Re: [PATCH v3 bpf-next 15/18] bpf: Support attaching tracing BPF program to other BPF programs

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Mon, Nov 11, 2019 at 3:04 PM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Sat, Nov 09, 2019 at 11:17:37PM -0800, Andrii Nakryiko wrote:
> > On Thu, Nov 7, 2019 at 10:41 PM Alexei Starovoitov <ast@kernel.org> wrote:
> > >
> > > Allow FENTRY/FEXIT BPF programs to attach to other BPF programs of any type
> > > including their subprograms. This feature allows snooping on input and output
> > > packets in XDP, TC programs including their return values. In order to do that
> > > the verifier needs to track types not only of vmlinux, but types of other BPF
> > > programs as well. The verifier also needs to translate uapi/linux/bpf.h types
> > > used by networking programs into kernel internal BTF types used by FENTRY/FEXIT
> > > BPF programs. In some cases LLVM optimizations can remove arguments from BPF
> > > subprograms without adjusting BTF info that LLVM backend knows. When BTF info
> > > disagrees with actual types that the verifiers sees the BPF trampoline has to
> > > fallback to conservative and treat all arguments as u64. The FENTRY/FEXIT
> > > program can still attach to such subprograms, but won't be able to recognize
> > > pointer types like 'struct sk_buff *' into won't be able to pass them to
> > > bpf_skb_output() for dumping to user space.
> > >
> > > The BPF_PROG_LOAD command is extended with attach_prog_fd field. When it's set
> > > to zero the attach_btf_id is one vmlinux BTF type ids. When attach_prog_fd
> > > points to previously loaded BPF program the attach_btf_id is BTF type id of
> > > main function or one of its subprograms.
> > >
> > > Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> > > ---
> > >  arch/x86/net/bpf_jit_comp.c |  3 +-
> > >  include/linux/bpf.h         |  2 +
> > >  include/linux/btf.h         |  1 +
> > >  include/uapi/linux/bpf.h    |  1 +
> > >  kernel/bpf/btf.c            | 58 +++++++++++++++++++---
> > >  kernel/bpf/core.c           |  2 +
> > >  kernel/bpf/syscall.c        | 19 +++++--
> > >  kernel/bpf/verifier.c       | 98 +++++++++++++++++++++++++++++--------
> > >  kernel/trace/bpf_trace.c    |  2 -
> > >  9 files changed, 151 insertions(+), 35 deletions(-)
> > >
> >
> > [...]
> >
> > > +
> > > +static bool btf_translate_to_vmlinux(struct bpf_verifier_log *log,
> > > +                                    struct btf *btf,
> > > +                                    const struct btf_type *t,
> > > +                                    struct bpf_insn_access_aux *info)
> > > +{
> > > +       const char *tname = __btf_name_by_offset(btf, t->name_off);
> > > +       int btf_id;
> > > +
> > > +       if (!tname) {
> > > +               bpf_log(log, "Program's type doesn't have a name\n");
> > > +               return false;
> > > +       }
> > > +       if (strcmp(tname, "__sk_buff") == 0) {
> >
> > might be a good idea to ensure that t's type is also a struct?
> >
> > > +               btf_id = btf_resolve_helper_id(log, &bpf_skb_output_proto, 0);
> >
> > This is kind of ugly and high-maintenance. Have you considered having
> > something like this, to do this mapping:
> >
> > struct bpf_ctx_mapping {
> >     struct sk_buff *__sk_buff;
> >     struct xdp_buff *xdp_md;
> > };
> >
> > So field name is a name you are trying to match, while field type is
> > actual type you are mapping to? You won't need to find special
> > function protos (like bpf_skb_output_proto), it will be easy to
> > extend, you'll have real vmlinux types automatically captured for you
> > (you'll just have to initially find bpf_ctx_mapping's btf_id).
>
> I was thinking something along these lines.
> The problem with single struct like above is that it's centralized.
> convert_ctx_access callbacks are all over the place.
> So I'm thinking to add macro like this to bpf.h
> +#define BPF_RECORD_CTX_CONVERSION(user_type, kernel_type) \
> +       ({typedef kernel_type (*bpf_ctx_convert)(user_type); \
> +        (void) (bpf_ctx_convert) (void *) 0;})
>
> and then do
> BPF_RECORD_CTX_CONVERSION(struct bpf_xdp_sock, struct xdp_sock);
> inside convert_ctx_access functions (like bpf_xdp_sock_convert_ctx_access).
> There will be several typedefs with 'bpf_ctx_convert' name. The
> btf_translate_to_vmlinux() will iterate over them. Speed is not criticial here,

I guess that works as well. Please leave a comment explaining the idea
behind this distributed mapping :)

> but long term we probably need to merge prog's BTF with vmlinux's BTF, so most
> of the type comparison is done during prog load. It probably should reduce the
> size of prog's BTF too. Renumbering of prog's BTF will be annoying though.
> Something to consider long term.
>
> >
> > > +               if (btf_id < 0)
> > > +                       return false;
> > > +               info->btf_id = btf_id;
> > > +               return true;
> > > +       }
> > > +       return false;
> > > +}
> > >
> >
> > [...]
> >
> > > +               if (tgt_prog && conservative) {
> > > +                       struct btf_func_model *m = &tr->func.model;
> > > +
> > > +                       /* BTF function prototype doesn't match the verifier types.
> > > +                        * Fall back to 5 u64 args.
> > > +                        */
> > > +                       for (i = 0; i < 5; i++)
> > > +                               m->arg_size[i] = 8;
> > > +                       m->ret_size = 8;
> > > +                       m->nr_args = 5;
> > > +                       prog->aux->attach_func_proto = NULL;
> > > +               } else {
> > > +                       ret = btf_distill_func_proto(&env->log, btf, t,
> > > +                                                    tname, &tr->func.model);
> >
> > there is nothing preventing some parallel thread to modify
> > tr->func.model in parallel, right? Should these modifications be
> > either locked or at least WRITE_ONCE, similar to
> > btf_resolve_helper_id?
>
> hmm. Right. There is a race with bpf_trampoline_lookup. One thread could have
> just created the trampoline and still doing distill, while another thread is
> trying to use it after getting it from bpf_trampoline_lookup. The fix choices
> are not pretty. Either to add a mutex to check_attach_btf_id() or do
> bpf_trampoline_lookup_or_create() with extra callback that does
> btf_distill_func_proto while bpf_trampoline_lookup_or_create is holding
> trampoline_mutex or move most of the check_attach_btf_id() logic into
> bpf_trampoline_lookup_or_create().
> I tried to keep trampoline as abstract concept, but with callback or move
> the verifer and btf logic will bleed into trampoline. Hmm.

yeah, that sounds too intrusive. I'd change btf_distill_func_proto to
accept struct btf_func_model **m, allocate model dynamically, and then
compare_exchange the final constructed model pointer.

Similarly for "fallback to conservative" case.

>