netdev - Re: [PATCH bpf 2/2] bpf: selftests: Add non function pointer test to struct

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Date:   Wed, 10 Feb 2021 14:54:40 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf 2/2] bpf: selftests: Add non function pointer test to struct_ops

On Wed, Feb 10, 2021 at 1:17 PM Martin KaFai Lau <kafai@...com> wrote:
>
> On Wed, Feb 10, 2021 at 12:27:38PM -0800, Andrii Nakryiko wrote:
> > On Tue, Feb 9, 2021 at 12:11 PM Martin KaFai Lau <kafai@...com> wrote:
> > >
> > > This patch adds a "void *owner" member.  The existing
> > > bpf_tcp_ca test will ensure the bpf_cubic.o and bpf_dctcp.o
> > > can be loaded.
> > >
> > > Signed-off-by: Martin KaFai Lau <kafai@...com>
> > > ---
> >
> > Acked-by: Andrii Nakryiko <andrii@...nel.org>
> >
> > What will happen if BPF code initializes such non-func ptr member?
> > Will libbpf complain or just ignore those values? Ignoring initialized
> > members isn't great.
> The latter. libbpf will ignore non-func ptr member.  The non-func ptr
> member stays zero when it is passed to the kernel.
>
> libbpf can be changed to copy this non-func ptr value.
> The kernel will decide what to do with it.  It will
> then be consistent with int/array member like ".name"
> and ".flags" where the kernel will verify the value.
> I can spin v2 to do that.

I was thinking about erroring out on non-zero fields, but if you think
it's useful to pass through values, it could be done, but will require
more and careful code, probably. So, basically, don't feel obligated
to do this in this patch set.

>
> >
> > >  tools/testing/selftests/bpf/bpf_tcp_helpers.h | 1 +
> > >  1 file changed, 1 insertion(+)
> > >
> > > diff --git a/tools/testing/selftests/bpf/bpf_tcp_helpers.h b/tools/testing/selftests/bpf/bpf_tcp_helpers.h
> > > index 6a9053162cf2..91f0fac632f4 100644
> > > --- a/tools/testing/selftests/bpf/bpf_tcp_helpers.h
> > > +++ b/tools/testing/selftests/bpf/bpf_tcp_helpers.h
> > > @@ -177,6 +177,7 @@ struct tcp_congestion_ops {
> > >          * after all the ca_state processing. (optional)
> > >          */
> > >         void (*cong_control)(struct sock *sk, const struct rate_sample *rs);
> > > +       void *owner;
> > >  };
> > >
> > >  #define min(a, b) ((a) < (b) ? (a) : (b))
> > > --
> > > 2.24.1
> > >