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

[Ilya Leoshkevich <iii@linux.ibm.com>]

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?

[...]

Best regards,
Ilya