Re: [PATCH bpf-next v1 6/8] bpf: Introduce bpf_per_cpu_ptr()

[Andrii Nakryiko]

On Sat, Aug 22, 2020 at 12:49 AM Hao Luo <haoluo@xxxxxxxxxx> wrote:
>
> On Fri, Aug 21, 2020 at 8:31 PM Andrii Nakryiko
> <andrii.nakryiko@xxxxxxxxx> wrote:
> >
> > On Fri, Aug 21, 2020 at 8:26 PM Andrii Nakryiko
> > <andrii.nakryiko@xxxxxxxxx> wrote:
> > >
> > > On Wed, Aug 19, 2020 at 3:42 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
> > > >
> > > > Add bpf_per_cpu_ptr() to help bpf programs access percpu vars.
> > > > bpf_per_cpu_ptr() has the same semantic as per_cpu_ptr() in the kernel
> > > > except that it may return NULL. This happens when the cpu parameter is
> > > > out of range. So the caller must check the returned value.
> > > >
> > > > Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
> > > > ---
> > >
> > > The logic looks correct, few naming nits, but otherwise:
> > >
> > > Acked-by: Andrii Nakryiko <andriin@xxxxxx>
> > >
> > > >  include/linux/bpf.h      |  3 ++
> > > >  include/linux/btf.h      | 11 +++++++
> > > >  include/uapi/linux/bpf.h | 14 +++++++++
> > > >  kernel/bpf/btf.c         | 10 -------
> > > >  kernel/bpf/verifier.c    | 64 ++++++++++++++++++++++++++++++++++++++--
> > > >  kernel/trace/bpf_trace.c | 18 +++++++++++
> > > >  6 files changed, 107 insertions(+), 13 deletions(-)
> [...]
> >
> > btw, having bpf_this_cpu_ptr(const void *ptr) seems worthwhile as well, WDYT?
> >
>
> It's probably not a good idea, IMHO. How does it interact with
> preemption? Should we treat it as __this_cpu_ptr()? If so, I feel it's
> easy to be misused, if the bpf program is called in a preemptible
> context.
>
> Btw, is bpf programs always called with preemption disabled? How about
> interrupts? I haven't thought about these questions before but I think
> they matter as we start to have more ways for bpf programs to interact
> with the kernel.

non-sleepable BPF is always disabling CPU migration, so there is no
problem with this_cpu_ptr. For sleepable not sure, but we can disable
this helper for sleepable BPF programs, if that's a problem.

>
> Best,
> Hao