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Date: Fri, 15 Jan 2021 17:50:03 -0800
From: Yonghong Song <yhs@...com>
To: Song Liu <songliubraving@...com>
CC: KP Singh <kpsingh@...nel.org>, Martin Lau <kafai@...com>,
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 1/15/21 5:12 PM, Song Liu wrote:
>
>
>> On Jan 15, 2021, at 4:55 PM, Yonghong Song <yhs@...com> wrote:
>>
>>
>>
>> On 1/15/21 3:34 PM, Song Liu wrote:
>>>> On Jan 12, 2021, at 8:53 AM, KP Singh <kpsingh@...nel.org> wrote:
>>>>
>>>> On Tue, Jan 12, 2021 at 5:32 PM Yonghong Song <yhs@...com> wrote:
>>>>>
>>>>>
>>>>>
>>>>> 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
>>>>
>>>> [...]
>>>>
>>>>>>>>>> +#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()?
>>>>
>>>> Yeah, something like, or if you find a more elegant alternative :)
>>>>
>>>> --- a/include/linux/sched/task.h
>>>> +++ b/include/linux/sched/task.h
>>>> @@ -107,13 +107,20 @@ extern void __put_task_struct(struct task_struct *t);
>>>>
>>>> static inline void put_task_struct(struct task_struct *t)
>>>> {
>>>> - if (refcount_dec_and_test(&t->usage))
>>>> +
>>>> + if (rcu_access_pointer(t->bpf_storage)) {
>>>> + if (refcount_sub_and_test(2, &t->usage))
>>>> + __put_task_struct(t);
>>>> + } else if (refcount_dec_and_test(&t->usage))
>>>> __put_task_struct(t);
>>>> }
>>>>
>>>> static inline void put_task_struct_many(struct task_struct *t, int nr)
>>>> {
>>>> - if (refcount_sub_and_test(nr, &t->usage))
>>>> + if (rcu_access_pointer(t->bpf_storage)) {
>>>> + if (refcount_sub_and_test(nr + 1, &t->usage))
>>>> + __put_task_struct(t);
>>>> + } else if (refcount_sub_and_test(nr, &t->usage))
>>>> __put_task_struct(t);
>>>> }
>>> It is not ideal to leak bpf_storage here. How about we only add the
>>> following:
>>> diff --git i/kernel/bpf/bpf_task_storage.c w/kernel/bpf/bpf_task_storage.c
>>> index f654b56907b69..2811b9fc47233 100644
>>> --- i/kernel/bpf/bpf_task_storage.c
>>> +++ w/kernel/bpf/bpf_task_storage.c
>>> @@ -216,6 +216,10 @@ 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) {
>>> + /* the task_struct is being freed, fail over*/
>>> + if (!refcount_read(&task->usage))
>>> + return -EBUSY;
>>
>> This may not work? Even we check here and task->usage is not 0, it could still become 0 immediately after the above refcount_read, right?
>
> We call bpf_task_storage_get() with "task" that has valid BTF, so "task"
> should not go away during the BPF program? Whatever mechanism that
Oh, right. this is true. Otherwise, we cannot use task ptr in the helper.
> triggers the BPF program should either hold a reference to task (usage > 0)
> or be the only one owning it (usage == 0, in __put_task_struct). Did I miss
> anything?
Sorry. I think you are right. Not sure lsm requirement. There are two
more possible ways to check task is exiting which happens before
__put_task_struct():
. check task->exit_state
. check task->flags & PF_EXITING (used in bpf_trace.c)
Not sure which condition is the correct one to check.
>
> Thanks,
> Song
>
>>
>>> +
>>> sdata = bpf_local_storage_update(
>>> task, (struct bpf_local_storage_map *)map, value,
>>> BPF_NOEXIST);
>>>>
>>>>
>>>> I may be missing something but shouldn't bpf_storage be an __rcu
>>>> member like we have for sk_bpf_storage?
>>> Good catch! I will fix this in v2.
>>> Thanks,
>>> Song
>
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