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

[Yonghong Song]

On 1/15/21 3:34 PM, Song Liu wrote:
> > On Jan 12, 2021, at 8:53 AM, KP Singh <kpsingh@xxxxxxxxxx> wrote:
> >
> > 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);
> > }
>
> 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?

> +
>                  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