Re: [RFC bpf-next v4 5/6] bpf: Create argument information for nullable arguments.

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

On 2/2/24 16:40, Martin KaFai Lau wrote:
> On 2/2/24 2:05 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.
> 
> I looked at the high level. Some comments below.
> 
>>
>> Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
>> ---
>>   include/linux/bpf.h         |  17 ++++
>>   kernel/bpf/bpf_struct_ops.c | 166 ++++++++++++++++++++++++++++++++++--
>>   kernel/bpf/btf.c            |  14 +++
>>   kernel/bpf/verifier.c       |   6 ++
>>   4 files changed, 198 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index 9a2ee9456989..63ef5cbfd213 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 {
>> +    struct bpf_ctx_arg_aux *arg_info;
>> +    u32 arg_info_cnt;
>> +};
>> +
>>   struct bpf_struct_ops_desc {
>>       struct bpf_struct_ops *st_ops;
>> @@ -1716,6 +1729,10 @@ 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;
>> +    u32 member_arg_info_cnt;
>>   };
>>   enum bpf_struct_ops_state {
>> diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
>> index f98f580de77a..313f6ceabcf4 100644
>> --- a/kernel/bpf/bpf_struct_ops.c
>> +++ b/kernel/bpf/bpf_struct_ops.c
>> @@ -116,17 +116,148 @@ 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 int match_nullable_suffix(const char *name)
>> +{
>> +    int suffix_len, len;
>> +
>> +    if (!name)
>> +        return 0;
>> +
>> +    suffix_len = sizeof(MAYBE_NULL_SUFFIX) - 1;
>> +    len = strlen(name);
>> +    if (len < suffix_len)
>> +        return 0;
>> +
>> +    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 *
>> +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);
>> +    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);
>> +
>> +    return func_proto;
>> +}
>> +
>> +/* Prepare argument info for every nullable argument of a member of a
>> + * struct_ops type.
>> + *
>> + * Create and initialize a list of struct bpf_struct_ops_member_arg_info
>> + * according to type info of the arguments of the stub functions. (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())
>> + */
>> +static int prepare_arg_info(struct btf *btf,
>> +                const char *st_ops_name,
>> +                const char *member_name,
>> +                struct bpf_struct_ops_member_arg_info *member_arg_info)
>> +{
>> +    const struct btf_type *stub_func_proto, *ptr_type;
>> +    struct bpf_ctx_arg_aux *arg_info, *ai_buf = NULL;
>> +    const struct btf_param *args;
>> +    u32 nargs, arg_no = 0;
>> +    const char *arg_name;
>> +    s32 arg_btf_id;
>> +
>> +    stub_func_proto = find_stub_func_proto(btf, st_ops_name, 
>> member_name);
>> +    if (!stub_func_proto)
>> +        return 0;
>> +
>> +    nargs = btf_type_vlen(stub_func_proto);
>> +    if (nargs > MAX_BPF_FUNC_REG_ARGS) {
> 
> Checking MAX_BPF_FUNC_REG_ARGS on the stub_func_proto may not be the 
> right check. It should have been done on the origin func_proto (i.e. 
> non-stub) when preparing the func_model in btf_distill_func_proto(). 
> Please double check.

Got it!

> 
> If it needs to do sanity check on nargs of stub_func_proto, a better 
> check is to ensure the narg of the stub_func_proto is the same as the 
> orig_func_proto instead. This discrepancy probably should have been 
> complained by the compiler already but does not harm to check (==) here 
> in case the argument type is changed and a force cast is used (more below).

Yes, it should be complained by the compiler. However, we are not sure
if the stub function found is the one assign to .cfi_stubs, or a random
function happening to have a matched name.

> 
>> +        pr_warn("Cannot support #%u args in stub func %s_stub_%s\n",
>> +            nargs, st_ops_name, member_name);
>> +        return -EINVAL;
>> +    }
>> +
>> +    ai_buf = kcalloc(nargs, sizeof(*ai_buf), GFP_KERNEL);
>> +    if (!ai_buf)
>> +        return -ENOMEM;
>> +
>> +    args = btf_params(stub_func_proto);
>> +    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))
> 
> I have a question/request.
> 
> On top of tagging nullable, can we extend the ctx_arg_info idea here to 
> allow changing the pointer type?
> 
> In particular, take a stub function in bpf_tcp_ca.c:
> 
> static u32 bpf_tcp_ca_ssthresh(struct tcp_sock *tp)
> {
>          return 0;
> }
> 
> Instead of the "struct sock *sk" argument as defined in the 
> tcp_congestion_ops, the stub function uses "struct tcp_sock *tp'. If we 
> can reuse the ctx_arg_info idea here, then it can remove the existing 
> way of changing the pointer type from bpf_tcp_ca_is_valid_access.

Yes, it can be. We need a way to annotate the argument we want to
override/promote its type, or generate ctx_arg_info for each
argument of a stub function.

> 
>> +            continue;
>> +
>> +        /* Should be a pointer to struct, array, scalar, or enum */
>> +        ptr_type = btf_type_resolve_ptr(btf, args[arg_no].type,
>> +                        &arg_btf_id);
>> +        if (!ptr_type ||
>> +            (!btf_type_is_struct(ptr_type) &&
>> +             !btf_type_is_array(ptr_type) &&
>> +             !btf_type_is_scalar(ptr_type) &&
>> +             !btf_is_any_enum(ptr_type))) {
>> +            kfree(ai_buf);
>> +            return -EINVAL;
>> +        }
>> +
>> +        /* Fill the information of the new argument */
>> +        arg_info = ai_buf + member_arg_info->arg_info_cnt++;
>> +        arg_info->reg_type =
>> +            PTR_TRUSTED | PTR_MAYBE_NULL | PTR_TO_BTF_ID;
>> +        arg_info->btf_id = arg_btf_id;
>> +        arg_info->btf = btf;
>> +        arg_info->offset = arg_no * sizeof(u64);
> 
> I think for the current struct_ops users should be fine to assume 
> sizeof(u64). The current struct_ops users should only have 
> pointer/scalar argument (meaning there is no struct passed-by-value 
> argument).
> 
> I still think it is better to get it correct for all trampoline 
> supported argument here. Take a look at 720e6a435194 ("bpf: Allow struct 
> argument in trampoline based programs") and get_ctx_arg_idx(). It may be 

I will add another function to translate arg_no to offset.

> easier (not sure if it is cleaner) to directly store the arg_no into 
> arg_info here but arg_info only has offset now. Please think about what 
> could be a cleaner way to do it.

The offset here is an offset from the start of a context where
the argument is. The BPF opcode access an argument with it's offset, so
we eventually need to translate the arg_no into the offset. The
difference is translating here or in btf_ctx_access().

The question here is "what is OFFSET for?"  Without explanation, it
is hard for people to tell what it is. Maybe, we need to change the its
to ctx_offset or alike.

> 
>> +    }
>> +
>> +    if (!member_arg_info->arg_info_cnt)
>> +        kfree(ai_buf);
>> +    else
>> +        member_arg_info->arg_info = ai_buf;
>> +
>> +    return 0;
>> +}
>> +
>>   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 +291,11 @@ 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;
>> @@ -167,13 +303,15 @@ 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,
>> @@ -185,14 +323,24 @@ int bpf_struct_ops_desc_init(struct 
>> bpf_struct_ops_desc *st_ops_desc,
>>                          &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,
>> +                       member_arg_info + i);
>> +        if (err)
>> +            goto errout;
>>       }
>> +    st_ops_desc->member_arg_info = member_arg_info;
>> +    st_ops_desc->member_arg_info_cnt = btf_type_vlen(t);
> 
> It should be the same as btf_type_vlen(st_ops_desc->type). I would avoid 
> this duplicated info within the same st_ops_desc.

Will remove it.

> 
>> +
>>       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 +349,14 @@ 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:
>> +    while (i > 0)
>> +        kfree(member_arg_info[--i].arg_info);
>> +    kfree(member_arg_info);
>> +    st_ops_desc->member_arg_info = NULL;
>> +
>> +    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 20d2160b3db5..fd192f69eb78 100644
>> --- a/kernel/bpf/btf.c
>> +++ b/kernel/bpf/btf.c
>> @@ -1699,6 +1699,20 @@ 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;
>> +    int i, j;
>> +    u32 cnt;
>> +
>> +    if (tab)
>> +        for (i = 0; i < tab->cnt; i++) {
>> +            ma_info = tab->ops[i].member_arg_info;
>> +            if (ma_info) {
>> +                cnt = tab->ops[i].member_arg_info_cnt;
>> +                for (j = 0; j < cnt; j++)
>> +                    kfree(ma_info[j].arg_info);
>> +            }
>> +            kfree(ma_info);
>> +        }
>>       kfree(tab);
>>       btf->struct_ops_tab = NULL;
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index cd4d780e5400..d1d1c2836bc2 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -20373,6 +20373,12 @@ 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;
>