Re: [PATCH bpf-next v5 2/3] bpf: Create argument information for nullable arguments.

[Martin KaFai Lau <martin.lau@linux.dev>]

On 2/5/24 10:38 PM, thinker.li@gmail.com wrote:
> From: Kui-Feng Lee <thinker.li@gmail.com>
> 
> Collect argument information from the type information of stub functions to
> mark arguments of BPF struct_ops programs with PTR_MAYBE_NULL if they are
> nullable.  A nullable argument is annotated by suffixing "__nullable" at
> the argument name of stub function.
> 
> For nullable arguments, this patch sets an arg_info to label their reg_type
> with PTR_TO_BTF_ID | PTR_TRUSTED | PTR_MAYBE_NULL. This makes the verifier
> to check programs and ensure that they properly check the pointer. The
> programs should check if the pointer is null before accessing the pointed
> memory.
> 
> The implementer of a struct_ops type should annotate the arguments that can
> be null. The implementer should define a stub function (empty) as a
> placeholder for each defined operator. The name of a stub function should
> be in the pattern "<st_op_type>__<operator name>". For example, for
> test_maybe_null of struct bpf_testmod_ops, it's stub function name should
> be "bpf_testmod_ops__test_maybe_null". You mark an argument nullable by
> suffixing the argument name with "__nullable" at the stub function.
> 
> Since we already has stub functions for kCFI, we just reuse these stub
> functions with the naming convention mentioned earlier. These stub
> functions with the naming convention is only required if there are nullable
> arguments to annotate. For functions having not nullable arguments, stub
> functions are not necessary for the purpose of this patch.
> 
> This patch will prepare a list of struct bpf_ctx_arg_aux, aka arg_info, for
> each member field of a struct_ops type.  "arg_info" will be assigned to
> "prog->aux->ctx_arg_info" of BPF struct_ops programs in
> check_struct_ops_btf_id() so that it can be used by btf_ctx_access() later
> to set reg_type properly for the verifier.
> 
> Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
> ---
>   include/linux/bpf.h         |  18 ++++
>   kernel/bpf/bpf_struct_ops.c | 185 ++++++++++++++++++++++++++++++++++--
>   kernel/bpf/btf.c            |  40 ++++++++
>   kernel/bpf/verifier.c       |   6 ++
>   4 files changed, 242 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 9a2ee9456989..29d9ec1c4fd9 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1709,6 +1709,19 @@ struct bpf_struct_ops {
>   	struct btf_func_model func_models[BPF_STRUCT_OPS_MAX_NR_MEMBERS];
>   };
>   
> +/* Every member of a struct_ops type has an instance even the member is not
> + * an operator (function pointer). The "arg_info" field will be assigned to
> + * prog->aux->arg_info of BPF struct_ops programs to provide the argument
> + * information required by the verifier to verify the program.
> + *
> + * btf_ctx_access() will lookup prog->aux->arg_info to find the
> + * corresponding entry for an given argument.
> + */
> +struct bpf_struct_ops_member_arg_info {

nit. Shorten it to bpf_struct_ops_arg_info.

> +	struct bpf_ctx_arg_aux *arg_info;
> +	u32 arg_info_cnt;
> +};
> +
>   struct bpf_struct_ops_desc {
>   	struct bpf_struct_ops *st_ops;
>   
> @@ -1716,6 +1729,9 @@ struct bpf_struct_ops_desc {
>   	const struct btf_type *value_type;
>   	u32 type_id;
>   	u32 value_id;
> +
> +	/* Collection of argument information for each member */
> +	struct bpf_struct_ops_member_arg_info *member_arg_info;

nit. Same here on the struct's field name. s/member_arg_info/arg_info(s)/.

>   };
>   
>   enum bpf_struct_ops_state {
> @@ -2500,6 +2516,8 @@ int bpf_prog_test_run_sk_lookup(struct bpf_prog *prog,
>   int bpf_prog_test_run_nf(struct bpf_prog *prog,
>   			 const union bpf_attr *kattr,
>   			 union bpf_attr __user *uattr);
> +int btf_ctx_arg_offset(struct btf *btf, const struct btf_type *func_proto,
> +		       u32 arg_no);

This should be in btf.h. Its implementation is also in btf.c in this patch.

>   bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>   		    const struct bpf_prog *prog,
>   		    struct bpf_insn_access_aux *info);
> diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
> index f98f580de77a..0db7e12a9244 100644
> --- a/kernel/bpf/bpf_struct_ops.c
> +++ b/kernel/bpf/bpf_struct_ops.c
> @@ -116,17 +116,162 @@ static bool is_valid_value_type(struct btf *btf, s32 value_id,
>   	return true;
>   }
>   
> +#define MAYBE_NULL_SUFFIX "__nullable"
> +#define MAX_STUB_NAME 128
> +
> +static bool match_nullable_suffix(const char *name)
> +{
> +	int suffix_len, len;
> +
> +	if (!name)
> +		return false;
> +
> +	suffix_len = sizeof(MAYBE_NULL_SUFFIX) - 1;
> +	len = strlen(name);
> +	if (len <= suffix_len)

This function looks almost the same as __kfunc_param_match_suffix. How about 
refactor __kfunc_param_match_suffix (to btf.c?) and resue here to avoid 
duplicating code?

The only difference is between the "<=" here and the "<" there. I think the "<=" 
here may be a little better.

> +		return false;
> +
> +	return !strcmp(name + len - suffix_len, MAYBE_NULL_SUFFIX);
> +}
> +
> +/* Return the type info of a stub function, if it exists.
> + *
> + * The name of the stub function is made up of the name of the struct_ops
> + * and the name of the function pointer member, separated by "__". For
> + * example, if the struct_ops is named "foo_ops" and the function pointer
> + * member is named "bar", the stub function name would be "foo_ops__bar".
> + */
> +static const  struct btf_type *

I think there is double spaces.

> +find_stub_func_proto(struct btf *btf, const char *st_op_name,
> +		     const char *member_name)
> +{
> +	char stub_func_name[MAX_STUB_NAME];
> +	const struct btf_type *t, *func_proto;
> +	s32 btf_id;
> +
> +	snprintf(stub_func_name, MAX_STUB_NAME, "%s__%s",
> +		 st_op_name, member_name);

check truncated case.

> +	btf_id = btf_find_by_name_kind(btf, stub_func_name, BTF_KIND_FUNC);
> +	if (btf_id < 0)
> +		return NULL;
> +	t = btf_type_by_id(btf, btf_id);
> +	if (!t)
> +		return NULL;
> +	func_proto = btf_type_by_id(btf, t->type);

nit. directly "return btf_type_by_id(btf, t->type);"

> +
> +	return func_proto;
> +}
> +
> +/* Prepare argument info for every nullable argument of a member of a
> + * struct_ops type.
> + *
> + * Initialize a struct bpf_struct_ops_member_arg_info according to type
> + * info of the arguments of a stub function. (Check kCFI for more
> + * information about stub functions.)
> + *
> + * Each member in the struct_ops type has a struct
> + * bpf_struct_ops_member_arg_info to provide an array of struct
> + * bpf_ctx_arg_aux, which in turn provides the information that used by the
> + * verifier to check the arguments of the BPF struct_ops program assigned
> + * to the member. Here, we only care about the arguments that are marked as
> + * __nullable.
> + *
> + * The array of struct bpf_ctx_arg_aux is eventually assigned to
> + * prog->aux->ctx_arg_info of BPF struct_ops programs and passed to the
> + * verifier. (See check_struct_ops_btf_id())
> + *
> + * member_arg_info->arg_info will be the list of struct bpf_ctx_arg_aux if
> + * success. If fails, it will be kept untouched.
> + */
> +static int prepare_arg_info(struct btf *btf,
> +			    const char *st_ops_name,
> +			    const char *member_name,
> +			    const struct btf_type *func_proto,
> +			    struct bpf_struct_ops_member_arg_info *member_arg_info)
> +{
> +	const struct btf_type *stub_func_proto, *pointed_type;
> +	const struct btf_param *args, *member_args;
> +	struct bpf_ctx_arg_aux *arg_info, *ai_buf;
> +	u32 nargs, arg_no, arg_info_cnt = 0;
> +	const char *arg_name;
> +	s32 arg_btf_id;
> +	int offset;
> +
> +	stub_func_proto = find_stub_func_proto(btf, st_ops_name, member_name);
> +	if (!stub_func_proto)
> +		return 0;
> +
> +	args = btf_params(stub_func_proto);
> +	nargs = btf_type_vlen(stub_func_proto);
> +	if (nargs != btf_type_vlen(func_proto)) {
> +		pr_warn("the number of arguments of the stub function %s__%s does not match the number of arguments of the member %s of struct %s\n",
> +			st_ops_name, member_name, member_name, st_ops_name);
> +		return -EINVAL;
> +	}
> +
> +	member_args = btf_params(func_proto);

hmm... the "member_args" naming makes me come back here a few times to figure 
out "member_args" is the original func_proto or the stub func_proto.

How about using a similar convention like you did for func_proto: "func_proto" 
means the original func_proto and "stub_func_proto" means the stub func_proto,

so something like:

"args" means the args of original func_proto and
"stub_args" means the args of the stub_func_proto ?

> +
> +	ai_buf = kcalloc(nargs, sizeof(*ai_buf), GFP_KERNEL);

hmm... instead of ai, lets use a fuller name here like arg_info_buf.

> +	if (!ai_buf)
> +		return -ENOMEM;
> +
> +	for (arg_no = 0; arg_no < nargs; arg_no++) {
> +		/* Skip arguments that is not suffixed with
> +		 * "__nullable".
> +		 */
> +		arg_name = btf_name_by_offset(btf,
> +					      args[arg_no].name_off);
> +		if (!match_nullable_suffix(arg_name))
> +			continue;
> +
> +		/* Should be a pointer to struct, array, scalar, or enum */
> +		pointed_type = btf_type_resolve_ptr(btf,
> +						    member_args[arg_no].type,
> +						    &arg_btf_id);
> +		if (!pointed_type ||
> +		    !btf_type_is_struct(pointed_type))
> +			goto err_out;
> +
> +		offset = btf_ctx_arg_offset(btf, stub_func_proto, arg_no);

The original "func_proto" should be used here.

> +		if (offset < 0)
> +			goto err_out;
> +
> +		/* Fill the information of the new argument */
> +		arg_info = ai_buf + arg_info_cnt++;

nit. just init arg_info = arg_info_buf and advance the arg_info++ and 
arg_info_cnt++ after the offset assignment below.

> +		arg_info->reg_type =
> +			PTR_TRUSTED | PTR_TO_BTF_ID | PTR_MAYBE_NULL;
> +		arg_info->btf_id = arg_btf_id;
> +		arg_info->btf = btf;
> +		arg_info->offset = offset;
> +	}
> +
> +	if (arg_info_cnt) {
> +		member_arg_info->arg_info = ai_buf;
> +		member_arg_info->arg_info_cnt = arg_info_cnt;
> +	} else {
> +		kfree(ai_buf);
> +	}
> +
> +	return 0;
> +
> +err_out:
> +	kfree(ai_buf);
> +
> +	return -EINVAL;
> +}
> +
>   int bpf_struct_ops_desc_init(struct bpf_struct_ops_desc *st_ops_desc,
>   			     struct btf *btf,
>   			     struct bpf_verifier_log *log)
>   {
> +	struct bpf_struct_ops_member_arg_info *member_arg_info;
>   	struct bpf_struct_ops *st_ops = st_ops_desc->st_ops;
>   	const struct btf_member *member;
>   	const struct btf_type *t;
>   	s32 type_id, value_id;
>   	char value_name[128];
>   	const char *mname;
> -	int i;
> +	int i, err;
>   
>   	if (strlen(st_ops->name) + VALUE_PREFIX_LEN >=
>   	    sizeof(value_name)) {
> @@ -160,6 +305,11 @@ int bpf_struct_ops_desc_init(struct bpf_struct_ops_desc *st_ops_desc,
>   	if (!is_valid_value_type(btf, value_id, t, value_name))
>   		return -EINVAL;
>   
> +	member_arg_info = kcalloc(btf_type_vlen(t), sizeof(*member_arg_info),
> +				  GFP_KERNEL);
> +	if (!member_arg_info)
> +		return -ENOMEM;
> +
>   	for_each_member(i, t, member) {
>   		const struct btf_type *func_proto;
>   
> @@ -167,32 +317,44 @@ int bpf_struct_ops_desc_init(struct bpf_struct_ops_desc *st_ops_desc,
>   		if (!*mname) {
>   			pr_warn("anon member in struct %s is not supported\n",
>   				st_ops->name);
> -			return -EOPNOTSUPP;
> +			err = -EOPNOTSUPP;
> +			goto errout;
>   		}
>   
>   		if (__btf_member_bitfield_size(t, member)) {
>   			pr_warn("bit field member %s in struct %s is not supported\n",
>   				mname, st_ops->name);
> -			return -EOPNOTSUPP;
> +			err = -EOPNOTSUPP;
> +			goto errout;
>   		}
>   
>   		func_proto = btf_type_resolve_func_ptr(btf,
>   						       member->type,
>   						       NULL);
> -		if (func_proto &&
> -		    btf_distill_func_proto(log, btf,
> +		if (!func_proto)
> +			continue;
> +
> +		if (btf_distill_func_proto(log, btf,
>   					   func_proto, mname,
>   					   &st_ops->func_models[i])) {
>   			pr_warn("Error in parsing func ptr %s in struct %s\n",
>   				mname, st_ops->name);
> -			return -EINVAL;
> +			err = -EINVAL;
> +			goto errout;
>   		}
> +
> +		err = prepare_arg_info(btf, st_ops->name, mname,
> +				       func_proto,
> +				       member_arg_info + i);
> +		if (err)
> +			goto errout;
>   	}
>   
>   	if (st_ops->init(btf)) {
>   		pr_warn("Error in init bpf_struct_ops %s\n",
>   			st_ops->name);
> -		return -EINVAL;
> +		err = -EINVAL;
> +		goto errout;
>   	}
>   
>   	st_ops_desc->type_id = type_id;
> @@ -200,7 +362,16 @@ int bpf_struct_ops_desc_init(struct bpf_struct_ops_desc *st_ops_desc,
>   	st_ops_desc->value_id = value_id;
>   	st_ops_desc->value_type = btf_type_by_id(btf, value_id);
>   
> +	st_ops_desc->member_arg_info = member_arg_info;
> +
>   	return 0;
> +
> +errout:
> +	while (i > 0)
> +		kfree(member_arg_info[--i].arg_info);
> +	kfree(member_arg_info);
> +
> +	return err;
>   }
>   
>   static int bpf_struct_ops_map_get_next_key(struct bpf_map *map, void *key,
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index aa72674114af..6df390ade2c0 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -1699,6 +1699,21 @@ static void btf_free_struct_meta_tab(struct btf *btf)
>   static void btf_free_struct_ops_tab(struct btf *btf)
>   {
>   	struct btf_struct_ops_tab *tab = btf->struct_ops_tab;
> +	struct bpf_struct_ops_member_arg_info *ma_info;

Please try to stay consistent with the variable naming in the same patch. The 
other of this patch is using the "member_arg_info" name.

May be just use "arg_infos" everywhere.

> +	int i, j;
> +
> +	if (!tab)
> +		return;
> +
> +	for (i = 0; i < tab->cnt; i++) {
> +		ma_info = tab->ops[i].member_arg_info;
> +		if (!ma_info)
> +			continue;
> +
> +		for (j = 0; j < btf_type_vlen(tab->ops[i].type); j++)
> +			kfree(ma_info[j].arg_info);
> +		kfree(ma_info);


Lets refactor this part out to "bpf_struct_ops_desc_release(struct 
bpf_struct_ops_desc *)" such that it can be reused with the "errout" path of the 
"bpf_struct_ops_desc_init()" above.


> +	}
>   
>   	kfree(tab);
>   	btf->struct_ops_tab = NULL;
> @@ -6130,6 +6145,31 @@ static bool prog_args_trusted(const struct bpf_prog *prog)
>   	}
>   }
>   
> +int btf_ctx_arg_offset(struct btf *btf, const struct btf_type *func_proto,
> +		       u32 arg_no)
> +{
> +	const struct btf_param *args;
> +	const struct btf_type *t;
> +	int off = 0, i;
> +	u32 sz, nargs;
> +
> +	nargs = btf_type_vlen(func_proto);
> +	/* It is the return value if arg_no == nargs */
> +	if (arg_no > nargs)
> +		return -EINVAL;
> +
> +	args = btf_params(func_proto);
> +	for (i = 0; i < arg_no; i++) {
> +		t = btf_type_by_id(btf, args[i].type);
> +		t = btf_resolve_size(btf, t, &sz);
> +		if (IS_ERR(t))
> +			return -EINVAL;
> +		off += roundup(sz, 8);
> +	}
> +
> +	return off;
> +}
> +
>   bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>   		    const struct bpf_prog *prog,
>   		    struct bpf_insn_access_aux *info)
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 64fa188d00ad..8d7e761cda0d 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -20433,6 +20433,12 @@ static int check_struct_ops_btf_id(struct bpf_verifier_env *env)
>   		}
>   	}
>   
> +	/* btf_ctx_access() used this to provide argument type info */
> +	prog->aux->ctx_arg_info =
> +		st_ops_desc->member_arg_info[member_idx].arg_info;
> +	prog->aux->ctx_arg_info_size =
> +		st_ops_desc->member_arg_info[member_idx].arg_info_cnt;
> +
>   	prog->aux->attach_func_proto = func_proto;
>   	prog->aux->attach_func_name = mname;
>   	env->ops = st_ops->verifier_ops;