Re: [PATCH v13 bpf-next 10/10] selftests/bpf: tests for using dynptrs to parse skb and xdp buffers

From: Joanne Koong <joannelkoong@gmail.com>
To: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: "Alexei Starovoitov" <alexei.starovoitov@gmail.com>,
	bpf <bpf@vger.kernel.org>,
	"Martin KaFai Lau" <martin.lau@kernel.org>,
	"Andrii Nakryiko" <andrii@kernel.org>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Kumar Kartikeya Dwivedi" <memxor@gmail.com>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Network Development" <netdev@vger.kernel.org>,
	"Toke Høiland-Jørgensen" <toke@kernel.org>
Subject: Re: [PATCH v13 bpf-next 10/10] selftests/bpf: tests for using dynptrs to parse skb and xdp buffers
Date: Tue, 7 Mar 2023 23:22:30 -0800	[thread overview]
Message-ID: <CAJnrk1Y1ONmEJpwDqGzCUmyrkDf9s_HpDhR5mW=6fNKM6PiXew@mail.gmail.com> (raw)
In-Reply-To: <20230308015500.6pycr5i4nynyu22n@heavy>

On Tue, Mar 7, 2023 at 5:55 PM Ilya Leoshkevich <iii@linux.ibm.com> wrote:
>
> On Wed, Mar 01, 2023 at 08:28:40PM -0800, Joanne Koong wrote:
> > On Wed, Mar 1, 2023 at 10:08 AM Alexei Starovoitov
> > <alexei.starovoitov@gmail.com> wrote:
> > >
> > > On Wed, Mar 1, 2023 at 7:51 AM Joanne Koong <joannelkoong@gmail.com> wrote:
> > > >
> > > > 5) progs/dynptr_success.c
> > > >    * Add test case "test_skb_readonly" for testing attempts at writes
> > > >      on a prog type with read-only skb ctx.
> > > >    * Add "test_dynptr_skb_data" for testing that bpf_dynptr_data isn't
> > > >      supported for skb progs.
> > >
> > > I added
> > > +dynptr/test_dynptr_skb_data
> > > +dynptr/test_skb_readonly
> > > to DENYLIST.s390x and applied.
> >
> > Thanks, I'm still not sure why s390x cannot load these programs. It is
> > being loaded in the same way as other tests like
> > test_parse_tcp_hdr_opt() are loading programs. I will keep looking
> > some more into this
>
> Hi,
>
> I believe the culprit is:
>
>     insn->imm = BPF_CALL_IMM(bpf_dynptr_from_skb_rdonly);
>
> s390x needs to know the kfunc model in order to emit the call (like
> i386), but after this assignment it's no longer possible to look it
> up in kfunc_tab by insn->imm. x86_64 does not need this, because its
> ABI is exactly the same as BPF ABI.
>
> The simplest solution seems to be adding an artificial kfunc_desc
> like this:
>
>     {
>         .func_model = desc->func_model,  /* model must be compatible */
>         .func_id = 0,                    /* unused at this point */
>         .imm = insn->imm,                /* new target */
>         .offset = 0,                     /* unused at this point */
>     }
>
> here and also after this assignment:
>
>     insn->imm = BPF_CALL_IMM(xdp_kfunc);
>
> What do you think?

Ohh interesting! This makes sense to me. In particular, you're
referring to the bpf_jit_find_kfunc_model() call in bpf_jit_insn() (in
arch/s390/net/bpf_jit_comp.c) as the one that fails out whenever
insn->imm gets set, correct?

I like your proposed solution, I agree that this looks like the
simplest, though maybe we should replace the existing kfunc_desc
instead of adding it so we don't have to deal with the edge case of
reaching MAX_KFUNC_DESCS? To get the func model of the new insn->imm,
it seems pretty straightforward, it looks like we can just use
btf_distill_func_proto(). or call add_kfunc_call() directly, which
would do everything needed, but adds an additional unnecessary sort
and more overhead for replacing (eg we'd need to first swap the old
kfunc_desc with the last tab->descs[tab->nr_descs] entry and then
delete the old kfunc_desc before adding the new one). What are your
thoughts?

>
> [...]
>
> Best regards,
> Ilya