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

[Martin KaFai Lau]

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
> > > @@ -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?