Re: [PATCH bpf-next v6 3/4] bpf: Create argument information for nullable arguments.

[Kui-Feng Lee <sinquersw@gmail.com>]

On 2/8/24 15:01, Martin KaFai Lau wrote:
> On 2/7/24 10:51 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.
> 
> One more nit on the naming. It is my overlook in v5.
> 
> There are also things that need to address in btf_ctx_arg_offset(). 
> Comment inlined below.
> 
> Other patches of the set lgtm.
> 
>>
>> Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
>> ---
>>   include/linux/bpf.h         |  22 ++++
>>   include/linux/btf.h         |   2 +
>>   kernel/bpf/bpf_struct_ops.c | 197 ++++++++++++++++++++++++++++++++++--
>>   kernel/bpf/btf.c            |  33 ++++++
>>   kernel/bpf/verifier.c       |   6 ++
>>   5 files changed, 253 insertions(+), 7 deletions(-)
>>
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index 9a2ee9456989..6908bd2360ea 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 a member is 
>> not
>> + * an operator (function pointer). The "arg_info" field will be 
>> assigned to
>> + * prog->aux->ctx_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->ctx_arg_info to find the
>> + * corresponding entry for an given argument.
>> + */
>> +struct bpf_struct_ops_arg_info {
>> +    struct bpf_ctx_arg_aux *arg_info;
> 
> One more nit on naming,
> 
> It is my overlook in v5. After looking at how "arg_info" means both 
> "bpf_struct_ops_arg_info" and "bpf_ctx_arg_aux" in this patch, could you 
> do one more rename here and shorten the "*arg_info" here to "*info".


No problem!

> 
>> +    u32 arg_info_cnt;
> 
> and "info_cnt" or just "cnt" here.
> 
>> +};
>> +
>>   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_arg_info *arg_info;
>>   };
>>   enum bpf_struct_ops_state {
>> @@ -1790,6 +1806,8 @@ int bpf_struct_ops_desc_init(struct 
>> bpf_struct_ops_desc *st_ops_desc,
>>                    struct btf *btf,
>>                    struct bpf_verifier_log *log);
>>   void bpf_map_struct_ops_info_fill(struct bpf_map_info *info, struct 
>> bpf_map *map);
>> +void bpf_struct_ops_desc_release(struct bpf_struct_ops_desc 
>> *st_ops_desc,
>> +                 int len);
>>   #else
>>   #define register_bpf_struct_ops(st_ops, type) ({ (void *)(st_ops); 
>> 0; })
>>   static inline bool bpf_try_module_get(const void *data, struct 
>> module *owner)
>> @@ -1814,6 +1832,10 @@ static inline void 
>> bpf_map_struct_ops_info_fill(struct bpf_map_info *info, struc
>>   {
>>   }
>> +static inline void bpf_struct_ops_desc_release(struct 
>> bpf_struct_ops_desc *st_ops_desc, int len)
>> +{
>> +}
>> +
>>   #endif
>>   #if defined(CONFIG_CGROUP_BPF) && defined(CONFIG_BPF_LSM)
>> diff --git a/include/linux/btf.h b/include/linux/btf.h
>> index df76a14c64f6..15ee845e6b38 100644
>> --- a/include/linux/btf.h
>> +++ b/include/linux/btf.h
>> @@ -498,6 +498,8 @@ static inline void *btf_id_set8_contains(const 
>> struct btf_id_set8 *set, u32 id)
>>   bool btf_param_match_suffix(const struct btf *btf,
>>                   const struct btf_param *arg,
>>                   const char *suffix);
>> +int btf_ctx_arg_offset(struct btf *btf, const struct btf_type 
>> *func_proto,
>> +               u32 arg_no);
>>   struct bpf_verifier_log;
>> diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
>> index f98f580de77a..e9cc8c847736 100644
>> --- a/kernel/bpf/bpf_struct_ops.c
>> +++ b/kernel/bpf/bpf_struct_ops.c
>> @@ -116,17 +116,177 @@ static bool is_valid_value_type(struct btf 
>> *btf, s32 value_id,
>>       return true;
>>   }
>> +#define MAYBE_NULL_SUFFIX "__nullable"
>> +#define MAX_STUB_NAME 128
>> +
>> +/* Return the type info of a stub function, if it exists.
>> + *
>> + * The name of a 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 type 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 *
>> +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 *func_type;
>> +    s32 btf_id;
>> +    int cp;
>> +
>> +    cp = snprintf(stub_func_name, MAX_STUB_NAME, "%s__%s",
>> +              st_op_name, member_name);
>> +    if (cp >= MAX_STUB_NAME) {
>> +        pr_warn("Stub function name too long\n");
>> +        return NULL;
>> +    }
>> +    btf_id = btf_find_by_name_kind(btf, stub_func_name, BTF_KIND_FUNC);
>> +    if (btf_id < 0)
>> +        return NULL;
>> +    func_type = btf_type_by_id(btf, btf_id);
>> +    if (!func_type)
>> +        return NULL;
>> +
>> +    return btf_type_by_id(btf, func_type->type); /* FUNC_PROTO */
>> +}
>> +
>> +/* Prepare argument info for every nullable argument of a member of a
>> + * struct_ops type.
>> + *
>> + * Initialize a struct bpf_struct_ops_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_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())
>> + *
>> + * all_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_arg_info *all_arg_info)
> 
> s/all_arg_info/arg_info/
> 
>> +{
>> +    const struct btf_type *stub_func_proto, *pointed_type;
>> +    struct bpf_ctx_arg_aux *arg_info, *arg_info_buf;
> 
> s/arg_info/info/
> s/arg_info_buf/info_buf/
Make sense to me

> 
>> +    const struct btf_param *stub_args, *args;
>> +    u32 nargs, arg_no, arg_info_cnt = 0;
>> +    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;
>> +
>> +    /* Check if the number of arguments of the stub function is the same
>> +     * as the number of arguments of the function pointer.
>> +     */
>> +    nargs = btf_type_vlen(func_proto);
>> +    if (nargs != btf_type_vlen(stub_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;
>> +    }
>> +
>> +    args = btf_params(func_proto);
>> +    stub_args = btf_params(stub_func_proto);
>> +
>> +    arg_info_buf = kcalloc(nargs, sizeof(*arg_info_buf), GFP_KERNEL);
>> +    if (!arg_info_buf)
>> +        return -ENOMEM;
>> +
>> +    /* Prepare arg_info for every nullable argument */
>> +    arg_info = arg_info_buf;
>> +    for (arg_no = 0; arg_no < nargs; arg_no++) {
>> +        /* Skip arguments that is not suffixed with
>> +         * "__nullable".
>> +         */
>> +        if (!btf_param_match_suffix(btf, &stub_args[arg_no],
>> +                        MAYBE_NULL_SUFFIX))
>> +            continue;
>> +
>> +        /* Should be a pointer to struct */
>> +        pointed_type = btf_type_resolve_ptr(btf,
>> +                            args[arg_no].type,
>> +                            &arg_btf_id);
>> +        if (!pointed_type ||
>> +            !btf_type_is_struct(pointed_type))
> 
> pr_warn("stub function %s__%s has %s tagging to an unsupported type\n",
>      st_ops_name, member_name, MAYBE_NULL_SUFFIX);
> 
>> +            goto err_out;
>> +
>> +        offset = btf_ctx_arg_offset(btf, func_proto, arg_no);
>> +        if (offset < 0)
> 
> pr_warn("stub function %s__%s has invalid trampoline ctx offset for 
> arg#%u\n",
>      st_ops_name, member_name, arg_no);


OK!

> 
>> +            goto err_out;
>> +
>> +        /* Fill the information of the new argument */
>> +        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;
>> +
>> +        arg_info++;
>> +        arg_info_cnt++;
>> +    }
>> +
>> +    if (arg_info_cnt) {
>> +        all_arg_info->arg_info = arg_info_buf;
>> +        all_arg_info->arg_info_cnt = arg_info_cnt;
>> +    } else {
>> +        kfree(arg_info_buf);
>> +    }
>> +
>> +    return 0;
>> +
>> +err_out:
>> +    kfree(arg_info_buf);
>> +
>> +    return -EINVAL;
>> +}
>> +
>> +/* Clean up the arg_info in a struct bpf_struct_ops_desc.
>> + *
>> + * The callers should pass the length of st_ops_desc->arg_info.  The 
>> length
>> + * can not be derived from std_ops_desc->type since the list may be
>> + * incomplete.
>> + */
>> +void bpf_struct_ops_desc_release(struct bpf_struct_ops_desc 
>> *st_ops_desc,
>> +                 int len)
> 
> The "len" argument is not needed. It is the 
> btf_type_vlen(st_ops_desc->type). Initialize the 
> st_ops_desc->type/value_type/type_id/value_id earlier if necessary.


Sure!

> 
>> +{
>> +    struct bpf_struct_ops_arg_info *arg_info;
>> +    int i;
>> +
>> +    arg_info = st_ops_desc->arg_info;
>> +    if (!arg_info)
>> +        return;
>> +
>> +    for (i = 0; i < len; i++)
>> +        kfree(arg_info[i].arg_info);
>> +
>> +    kfree(arg_info);
>> +}
>> +
>>   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 *st_ops = st_ops_desc->st_ops;
>> +    struct bpf_struct_ops_arg_info *arg_info;
>>       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 +320,12 @@ 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;
>> +    arg_info = kcalloc(btf_type_vlen(t), sizeof(*arg_info),
>> +               GFP_KERNEL);
>> +    if (!arg_info)
>> +        return -ENOMEM;
>> +
>> +    st_ops_desc->arg_info = arg_info;
>>       for_each_member(i, t, member) {
>>           const struct btf_type *func_proto;
>> @@ -167,32 +333,44 @@ 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,
>> +                       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;
>> @@ -201,6 +379,11 @@ int bpf_struct_ops_desc_init(struct 
>> bpf_struct_ops_desc *st_ops_desc,
>>       st_ops_desc->value_type = btf_type_by_id(btf, value_id);
>>       return 0;
>> +
>> +errout:
>> +    bpf_struct_ops_desc_release(st_ops_desc, i);
>> +
>> +    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 e3508b8008a2..554a57a0eaa5 100644
>> --- a/kernel/bpf/btf.c
>> +++ b/kernel/bpf/btf.c
>> @@ -1699,6 +1699,14 @@ 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;
>> +    int i;
>> +
>> +    if (!tab)
>> +        return;
>> +
>> +    for (i = 0; i < tab->cnt; i++)
>> +        bpf_struct_ops_desc_release(&tab->ops[i],
>> +                        btf_type_vlen(tab->ops[i].type));
>>       kfree(tab);
>>       btf->struct_ops_tab = NULL;
>> @@ -6130,6 +6138,31 @@ 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 */
> 
> I forgot to mention this in v5. This comment is not accurate.
> 
> This function is trying to figure out the trampoline ctx offset for a 
> particular arg_no. arg_no cannot be the return value and arg_no cannot 
> be >= nargs.

After a clarification, I am going to remove this check.

> 
>> +    if (arg_no > nargs)
> 
> so remove this check all together.

I will remove this check.

> 
>> +        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;
> 
> return PTR_ERR(t);


Ok!

> 
>> +        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 7edd70eec7dd..7826d6e6a09b 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -20415,6 +20415,12 @@ 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->arg_info[member_idx].arg_info;
>> +    prog->aux->ctx_arg_info_size =
>> +        st_ops_desc->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;
>