Re: [PATCH bpf-next 1/4] bpf: enable task local storage for tracing programs

[Yonghong Song <yhs@fb.com>]

On 1/11/21 3:45 PM, Song Liu wrote:
> 
> 
>> On Jan 11, 2021, at 1:58 PM, Martin Lau <kafai@fb.com> wrote:
>>
>> On Mon, Jan 11, 2021 at 10:35:43PM +0100, KP Singh wrote:
>>> On Mon, Jan 11, 2021 at 7:57 PM Martin KaFai Lau <kafai@fb.com> wrote:
>>>>
>>>> On Fri, Jan 08, 2021 at 03:19:47PM -0800, Song Liu wrote:
>>>>
>>>> [ ... ]
>>>>
>>>>> diff --git a/kernel/bpf/bpf_local_storage.c b/kernel/bpf/bpf_local_storage.c
>>>>> index dd5aedee99e73..9bd47ad2b26f1 100644
>>>>> --- a/kernel/bpf/bpf_local_storage.c
>>>>> +++ b/kernel/bpf/bpf_local_storage.c
>>>>> @@ -140,17 +140,18 @@ static void __bpf_selem_unlink_storage(struct bpf_local_storage_elem *selem)
>>>>> {
>>>>>       struct bpf_local_storage *local_storage;
>>>>>       bool free_local_storage = false;
>>>>> +     unsigned long flags;
>>>>>
>>>>>       if (unlikely(!selem_linked_to_storage(selem)))
>>>>>               /* selem has already been unlinked from sk */
>>>>>               return;
>>>>>
>>>>>       local_storage = rcu_dereference(selem->local_storage);
>>>>> -     raw_spin_lock_bh(&local_storage->lock);
>>>>> +     raw_spin_lock_irqsave(&local_storage->lock, flags);
>>>> It will be useful to have a few words in commit message on this change
>>>> for future reference purpose.
>>>>
>>>> Please also remove the in_irq() check from bpf_sk_storage.c
>>>> to avoid confusion in the future.  It probably should
>>>> be in a separate patch.
>>>>
>>>> [ ... ]
>>>>
>>>>> diff --git a/kernel/bpf/bpf_task_storage.c b/kernel/bpf/bpf_task_storage.c
>>>>> index 4ef1959a78f27..f654b56907b69 100644
>>>>> diff --git a/kernel/fork.c b/kernel/fork.c
>>>>> index 7425b3224891d..3d65c8ebfd594 100644
>>>> [ ... ]
>>>>
>>>>> --- a/kernel/fork.c
>>>>> +++ b/kernel/fork.c
>>>>> @@ -96,6 +96,7 @@
>>>>> #include <linux/kasan.h>
>>>>> #include <linux/scs.h>
>>>>> #include <linux/io_uring.h>
>>>>> +#include <linux/bpf.h>
>>>>>
>>>>> #include <asm/pgalloc.h>
>>>>> #include <linux/uaccess.h>
>>>>> @@ -734,6 +735,7 @@ void __put_task_struct(struct task_struct *tsk)
>>>>>       cgroup_free(tsk);
>>>>>       task_numa_free(tsk, true);
>>>>>       security_task_free(tsk);
>>>>> +     bpf_task_storage_free(tsk);
>>>>>       exit_creds(tsk);
>>>> If exit_creds() is traced by a bpf and this bpf is doing
>>>> bpf_task_storage_get(..., BPF_LOCAL_STORAGE_GET_F_CREATE),
>>>> new task storage will be created after bpf_task_storage_free().
>>>>
>>>> I recalled there was an earlier discussion with KP and KP mentioned
>>>> BPF_LSM will not be called with a task that is going away.
>>>> It seems enabling bpf task storage in bpf tracing will break
>>>> this assumption and needs to be addressed?
>>>
>>> For tracing programs, I think we will need an allow list where
>>> task local storage can be used.
>> Instead of whitelist, can refcount_inc_not_zero(&tsk->usage) be used?
> 
> I think we can put refcount_inc_not_zero() in bpf_task_storage_get, like:
> 
> diff --git i/kernel/bpf/bpf_task_storage.c w/kernel/bpf/bpf_task_storage.c
> index f654b56907b69..93d01b0a010e6 100644
> --- i/kernel/bpf/bpf_task_storage.c
> +++ w/kernel/bpf/bpf_task_storage.c
> @@ -216,6 +216,9 @@ BPF_CALL_4(bpf_task_storage_get, struct bpf_map *, map, struct task_struct *,
>           * by an RCU read-side critical section.
>           */
>          if (flags & BPF_LOCAL_STORAGE_GET_F_CREATE) {
> +               if (!refcount_inc_not_zero(&task->usage))
> +                       return -EBUSY;
> +
>                  sdata = bpf_local_storage_update(
>                          task, (struct bpf_local_storage_map *)map, value,
>                          BPF_NOEXIST);
> 
> But where shall we add the refcount_dec()? IIUC, we cannot add it to
> __put_task_struct().

Maybe put_task_struct()?

> Thanks,
> Song
>