Commit a35b9af4 authored by Yonghong Song's avatar Yonghong Song Committed by Alexei Starovoitov
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bpf: Add a kfunc for generic type cast 



Implement bpf_rdonly_cast() which tries to cast the object
to a specified type. This tries to support use case like below:
  #define skb_shinfo(SKB) ((struct skb_shared_info *)(skb_end_pointer(SKB)))
where skb_end_pointer(SKB) is a 'unsigned char *' and needs to
be casted to 'struct skb_shared_info *'.

The signature of bpf_rdonly_cast() looks like
   void *bpf_rdonly_cast(void *obj, __u32 btf_id)
The function returns the same 'obj' but with PTR_TO_BTF_ID with
btf_id. The verifier will ensure btf_id being a struct type.

Since the supported type cast may not reflect what the 'obj'
represents, the returned btf_id is marked as PTR_UNTRUSTED, so
the return value and subsequent pointer chasing cannot be
used as helper/kfunc arguments.

Signed-off-by: default avatarYonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221120195437.3114585-1-yhs@fb.com


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent fd264ca0

kernel/bpf/helpers.c

+6 −0
Original line number Diff line number Diff line
@@ -1884,6 +1884,11 @@ void *bpf_cast_to_kern_ctx(void *obj) 
	return obj;

}



void *bpf_rdonly_cast(void *obj__ign, u32 btf_id__k)

{

	return obj__ign;

}



__diag_pop();



BTF_SET8_START(generic_btf_ids)
@@ -1913,6 +1918,7 @@ BTF_ID(func, bpf_task_release) 


BTF_SET8_START(common_btf_ids)

BTF_ID_FLAGS(func, bpf_cast_to_kern_ctx)

BTF_ID_FLAGS(func, bpf_rdonly_cast)

BTF_SET8_END(common_btf_ids)



static const struct btf_kfunc_id_set common_kfunc_set = {

kernel/bpf/verifier.c

+18 −2
Original line number Diff line number Diff line
@@ -8153,6 +8153,7 @@ enum special_kfunc_type { 
	KF_bpf_list_pop_front,

	KF_bpf_list_pop_back,

	KF_bpf_cast_to_kern_ctx,

	KF_bpf_rdonly_cast,

};



BTF_SET_START(special_kfunc_set)
@@ -8163,6 +8164,7 @@ 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_ID(func, bpf_rdonly_cast)

BTF_SET_END(special_kfunc_set)



BTF_ID_LIST(special_kfunc_list)
@@ -8173,6 +8175,7 @@ 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_ID(func, bpf_rdonly_cast)



static enum kfunc_ptr_arg_type

get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
@@ -8809,6 +8812,7 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn, 
	u32 i, nargs, func_id, ptr_type_id;

	int err, insn_idx = *insn_idx_p;

	const struct btf_param *args;

	const struct btf_type *ret_t;

	struct btf *desc_btf;

	u32 *kfunc_flags;
@@ -8888,7 +8892,6 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn, 


		if (meta.btf == btf_vmlinux && btf_id_set_contains(&special_kfunc_set, meta.func_id)) {

			if (meta.func_id == special_kfunc_list[KF_bpf_obj_new_impl]) {

				const struct btf_type *ret_t;

				struct btf *ret_btf;

				u32 ret_btf_id;
@@ -8941,6 +8944,18 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn, 
				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 if (meta.func_id == special_kfunc_list[KF_bpf_rdonly_cast]) {

				ret_t = btf_type_by_id(desc_btf, meta.arg_constant.value);

				if (!ret_t || !btf_type_is_struct(ret_t)) {

					verbose(env,

						"kfunc bpf_rdonly_cast type ID argument must be of a struct\n");

					return -EINVAL;

				}



				mark_reg_known_zero(env, regs, BPF_REG_0);

				regs[BPF_REG_0].type = PTR_TO_BTF_ID | PTR_UNTRUSTED;

				regs[BPF_REG_0].btf = desc_btf;

				regs[BPF_REG_0].btf_id = meta.arg_constant.value;

			} else {

				verbose(env, "kernel function %s unhandled dynamic return type\n",

					meta.func_name);
@@ -15194,7 +15209,8 @@ 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]) {

	} else if (desc->func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx] ||

		   desc->func_id == special_kfunc_list[KF_bpf_rdonly_cast]) {

		insn_buf[0] = BPF_MOV64_REG(BPF_REG_0, BPF_REG_1);

		*cnt = 1;

	}