Re: [PATCH bpf-next v2 3/4] bpf, x86: Support BPF cookie for fentry/fexit/fmod_ret.

[Kui-Feng Lee <kuifeng@fb.com>]

On Mon, 2022-03-21 at 16:18 -0700, Andrii Nakryiko wrote:
> On Tue, Mar 15, 2022 at 5:44 PM Kui-Feng Lee <kuifeng@fb.com> wrote:
> > 
> > Add a bpf_cookie field to attach a cookie to an instance of struct
> > bpf_link.  The cookie of a bpf_link will be installed when calling
> > the
> > associated program to make it available to the program.
> > 
> > Signed-off-by: Kui-Feng Lee <kuifeng@fb.com>
> > ---
> >  arch/x86/net/bpf_jit_comp.c    |  4 ++--
> >  include/linux/bpf.h            |  1 +
> >  include/uapi/linux/bpf.h       |  1 +
> >  kernel/bpf/syscall.c           | 11 +++++++----
> >  kernel/trace/bpf_trace.c       | 17 +++++++++++++++++
> >  tools/include/uapi/linux/bpf.h |  1 +
> >  tools/lib/bpf/bpf.c            | 14 ++++++++++++++
> >  tools/lib/bpf/bpf.h            |  1 +
> >  tools/lib/bpf/libbpf.map       |  1 +
> >  9 files changed, 45 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/x86/net/bpf_jit_comp.c
> > b/arch/x86/net/bpf_jit_comp.c
> > index 29775a475513..5fab8530e909 100644
> > --- a/arch/x86/net/bpf_jit_comp.c
> > +++ b/arch/x86/net/bpf_jit_comp.c
> > @@ -1753,8 +1753,8 @@ static int invoke_bpf_prog(const struct
> > btf_func_model *m, u8 **pprog,
> > 
> >         EMIT1(0x52);             /* push rdx */
> > 
> > -       /* mov rdi, 0 */
> > -       emit_mov_imm64(&prog, BPF_REG_1, 0, 0);
> > +       /* mov rdi, cookie */
> > +       emit_mov_imm64(&prog, BPF_REG_1, (long) l->cookie >> 32,
> > (u32) (long) l->cookie);
> 
> why __u64 to long casting? I don't think you need to cast anything at
> all, but if you want to make that more explicit than just casting to
> (u32) should be fine, no?
> 
> > 
> >         /* Prepare struct bpf_trace_run_ctx.
> >          * sub rsp, sizeof(struct bpf_trace_run_ctx)
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index d20a23953696..9469f9264b4f 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -1040,6 +1040,7 @@ struct bpf_link {
> >         struct bpf_prog *prog;
> >         struct work_struct work;
> >         struct hlist_node tramp_hlist;
> > +       u64 cookie;
> 
> I was a bit hesitant about adding tramp_hlist into generic struct
> bpf_link, but now with also cookie there I'm even more convinced that
> it's not the right thing to do... Some BPF links won't have cookie,
> some (like multi-kprobe) will have lots of them.
> 
> Should we create struct bpf_tramp_link {} which will have tramp_hlist
> and cookie? As for tramp_hlist, we can probably also keep it back in
> bpf_prog_aux and just fetch it through link->prog->aux->tramp_hlist
> in
> trampoline code. This might reduce amount of code churn in patch 1.

Do you mean a struct likes like?

struct bpf_tramp_link {
  struct bpf_link link;
  struct hlist_node tramp_hlist;
  u64 cookie;
};

I like this idea since we don't use cookie for every bpf_link.
But, could you give me an example that we don't want a cookie?