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

[KP Singh]

On Tue, Jan 12, 2021 at 5:32 PM Yonghong Song <yhs@xxxxxx> wrote:
>
>
>
> On 1/11/21 3:45 PM, Song Liu wrote:
> >
> >
> >> On Jan 11, 2021, at 1:58 PM, Martin Lau <kafai@xxxxxx> 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@xxxxxx> 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);
 }


I may be missing something but shouldn't bpf_storage be an __rcu
member like we have for sk_bpf_storage?

#ifdef CONFIG_BPF_SYSCALL
struct bpf_local_storage __rcu *sk_bpf_storage;
#endif


>
> > Thanks,
> > Song
> >