Re: [RFC patch 09/19] bpf: Use BPF_PROG_RUN_PIN_ON_CPU() at simple call sites.

[Alexei Starovoitov]

On Wed, Feb 19, 2020 at 1:01 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> Vinicius Costa Gomes <vinicius.gomes@xxxxxxxxx> writes:
>
> Cc+: seccomp folks
>
> > Thomas Gleixner <tglx@xxxxxxxxxxxxx> writes:
> >
> >> From: David Miller <davem@xxxxxxxxxxxxx>
>
> Leaving content for reference
>
> >> All of these cases are strictly of the form:
> >>
> >>      preempt_disable();
> >>      BPF_PROG_RUN(...);
> >>      preempt_enable();
> >>
> >> Replace this with BPF_PROG_RUN_PIN_ON_CPU() which wraps BPF_PROG_RUN()
> >> with:
> >>
> >>      migrate_disable();
> >>      BPF_PROG_RUN(...);
> >>      migrate_enable();
> >>
> >> On non RT enabled kernels this maps to preempt_disable/enable() and on RT
> >> enabled kernels this solely prevents migration, which is sufficient as
> >> there is no requirement to prevent reentrancy to any BPF program from a
> >> preempting task. The only requirement is that the program stays on the same
> >> CPU.
> >>
> >> Therefore, this is a trivially correct transformation.
> >>
> >> [ tglx: Converted to BPF_PROG_RUN_PIN_ON_CPU() ]
> >>
> >> Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
> >> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> >>
> >> ---
> >>  include/linux/filter.h    |    4 +---
> >>  kernel/seccomp.c          |    4 +---
> >>  net/core/flow_dissector.c |    4 +---
> >>  net/core/skmsg.c          |    8 ++------
> >>  net/kcm/kcmsock.c         |    4 +---
> >>  5 files changed, 6 insertions(+), 18 deletions(-)
> >>
> >> --- a/include/linux/filter.h
> >> +++ b/include/linux/filter.h
> >> @@ -713,9 +713,7 @@ static inline u32 bpf_prog_run_clear_cb(
> >>      if (unlikely(prog->cb_access))
> >>              memset(cb_data, 0, BPF_SKB_CB_LEN);
> >>
> >> -    preempt_disable();
> >> -    res = BPF_PROG_RUN(prog, skb);
> >> -    preempt_enable();
> >> +    res = BPF_PROG_RUN_PIN_ON_CPU(prog, skb);
> >>      return res;
> >>  }
> >>
> >> --- a/kernel/seccomp.c
> >> +++ b/kernel/seccomp.c
> >> @@ -268,16 +268,14 @@ static u32 seccomp_run_filters(const str
> >>       * All filters in the list are evaluated and the lowest BPF return
> >>       * value always takes priority (ignoring the DATA).
> >>       */
> >> -    preempt_disable();
> >>      for (; f; f = f->prev) {
> >> -            u32 cur_ret = BPF_PROG_RUN(f->prog, sd);
> >> +            u32 cur_ret = BPF_PROG_RUN_PIN_ON_CPU(f->prog, sd);
> >>
> >
> > More a question really, isn't the behavior changing here? i.e. shouldn't
> > migrate_disable()/migrate_enable() be moved to outside the loop? Or is
> > running seccomp filters on different cpus not a problem?
>
> In my understanding this is a list of filters and they are independent
> of each other.

Yes. It's fine to be preempted between filters.