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

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Date:   Mon, 11 Jan 2021 22:35:43 +0100
From:   KP Singh <kpsingh@...nel.org>
To:     Martin KaFai Lau <kafai@...com>
Cc:     Song Liu <songliubraving@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>, 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 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.