bpf: Add a kfunc to type cast from bpf uapi ctx to kernel ctx

Implement bpf_cast_to_kern_ctx() kfunc which does a type cast
of a uapi ctx object to the corresponding kernel ctx. Previously
if users want to access some data available in kctx but not
in uapi ctx, bpf_probe_read_kernel() helper is needed.
The introduction of bpf_cast_to_kern_ctx() allows direct
memory access which makes code simpler and easier to understand.

Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221120195432.3113982-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/btf.h

@@ -487,6 +487,7 @@ const struct btf_member *
 btf_get_prog_ctx_type(struct bpf_verifier_log *log, const struct btf *btf,
 		      const struct btf_type *t, enum bpf_prog_type prog_type,
 		      int arg);
+int get_kern_ctx_btf_id(struct bpf_verifier_log *log, enum bpf_prog_type prog_type);
 bool btf_types_are_same(const struct btf *btf1, u32 id1,
 			const struct btf *btf2, u32 id2);
 #else
@@ -531,6 +532,10 @@ btf_get_prog_ctx_type(struct bpf_verifier_log *log, const struct btf *btf,
 {
 	return NULL;
 }
+static inline int get_kern_ctx_btf_id(struct bpf_verifier_log *log,
+				      enum bpf_prog_type prog_type) {
+	return -EINVAL;
+}
 static inline bool btf_types_are_same(const struct btf *btf1, u32 id1,
 				      const struct btf *btf2, u32 id2)
 {

kernel/bpf/btf.c

@@ -5603,6 +5603,26 @@ static int btf_translate_to_vmlinux(struct bpf_verifier_log *log,
 	return kern_ctx_type->type;
 }
 
+int get_kern_ctx_btf_id(struct bpf_verifier_log *log, enum bpf_prog_type prog_type)
+{
+	const struct btf_member *kctx_member;
+	const struct btf_type *conv_struct;
+	const struct btf_type *kctx_type;
+	u32 kctx_type_id;
+
+	conv_struct = bpf_ctx_convert.t;
+	/* get member for kernel ctx type */
+	kctx_member = btf_type_member(conv_struct) + bpf_ctx_convert_map[prog_type] * 2 + 1;
+	kctx_type_id = kctx_member->type;
+	kctx_type = btf_type_by_id(btf_vmlinux, kctx_type_id);
+	if (!btf_type_is_struct(kctx_type)) {
+		bpf_log(log, "kern ctx type id %u is not a struct\n", kctx_type_id);
+		return -EINVAL;
+	}
+
+	return kctx_type_id;
+}
+
 BTF_ID_LIST(bpf_ctx_convert_btf_id)
 BTF_ID(struct, bpf_ctx_convert)
 

kernel/bpf/helpers.c

@@ -1879,6 +1879,11 @@ void bpf_task_release(struct task_struct *p)
 	put_task_struct_rcu_user(p);
 }
 
+void *bpf_cast_to_kern_ctx(void *obj)
+{
+	return obj;
+}
+
 __diag_pop();
 
 BTF_SET8_START(generic_btf_ids)
@@ -1907,6 +1912,7 @@ BTF_ID(struct, task_struct)
 BTF_ID(func, bpf_task_release)
 
 BTF_SET8_START(common_btf_ids)
+BTF_ID_FLAGS(func, bpf_cast_to_kern_ctx)
 BTF_SET8_END(common_btf_ids)
 
 static const struct btf_kfunc_id_set common_kfunc_set = {

kernel/bpf/verifier.c

@@ -7907,6 +7907,7 @@ struct bpf_kfunc_call_arg_meta {
 	u32 ref_obj_id;
 	u8 release_regno;
 	bool r0_rdonly;
+	u32 ret_btf_id;
 	u64 r0_size;
 	struct {
 		u64 value;
@@ -8151,6 +8152,7 @@ enum special_kfunc_type {
 	KF_bpf_list_push_back,
 	KF_bpf_list_pop_front,
 	KF_bpf_list_pop_back,
+	KF_bpf_cast_to_kern_ctx,
 };
 
 BTF_SET_START(special_kfunc_set)
@@ -8160,6 +8162,7 @@ BTF_ID(func, bpf_list_push_front)
 BTF_ID(func, bpf_list_push_back)
 BTF_ID(func, bpf_list_pop_front)
 BTF_ID(func, bpf_list_pop_back)
+BTF_ID(func, bpf_cast_to_kern_ctx)
 BTF_SET_END(special_kfunc_set)
 
 BTF_ID_LIST(special_kfunc_list)
@@ -8169,6 +8172,7 @@ BTF_ID(func, bpf_list_push_front)
 BTF_ID(func, bpf_list_push_back)
 BTF_ID(func, bpf_list_pop_front)
 BTF_ID(func, bpf_list_pop_back)
+BTF_ID(func, bpf_cast_to_kern_ctx)
 
 static enum kfunc_ptr_arg_type
 get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
@@ -8182,6 +8186,9 @@ get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
 	struct bpf_reg_state *reg = &regs[regno];
 	bool arg_mem_size = false;
 
+	if (meta->func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx])
+		return KF_ARG_PTR_TO_CTX;
+
 	/* In this function, we verify the kfunc's BTF as per the argument type,
 	 * leaving the rest of the verification with respect to the register
 	 * type to our caller. When a set of conditions hold in the BTF type of
@@ -8668,6 +8675,13 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 				verbose(env, "arg#%d expected pointer to ctx, but got %s\n", i, btf_type_str(t));
 				return -EINVAL;
 			}
+
+			if (meta->func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx]) {
+				ret = get_kern_ctx_btf_id(&env->log, resolve_prog_type(env->prog));
+				if (ret < 0)
+					return -EINVAL;
+				meta->ret_btf_id  = ret;
+			}
 			break;
 		case KF_ARG_PTR_TO_ALLOC_BTF_ID:
 			if (reg->type != (PTR_TO_BTF_ID | MEM_ALLOC)) {
@@ -8922,6 +8936,11 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 				regs[BPF_REG_0].btf = field->list_head.btf;
 				regs[BPF_REG_0].btf_id = field->list_head.value_btf_id;
 				regs[BPF_REG_0].off = field->list_head.node_offset;
+			} else if (meta.func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx]) {
+				mark_reg_known_zero(env, regs, BPF_REG_0);
+				regs[BPF_REG_0].type = PTR_TO_BTF_ID | PTR_TRUSTED;
+				regs[BPF_REG_0].btf = desc_btf;
+				regs[BPF_REG_0].btf_id = meta.ret_btf_id;
 			} else {
 				verbose(env, "kernel function %s unhandled dynamic return type\n",
 					meta.func_name);
@@ -15175,6 +15194,9 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 		insn_buf[1] = addr[1];
 		insn_buf[2] = *insn;
 		*cnt = 3;
+	} else if (desc->func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx]) {
+		insn_buf[0] = BPF_MOV64_REG(BPF_REG_0, BPF_REG_1);
+		*cnt = 1;
 	}
 	return 0;
 }