_struct_ops_tcp_congestion_ops" of the
   running kernel.
   Instead of reusing the attr->btf_value_type_id,
   btf_vmlinux_value_type_id s added such that attr->btf_fd can still be
   used as the "user" btf which could store other useful sysadmin/debug
   info that may be introduced in the furture,
   e.g. creation-date/compiler-details/map-creator...etc.
3. Create a "struct bpf_struct_ops_tcp_congestion_ops" object as described
   in the running kernel btf.  Populate the value of this object.
   The function ptr should be populated with the prog fds.
4. Call BPF_MAP_UPDATE with the object created in (3) as
   the map value.  The key is always "0".

During BPF_MAP_UPDATE, the code that saves the kernel-func-ptr's
args as an array of u64 is generated.  BPF_MAP_UPDATE also allows
the specific struct_ops to do some final checks in "st_ops->init_member()"
(e.g. ensure all mandatory func ptrs are implemented).
If everything looks good, it will register this kernel struct
to the kernel subsystem.  The map will not allow further update
from this point.

Unregister a struct_ops from the kernel subsystem:
BPF_MAP_DELETE with key "0".

Introspect a struct_ops:
BPF_MAP_LOOKUP_ELEM with key "0".  The map value returned will
have the prog _id_ populated as the func ptr.

The map value state (enum bpf_struct_ops_state) will transit from:
INIT (map created) =>
INUSE (map updated, i.e. reg) =>
TOBEFREE (map value deleted, i.e. unreg)

The kernel subsystem needs to call bpf_struct_ops_get() and
bpf_struct_ops_put() to manage the "refcnt" in the
"struct bpf_struct_ops_XYZ".  This patch uses a separate refcnt
for the purose of tracking the subsystem usage.  Another approach
is to reuse the map->refcnt and then "show" (i.e. during map_lookup)
the subsystem's usage by doing map->refcnt - map->usercnt to filter out
the map-fd/pinned-map usage.  However, that will also tie down the
future semantics of map->refcnt and map->usercnt.

The very first subsystem's refcnt (during reg()) holds one
count to map->refcnt.  When the very last subsystem's refcnt
is gone, it will also release the map->refcnt.  All bpf_prog will be
freed when the map->refcnt reaches 0 (i.e. during map_free()).

Here is how the bpftool map command will look like:
[root@arch-fb-vm1 bpf]# bpftool map show
6: struct_ops  name dctcp  flags 0x0
	key 4B  value 256B  max_entries 1  memlock 4096B
	btf_id 6
[root@arch-fb-vm1 bpf]# bpftool map dump id 6
[{
        "value": {
            "refcnt": {
                "refs": {
                    "counter": 1
                }
            },
            "state": 1,
            "data": {
                "list": {
                    "next": 0,
                    "prev": 0
                },
                "key": 0,
                "flags": 2,
                "init": 24,
                "release": 0,
                "ssthresh": 25,
                "cong_avoid": 30,
                "set_state": 27,
                "cwnd_event": 28,
                "in_ack_event": 26,
                "undo_cwnd": 29,
                "pkts_acked": 0,
                "min_tso_segs": 0,
                "sndbuf_expand": 0,
                "cong_control": 0,
                "get_info": 0,
                "name": [98,112,102,95,100,99,116,99,112,0,0,0,0,0,0,0
                ],
                "owner": 0
            }
        }
    }
]

Misc Notes:
* bpf_struct_ops_map_sys_lookup_elem() is added for syscall lookup.
  It does an inplace update on "*value" instead returning a pointer
  to syscall.c.  Otherwise, it needs a separate copy of "zero" value
  for the BPF_STRUCT_OPS_STATE_INIT to avoid races.

* The bpf_struct_ops_map_delete_elem() is also called without
  preempt_disable() from map_delete_elem().  It is because
  the "->unreg()" may requires sleepable context, e.g.
  the "tcp_unregister_congestion_control()".

* "const" is added to some of the existing "struct btf_func_model *"
  function arg to avoid a compiler warning caused by this patch.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200109003505.3855919-1-kafai@fb.com

1 files changed, 47 insertions, 2 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 50f3b20ae284..a7bfe8a388c6 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -17,6 +17,7 @@
 #include <linux/u64_stats_sync.h>
 #include <linux/refcount.h>
 #include <linux/mutex.h>
+#include <linux/module.h>
 
 struct bpf_verifier_env;
 struct bpf_verifier_log;
@@ -106,6 +107,7 @@ struct bpf_map {
 	struct btf *btf;
 	struct bpf_map_memory memory;
 	char name[BPF_OBJ_NAME_LEN];
+	u32 btf_vmlinux_value_type_id;
 	bool unpriv_array;
 	bool frozen; /* write-once; write-protected by freeze_mutex */
 	/* 22 bytes hole */
@@ -183,7 +185,8 @@ static inline bool bpf_map_offload_neutral(const struct bpf_map *map)
 
 static inline bool bpf_map_support_seq_show(const struct bpf_map *map)
 {
-	return map->btf && map->ops->map_seq_show_elem;
+	return (map->btf_value_type_id || map->btf_vmlinux_value_type_id) &&
+		map->ops->map_seq_show_elem;
 }
 
 int map_check_no_btf(const struct bpf_map *map,
@@ -441,7 +444,8 @@ struct btf_func_model {
  *      fentry = a set of program to run before calling original function
  *      fexit = a set of program to run after original function
  */
-int arch_prepare_bpf_trampoline(void *image, struct btf_func_model *m, u32 flags,
+int arch_prepare_bpf_trampoline(void *image, void *image_end,
+				const struct btf_func_model *m, u32 flags,
 				struct bpf_prog **fentry_progs, int fentry_cnt,
 				struct bpf_prog **fexit_progs, int fexit_cnt,
 				void *orig_call);
@@ -672,6 +676,7 @@ struct bpf_array_aux {
 	struct work_struct work;
 };
 
+struct bpf_struct_ops_value;
 struct btf_type;
 struct btf_member;
 
@@ -681,21 +686,61 @@ struct bpf_struct_ops {
 	int (*init)(struct btf *btf);
 	int (*check_member)(const struct btf_type *t,
 			    const struct btf_member *member);
+	int (*init_member)(const struct btf_type *t,
+			   const struct btf_member *member,
+			   void *kdata, const void *udata);
+	int (*reg)(void *kdata);
+	void (*unreg)(void *kdata);
 	const struct btf_type *type;
+	const struct btf_type *value_type;
 	const char *name;
 	struct btf_func_model func_models[BPF_STRUCT_OPS_MAX_NR_MEMBERS];
 	u32 type_id;
+	u32 value_id;
 };
 
 #if defined(CONFIG_BPF_JIT) && defined(CONFIG_BPF_SYSCALL)
+#define BPF_MODULE_OWNER ((void *)((0xeB9FUL << 2) + POISON_POINTER_DELTA))
 const struct bpf_struct_ops *bpf_struct_ops_find(u32 type_id);
 void bpf_struct_ops_init(struct btf *btf);
+bool bpf_struct_ops_get(const void *kdata);
+void bpf_struct_ops_put(const void *kdata);
+int bpf_struct_ops_map_sys_lookup_elem(struct bpf_map *map, void *key,
+				       void *value);
+static inline bool bpf_try_module_get(const void *data, struct module *owner)
+{
+	if (owner == BPF_MODULE_OWNER)
+		return bpf_struct_ops_get(data);
+	else
+		return try_module_get(owner);
+}
+static inline void bpf_module_put(const void *data, struct module *owner)
+{
+	if (owner == BPF_MODULE_OWNER)
+		bpf_struct_ops_put(data);
+	else
+		module_put(owner);
+}
 #else
 static inline const struct bpf_struct_ops *bpf_struct_ops_find(u32 type_id)
 {
 	return NULL;
 }
 static inline void bpf_struct_ops_init(struct btf *btf) { }
+static inline bool bpf_try_module_get(const void *data, struct module *owner)
+{
+	return try_module_get(owner);
+}
+static inline void bpf_module_put(const void *data, struct module *owner)
+{
+	module_put(owner);
+}
+static inline int bpf_struct_ops_map_sys_lookup_elem(struct bpf_map *map,
+						     void *key,
+						     void *value)
+{
+	return -EINVAL;
+}
 #endif
 
 struct bpf_array {