Re: [PATCH bpf-next v3 06/11] bpf: Introduce BPF_MAP_TYPE_STRUCT_OPS

[Yonghong Song <yhs@fb.com>]

On 12/30/19 10:20 PM, Martin KaFai Lau wrote:
> 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.
> 
> Acked-by: Andrii Nakryiko <andriin@fb.com>
> Signed-off-by: Martin KaFai Lau <kafai@fb.com>

Acked-by: Yonghong Song <yhs@fb.com>