Date:   Thu, 9 Mar 2023 00:13:28 -0800
From:   Joanne Koong <joannelkoong@...il.com>
To:     Ilya Leoshkevich <iii@...ux.ibm.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <martin.lau@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Network Development <netdev@...r.kernel.org>,
        Toke Høiland-Jørgensen <toke@...nel.org>
Subject: Re: [PATCH v13 bpf-next 10/10] selftests/bpf: tests for using dynptrs
 to parse skb and xdp buffers

On Wed, Mar 8, 2023 at 6:24 AM Ilya Leoshkevich <iii@...ux.ibm.com> wrote:
>
> On Tue, 2023-03-07 at 23:22 -0800, Joanne Koong wrote:
> > On Tue, Mar 7, 2023 at 5:55 PM Ilya Leoshkevich <iii@...ux.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@...il.com> wrote:
> > > > >
> > > > > On Wed, Mar 1, 2023 at 7:51 AM Joanne Koong
> > > > > <joannelkoong@...il.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?
>
> Precisely.
>
> > 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,
>
> I wonder whether replacement is safe? This would depend on the
> following functions returning the same value for the same inputs:
>
> - may_access_direct_pkt_data() - this looks ok;
> - bpf_dev_bound_resolve_kfunc() - I'm not so sure, any insights?

For the bpf_dev_bound_resolve_kfunc() case (in fixup_kfunc_call()), I
think directly replacing the kfunc_desc here is okay because
bpf_dev_bound_resolve_kfunc() is findingthe target device-specific
version of the kfunc (if it exists) to replace the generic version of
the kfunc with, and we're using that target device-specific version of
the kfunc as the new updated insn->imm to call

>
> If it's not, then MAX_KFUNC_DESCS indeed becomes a concern.
>
> > 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?
>
> Is there a way to find BTF by function pointer?
> IIUC bpf_dev_bound_resolve_kfunc() can return many different things,
> and btf_distill_func_proto() and add_kfunc_call() need BTF.
> A straightforward way that immediately comes to mind is to do kallsyms
> lookup and then resolve by name, but this sounds clumsy.
>

I'm not sure whether there's a way to find the function's BTF by its
pointer, but I think maybe we can use the vmlinux btf (which we can
get through the bpf_get_btf_vmlinux() api) to get the func proto?

>
>
> I've been looking into this in context of fixing (kfunc
> __bpf_call_base) not fitting into 32 bits on s390x. A solution that

Sorry, I'm not fully understanding - can you elaborate a little on
what the issue is? why doesn't the __bpf_call_base address fit on
s390x? my understanding is that s390x is a 64-bit architecture?

> would solve both problems that I'm currently thinking about is to
> associate
>
> struct {
>     struct btf_func_model *m;
>     unsigned long addr;
> } kfunc_callee;
>
> with every insn - during verification it could live in
> bpf_insn_aux_data, during jiting in bpf_prog, and afterwards it can
> be freed. Any thoughts about this?

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