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

[Andrii Nakryiko]

On Sun, Mar 6, 2022 at 9:29 AM Jiri Olsa <olsajiri@xxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> > > ---
> > >  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

>
> >
> > > +       u64 *cookies;
> > > +       u32 cnt;
> > >  };
> > >

[...]

> >
> > >  {
> > >         do {
> > >  #ifdef CONFIG_64BIT
> > > diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> > > index 6c66138c1b9b..d18996502aac 100644
> > > --- a/tools/include/uapi/linux/bpf.h
> > > +++ b/tools/include/uapi/linux/bpf.h
> > > @@ -1482,6 +1482,7 @@ union bpf_attr {
> > >                         struct {
> > >                                 __aligned_u64   syms;
> > >                                 __aligned_u64   addrs;
> > > +                               __aligned_u64   cookies;
> >
> > looks a bit weird to change layout of UAPI. That's not really a
> > problem, because both patches will land at the same time. But if you
> > move flags and cnt to the front of the struct it would a bit better.
>
> I was following your previous comment:
>   https://lore.kernel.org/bpf/CAEf4BzbPeQbURZOD93TgPudOk3JD4odsZ9uwriNkrphes9V4dg@xxxxxxxxxxxxxx/
>

yeah, I didn't anticipate the cookies change at that time, but now it
became obvious

> I like the idea that syms/addrs/cookies stay together,
> because they are all related to cnt.. but yes, it's
> 'breaking' KABI in between these patches
>
> jirka
>
> >
> >
> > >                                 __u32           cnt;
> > >                                 __u32           flags;
> > >                         } kprobe_multi;
> > > --
> > > 2.35.1
> > >