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

[Song Liu]

> On Jan 11, 2021, at 1:58 PM, Martin Lau <ka...@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 <ka...@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(). 

Thanks,
Song