Subject: Re: [PATCH 10/14] bpf: Re-initialize lnode in bpf_ksym_del

From: Jiri Olsa <[email-masked]>
Date: Wed, 12 Feb 2020 11:43:24 +0100

On Tue, Feb 11, 2020 at 10:28:50AM -0800, Andrii Nakryiko wrote:
> On Sat, Feb 8, 2020 at 7:43 AM Jiri Olsa <[email-masked]> wrote:
> >
> > When bpf_prog is removed from kallsyms it's on the way
> > out to be removed, so we don't care about lnode state.
> >
> > However the bpf_ksym_del will be used also by bpf_trampoline
> > and bpf_dispatcher objects, which stay allocated even when
> > they are not in kallsyms list, hence the lnode re-init.
> >
> > The list_del_rcu commentary states that we need to call
> > synchronize_rcu, before we can change/re-init the list_head
> > pointers.
> >
> > Signed-off-by: Jiri Olsa <[email-masked]>
> > ---
> 
> Wouldn't it make more sense to have patches 7 though 10 as a one
> patch? It's a generalization of ksym from being bpf_prog-specific to
> be more general (which this initialization fix is part of, arguably).

it was my initial change ;-) but then I realized I have to explain
several things in the changelog, and that's usually the sign that
you need to split the patch.. also I think now it's easier for review
and backporting

so I prefer it split like this, but if you guys want to squash it
together, I'll do it ;-)

jirka

> 
> >  kernel/bpf/core.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> >
> > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > index 73242fd07893..66b17bea286e 100644
> > --- a/kernel/bpf/core.c
> > +++ b/kernel/bpf/core.c
> > @@ -676,6 +676,13 @@ void bpf_ksym_del(struct bpf_ksym *ksym)
> >         spin_lock_bh(&bpf_lock);
> >         __bpf_ksym_del(ksym);
> >         spin_unlock_bh(&bpf_lock);
> > +
> > +       /*
> > +        * As explained in list_del_rcu, We must call synchronize_rcu
> > +        * before changing list_head pointers.
> > +        */
> > +       synchronize_rcu();
> > +       INIT_LIST_HEAD_RCU(&ksym->lnode);
> >  }
> >
> >  static bool bpf_prog_kallsyms_candidate(const struct bpf_prog *fp)
> > --
> > 2.24.1
> >
>