Re: [PATCH 05/10] bpf: Add cookie support to programs attached with kprobe multi link

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Mar 07, 2022 at 05:23:31PM -0800, Andrii Nakryiko wrote:
> On Sun, Mar 6, 2022 at 9:29 AM Jiri Olsa <olsajiri@gmail.com> wrote:
> >
> > On Fri, Mar 04, 2022 at 03:11:08PM -0800, Andrii Nakryiko wrote:
> > > On Tue, Feb 22, 2022 at 9:07 AM Jiri Olsa <jolsa@kernel.org> wrote:
> > > >
> > > > Adding support to call bpf_get_attach_cookie helper from
> > > > kprobe programs attached with kprobe multi link.
> > > >
> > > > The cookie is provided by array of u64 values, where each
> > > > value is paired with provided function address or symbol
> > > > with the same array index.
> > > >
> > > > Suggested-by: Andrii Nakryiko <andrii@kernel.org>
> > > > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > > > ---
> > > >  include/linux/sort.h           |   2 +
> > > >  include/uapi/linux/bpf.h       |   1 +
> > > >  kernel/trace/bpf_trace.c       | 103 ++++++++++++++++++++++++++++++++-
> > > >  lib/sort.c                     |   2 +-
> > > >  tools/include/uapi/linux/bpf.h |   1 +
> > > >  5 files changed, 107 insertions(+), 2 deletions(-)
> > > >
> > >
> > > [...]
> > >
> > > >  BPF_CALL_1(bpf_get_attach_cookie_trace, void *, ctx)
> > > >  {
> > > >         struct bpf_trace_run_ctx *run_ctx;
> > > > @@ -1297,7 +1312,9 @@ kprobe_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> > > >                         &bpf_get_func_ip_proto_kprobe_multi :
> > > >                         &bpf_get_func_ip_proto_kprobe;
> > > >         case BPF_FUNC_get_attach_cookie:
> > > > -               return &bpf_get_attach_cookie_proto_trace;
> > > > +               return prog->expected_attach_type == BPF_TRACE_KPROBE_MULTI ?
> > > > +                       &bpf_get_attach_cookie_proto_kmulti :
> > > > +                       &bpf_get_attach_cookie_proto_trace;
> > > >         default:
> > > >                 return bpf_tracing_func_proto(func_id, prog);
> > > >         }
> > > > @@ -2203,6 +2220,9 @@ struct bpf_kprobe_multi_link {
> > > >         struct bpf_link link;
> > > >         struct fprobe fp;
> > > >         unsigned long *addrs;
> > > > +       struct bpf_run_ctx run_ctx;
> > >
> > > clever, I like it! Keep in mind, though, that this trick can only be
> > > used here because this run_ctx is read-only (I'd leave the comment
> > > here about this, I didn't realize immediately that this approach can't
> > > be used for run_ctx that needs to be modified).
> >
> > hum, I don't see it at the moment.. I'll check on that and add the
> > comment or come up with more questions ;-)
> 
> if run_ctx is used to store some information, it has to be per program
> execution (private to a single bpf program run, just like bpf
> program's stack). So you can't just reuse bpf_link for that, because
> bpf_link is shared across all CPUs and thus (potentially) across
> multiple simultaneous prog runs

ok, I'll put some comments in here about that

thanks,
jirka