Re: [PATCH bpf-next v3 6/7] bpf: Let bpf_iter_task_new accept null task ptr

[Chuyi Zhou <zhouchuyi@bytedance.com>]

Hello,

在 2023/9/28 07:37, Andrii Nakryiko 写道:
> On Mon, Sep 25, 2023 at 3:56 AM Chuyi Zhou <zhouchuyi@bytedance.com> wrote:
>>
>> When using task_iter to iterate all threads of a specific task, we enforce
>> that the user must pass a valid task pointer to ensure safety. However,
>> when iterating all threads/process in the system, BPF verifier still
>> require a valid ptr instead of "nullable" pointer, even though it's
>> pointless, which is a kind of surprising from usability standpoint. It
>> would be nice if we could let that kfunc accept a explicit null pointer
>> when we are using BPF_TASK_ITER_ALL/BPF_TASK_ITER_PROC and a valid pointer
>> when using BPF_TASK_ITER_THREAD.
>>
>> Given a trival kfunc:
>>          __bpf_kfunc void FN(struct TYPE_A *obj)
>>
>> BPF Prog would reject a nullptr for obj. The error info is:
>> "arg#x pointer type xx xx must point to scalar, or struct with scalar"
>> reported by get_kfunc_ptr_arg_type(). The reg->type is SCALAR_VALUE and
>> the btf type of ref_t is not scalar or scalar_struct which leads to the
>> rejection of get_kfunc_ptr_arg_type.
>>
>> This patch reuse the __opt annotation which was used to indicate that
>> the buffer associated with an __sz or __szk argument may be null:
>>          __bpf_kfunc void FN(struct TYPE_A *obj__opt)
>> Here __opt indicates obj can be optional, user can pass a explicit nullptr
>> or a normal TYPE_A pointer. In get_kfunc_ptr_arg_type(), we will detect
>> whether the current arg is optional and register is null, If so, return
>> a new kfunc_ptr_arg_type KF_ARG_PTR_TO_NULL and skip to the next arg in
>> check_kfunc_args().
>>
>> Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
>> ---
>>   kernel/bpf/task_iter.c |  7 +++++--
>>   kernel/bpf/verifier.c  | 13 ++++++++++++-
>>   2 files changed, 17 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
>> index 9bcd3f9922b1..7ac007f161cc 100644
>> --- a/kernel/bpf/task_iter.c
>> +++ b/kernel/bpf/task_iter.c
>> @@ -867,7 +867,7 @@ struct bpf_iter_task_kern {
>>          unsigned int type;
>>   } __attribute__((aligned(8)));
>>
>> -__bpf_kfunc int bpf_iter_task_new(struct bpf_iter_task *it, struct task_struct *task, unsigned int type)
>> +__bpf_kfunc int bpf_iter_task_new(struct bpf_iter_task *it, struct task_struct *task__opt, unsigned int type)
>>   {
>>          struct bpf_iter_task_kern *kit = (void *)it;
>>          BUILD_BUG_ON(sizeof(struct bpf_iter_task_kern) != sizeof(struct bpf_iter_task));
>> @@ -877,14 +877,17 @@ __bpf_kfunc int bpf_iter_task_new(struct bpf_iter_task *it, struct task_struct *
>>          switch (type) {
>>          case BPF_TASK_ITER_ALL:
>>          case BPF_TASK_ITER_PROC:
>> +               break;
>>          case BPF_TASK_ITER_THREAD:
>> +               if (!task__opt)
>> +                       return -EINVAL;
>>                  break;
>>          default:
>>                  return -EINVAL;
>>          }
>>
>>          if (type == BPF_TASK_ITER_THREAD)
>> -               kit->task = task;
>> +               kit->task = task__opt;
>>          else
>>                  kit->task = &init_task;
>>          kit->pos = kit->task;
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index a065e18a0b3a..a79204c75a90 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -10331,6 +10331,7 @@ enum kfunc_ptr_arg_type {
>>          KF_ARG_PTR_TO_CALLBACK,
>>          KF_ARG_PTR_TO_RB_ROOT,
>>          KF_ARG_PTR_TO_RB_NODE,
>> +       KF_ARG_PTR_TO_NULL,
>>   };
>>
>>   enum special_kfunc_type {
>> @@ -10425,6 +10426,12 @@ static bool is_kfunc_bpf_rcu_read_unlock(struct bpf_kfunc_call_arg_meta *meta)
>>          return meta->func_id == special_kfunc_list[KF_bpf_rcu_read_unlock];
>>   }
>>
>> +static inline bool is_kfunc_arg_optional_null(struct bpf_reg_state *reg,
>> +                               const struct btf *btf, const struct btf_param *arg)
>> +{
>> +       return register_is_null(reg) && is_kfunc_arg_optional(btf, arg);
>> +}
>> +
>>   static enum kfunc_ptr_arg_type
>>   get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
>>                         struct bpf_kfunc_call_arg_meta *meta,
>> @@ -10497,6 +10504,8 @@ get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
>>           */
>>          if (!btf_type_is_scalar(ref_t) && !__btf_type_is_scalar_struct(env, meta->btf, ref_t, 0) &&
>>              (arg_mem_size ? !btf_type_is_void(ref_t) : 1)) {
>> +                       if (is_kfunc_arg_optional_null(reg, meta->btf, &args[argno]))
>> +                               return KF_ARG_PTR_TO_NULL;
> 
> This nested check seems misplaced. Maybe we shouldn't reuse __opt
> suffix which already has a different meaning (coupled with __sz). Why
> not add "__nullable" convention and just check it separately?
> 

IIUC, do you mean:

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index dbba2b806017..05d197365fcb 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10458,6 +10458,8 @@ get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
         if (is_kfunc_arg_callback(env, meta->btf, &args[argno]))
                 return KF_ARG_PTR_TO_CALLBACK;

+       if (is_kfunc_arg_nullable(meta->btf, &args[argno]) && 
register_is_null(reg))
+               return KF_ARG_PTR_TO_NULL;

         if (argno + 1 < nargs &&
             (is_kfunc_arg_mem_size(meta->btf, &args[argno + 1], 
&regs[regno + 1]) ||


OK, I would change in next version.

Thanks.