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Message-ID: <e4002f5c-6c2c-0945-9324-a8dc51125018@fb.com>
Date: Tue, 12 Jan 2021 08:32:18 -0800
From: Yonghong Song <yhs@...com>
To: Song Liu <songliubraving@...com>, Martin Lau <kafai@...com>
CC: KP Singh <kpsingh@...nel.org>, bpf <bpf@...r.kernel.org>,
Networking <netdev@...r.kernel.org>,
open list <linux-kernel@...r.kernel.org>,
"mingo@...hat.com" <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
John Fastabend <john.fastabend@...il.com>,
Kernel Team <Kernel-team@...com>, Hao Luo <haoluo@...gle.com>,
kernel test robot <lkp@...el.com>
Subject: Re: [PATCH bpf-next 1/4] bpf: enable task local storage for tracing
programs
On 1/11/21 3:45 PM, Song Liu wrote:
>
>
>> On Jan 11, 2021, at 1:58 PM, Martin Lau <kafai@...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@...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
>
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