netdev - Re: [PATCH bpf-next v4 05/11] bpf: Enable TCP congestion control kfunc from modules

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Date:   Wed, 22 Sep 2021 03:53:48 +0530
From:   Kumar Kartikeya Dwivedi <memxor@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v4 05/11] bpf: Enable TCP congestion control
 kfunc from modules

On Wed, Sep 22, 2021 at 03:48:55AM IST, Andrii Nakryiko wrote:
> On Mon, Sep 20, 2021 at 7:15 AM Kumar Kartikeya Dwivedi
> <memxor@...il.com> wrote:
> >
> > This commit moves BTF ID lookup into the newly added registration
> > helper, in a way that the bbr, cubic, and dctcp implementation set up
> > their sets in the bpf_tcp_ca kfunc_btf_set list, while the ones not
> > dependent on modules are looked up from the wrapper function.
> >
> > This lifts the restriction for them to be compiled as built in objects,
> > and can be loaded as modules if required. Also modify Makefile.modfinal
> > to resolve_btfids in TCP congestion control modules if the config option
> > is set, using the base BTF support added in the previous commit.
> >
> > See following commits for background on use of:
> >
> >  CONFIG_X86 ifdef:
> >  569c484f9995 (bpf: Limit static tcp-cc functions in the .BTF_ids list to x86)
> >
> >  CONFIG_DYNAMIC_FTRACE ifdef:
> >  7aae231ac93b (bpf: tcp: Limit calling some tcp cc functions to CONFIG_DYNAMIC_FTRACE)
> >
> > [ resolve_btfids uses --no-fail because some crypto kernel modules
> >   under arch/x86/crypto generated from ASM do not have the .BTF sections ]
> >
> > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
> > ---
> >  include/linux/btf.h       |  4 ++++
> >  kernel/bpf/btf.c          |  3 +++
> >  net/ipv4/bpf_tcp_ca.c     | 34 +++-------------------------------
> >  net/ipv4/tcp_bbr.c        | 28 +++++++++++++++++++++++++++-
> >  net/ipv4/tcp_cubic.c      | 26 +++++++++++++++++++++++++-
> >  net/ipv4/tcp_dctcp.c      | 26 +++++++++++++++++++++++++-
> >  scripts/Makefile.modfinal |  1 +
> >  7 files changed, 88 insertions(+), 34 deletions(-)
> >
>
> [...]
>
> > diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
> > index ff805777431c..b4f83533eda6 100644
> > --- a/scripts/Makefile.modfinal
> > +++ b/scripts/Makefile.modfinal
> > @@ -41,6 +41,7 @@ quiet_cmd_btf_ko = BTF [M] $@
> >        cmd_btf_ko =                                                     \
> >         if [ -f vmlinux ]; then                                         \
> >                 LLVM_OBJCOPY="$(OBJCOPY)" $(PAHOLE) -J --btf_base vmlinux $@; \
> > +               $(RESOLVE_BTFIDS) --no-fail -s vmlinux $@;              \
>
> I think I've asked that before, but I don't remember this being
> answered. Why is this --no-fail?
>

Sorry, the first time, I missed that mail, and then it was too late so I decided
to put the reason in the commit message above.

> > [ resolve_btfids uses --no-fail because some crypto kernel modules
> >   under arch/x86/crypto generated from ASM do not have the .BTF sections ]

I could add a mode that fails only when processing a .BTF section present in
object fails, would that be better?

--
Kartikeya