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

[Hao Luo <haoluo@google.com>]

On Fri, Aug 21, 2020 at 8:31 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Fri, Aug 21, 2020 at 8:26 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
> >
> > On Wed, Aug 19, 2020 at 3:42 PM Hao Luo <haoluo@google.com> 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@google.com>
> > > ---
> >
> > The logic looks correct, few naming nits, but otherwise:
> >
> > Acked-by: Andrii Nakryiko <andriin@fb.com>
> >
> > >  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.

Best,
Hao