On Tue, Mar 22, 2022 at 9:08 AM Kui-Feng Lee <kuifeng@xxxxxx> wrote:
>
> 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@xxxxxx> 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@xxxxxx>
> > > ---
> > > 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;
> };
something like this, yes. Keep in mind that we already use struct
bpf_tracing_link which is used for all trampoline-based programs,
except for struct_ops. So we can either somehow make struct_ops just
result struct bpf_tracing_link (cc Martin for ideas, he was thinking
about doing proper bpf_link support for struct_ops anyways), or we'll
need this kind of struct inheritance to reuse the same layout between
struct_ops and struct bpf_tracing_link.
>
> 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?
>
For example, currently cgroup-based programs don't have cookie
support. So doesn't raw_tp, btw. But it's not only cases when we don't
support cookie, it's also cases like bpf_kprobe_multi_link which has a
separate array of cookies, so this u64 cookie is useless in such case.
