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From:   Song Liu <songliubraving@fb.com>
To:     Martin Lau <kafai@fb.com>
CC:     KP Singh <kpsingh@kernel.org>, bpf <bpf@vger.kernel.org>,
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        Kernel Team <Kernel-team@fb.com>, Hao Luo <haoluo@google.com>,
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Subject: Re: [PATCH bpf-next 1/4] bpf: enable task local storage for tracing
 programs
Thread-Topic: [PATCH bpf-next 1/4] bpf: enable task local storage for tracing
 programs
Thread-Index: AQHW5hbxN1bu8adE10+5MJhm5YS4VaoiytsAgAAsZICAAAZZAIAAHdAA
Date:   Mon, 11 Jan 2021 23:45:08 +0000
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> On Jan 11, 2021, at 1:58 PM, Martin Lau <kafai@fb.com> wrote:
>=20
> 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:
>>>=20
>>> On Fri, Jan 08, 2021 at 03:19:47PM -0800, Song Liu wrote:
>>>=20
>>> [ ... ]
>>>=20
>>>> diff --git a/kernel/bpf/bpf_local_storage.c b/kernel/bpf/bpf_local_sto=
rage.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 bp=
f_local_storage_elem *selem)
>>>> {
>>>>      struct bpf_local_storage *local_storage;
>>>>      bool free_local_storage =3D false;
>>>> +     unsigned long flags;
>>>>=20
>>>>      if (unlikely(!selem_linked_to_storage(selem)))
>>>>              /* selem has already been unlinked from sk */
>>>>              return;
>>>>=20
>>>>      local_storage =3D 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.
>>>=20
>>> 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.
>>>=20
>>> [ ... ]
>>>=20
>>>> diff --git a/kernel/bpf/bpf_task_storage.c b/kernel/bpf/bpf_task_stora=
ge.c
>>>> index 4ef1959a78f27..f654b56907b69 100644
>>>> diff --git a/kernel/fork.c b/kernel/fork.c
>>>> index 7425b3224891d..3d65c8ebfd594 100644
>>> [ ... ]
>>>=20
>>>> --- 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>
>>>>=20
>>>> #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().
>>>=20
>>> 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?
>>=20
>> 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 =3D 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=20
__put_task_struct().=20

Thanks,
Song=