bpf: Introduce BPF_PROG_TYPE_STRUCT_OPS

This patch allows the kernel's struct ops (i.e. func ptr) to be
implemented in BPF.  The first use case in this series is the
"struct tcp_congestion_ops" which will be introduced in a
latter patch.

This patch introduces a new prog type BPF_PROG_TYPE_STRUCT_OPS.
The BPF_PROG_TYPE_STRUCT_OPS prog is verified against a particular
func ptr of a kernel struct.  The attr->attach_btf_id is the btf id
of a kernel struct.  The attr->expected_attach_type is the member
"index" of that kernel struct.  The first member of a struct starts
with member index 0.  That will avoid ambiguity when a kernel struct
has multiple func ptrs with the same func signature.

For example, a BPF_PROG_TYPE_STRUCT_OPS prog is written
to implement the "init" func ptr of the "struct tcp_congestion_ops".
The attr->attach_btf_id is the btf id of the "struct tcp_congestion_ops"
of the _running_ kernel.  The attr->expected_attach_type is 3.

The ctx of BPF_PROG_TYPE_STRUCT_OPS is an array of u64 args saved
by arch_prepare_bpf_trampoline that will be done in the next
patch when introducing BPF_MAP_TYPE_STRUCT_OPS.

"struct bpf_struct_ops" is introduced as a common interface for the kernel
struct that supports BPF_PROG_TYPE_STRUCT_OPS prog.  The supporting kernel
struct will need to implement an instance of the "struct bpf_struct_ops".

The supporting kernel struct also needs to implement a bpf_verifier_ops.
During BPF_PROG_LOAD, bpf_struct_ops_find() will find the right
bpf_verifier_ops by searching the attr->attach_btf_id.

A new "btf_struct_access" is also added to the bpf_verifier_ops such
that the supporting kernel struct can optionally provide its own specific
check on accessing the func arg (e.g. provide limited write access).

After btf_vmlinux is parsed, the new bpf_struct_ops_init() is called
to initialize some values (e.g. the btf id of the supporting kernel
struct) and it can only be done once the btf_vmlinux is available.

The R0 checks at BPF_EXIT is excluded for the BPF_PROG_TYPE_STRUCT_OPS prog
if the return type of the prog->aux->attach_func_proto is "void".

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/20200109003503.3855825-1-kafai@fb.com

include/linux/bpf.h

@@ -349,6 +349,10 @@ struct bpf_verifier_ops {
 				  const struct bpf_insn *src,
 				  struct bpf_insn *dst,
 				  struct bpf_prog *prog, u32 *target_size);
+	int (*btf_struct_access)(struct bpf_verifier_log *log,
+				 const struct btf_type *t, int off, int size,
+				 enum bpf_access_type atype,
+				 u32 *next_btf_id);
 };
 
 struct bpf_prog_offload_ops {
@@ -668,6 +672,32 @@ struct bpf_array_aux {
 	struct work_struct work;
 };
 
+struct btf_type;
+struct btf_member;
+
+#define BPF_STRUCT_OPS_MAX_NR_MEMBERS 64
+struct bpf_struct_ops {
+	const struct bpf_verifier_ops *verifier_ops;
+	int (*init)(struct btf *btf);
+	int (*check_member)(const struct btf_type *t,
+			    const struct btf_member *member);
+	const struct btf_type *type;
+	const char *name;
+	struct btf_func_model func_models[BPF_STRUCT_OPS_MAX_NR_MEMBERS];
+	u32 type_id;
+};
+
+#if defined(CONFIG_BPF_JIT) && defined(CONFIG_BPF_SYSCALL)
+const struct bpf_struct_ops *bpf_struct_ops_find(u32 type_id);
+void bpf_struct_ops_init(struct btf *btf);
+#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) { }
+#endif
+
 struct bpf_array {
 	struct bpf_map map;
 	u32 elem_size;