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

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Message-ID: <20210210211735.4snmhc7gofo6zrp5@kafai-mbp.dhcp.thefacebook.com>
Date:   Wed, 10 Feb 2021 13:17:35 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.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 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.

> 
> >  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
> >