netdev - Re: [PATCH bpf] libbpf: fix compatibility for kernels without need

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Message-ID: <CAADnVQKDr0u_YfB_eMYZEPKO1O=4hdzLye9FDMqjy4J3GL8Szg@mail.gmail.com>
Date:   Sat, 12 Oct 2019 09:54:50 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Magnus Karlsson <magnus.karlsson@...il.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Network Development <netdev@...r.kernel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf] libbpf: fix compatibility for kernels without need_wakeup

On Fri, Oct 11, 2019 at 1:58 PM John Fastabend <john.fastabend@...il.com> wrote:
>
> Magnus Karlsson wrote:
> > On Tue, Oct 8, 2019 at 9:29 PM John Fastabend <john.fastabend@...il.com> wrote:
> > >
> > > Magnus Karlsson wrote:
> > > > When the need_wakeup flag was added to AF_XDP, the format of the
> > > > XDP_MMAP_OFFSETS getsockopt was extended. Code was added to the kernel
> > > > to take care of compatibility issues arrising from running
> > > > applications using any of the two formats. However, libbpf was not
> > > > extended to take care of the case when the application/libbpf uses the
> > > > new format but the kernel only supports the old format. This patch
> > > > adds support in libbpf for parsing the old format, before the
> > > > need_wakeup flag was added, and emulating a set of static need_wakeup
> > > > flags that will always work for the application.
> > > >
> > > > Fixes: a4500432c2587cb2a ("libbpf: add support for need_wakeup flag in AF_XDP part")
> > > > Reported-by: Eloy Degen <degeneloy@...il.com>
> > > > Signed-off-by: Magnus Karlsson <magnus.karlsson@...el.com>
> > > > ---
> > > >  tools/lib/bpf/xsk.c | 109 +++++++++++++++++++++++++++++++++++++---------------
> > > >  1 file changed, 78 insertions(+), 31 deletions(-)
> > > >
> > > > diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
> > > > index a902838..46f9687 100644
> > > > --- a/tools/lib/bpf/xsk.c
> > > > +++ b/tools/lib/bpf/xsk.c
> > > > @@ -44,6 +44,25 @@
> > > >   #define PF_XDP AF_XDP
> > > >  #endif
> > > >
> > > > +#define is_mmap_offsets_v1(optlen) \
> > > > +     ((optlen) == sizeof(struct xdp_mmap_offsets_v1))
> > > > +
> > > > +#define get_prod_off(ring) \
> > > > +     (is_mmap_offsets_v1(optlen) ? \
> > > > +      ((struct xdp_mmap_offsets_v1 *)&off)->ring.producer : \
> > > > +      off.ring.producer)
> > > > +#define get_cons_off(ring) \
> > > > +     (is_mmap_offsets_v1(optlen) ? \
> > > > +      ((struct xdp_mmap_offsets_v1 *)&off)->ring.consumer : \
> > > > +      off.ring.consumer)
> > > > +#define get_desc_off(ring) \
> > > > +     (is_mmap_offsets_v1(optlen) ? \
> > > > +      ((struct xdp_mmap_offsets_v1 *)&off)->ring.desc : off.ring.desc)
> > > > +#define get_flags_off(ring) \
> > > > +     (is_mmap_offsets_v1(optlen) ? \
> > > > +      ((struct xdp_mmap_offsets_v1 *)&off)->ring.consumer + sizeof(u32) : \
> > > > +      off.ring.flags)
> > > > +
> > >
> > > It seems the only thing added was flags right? If so seems we
> > > only need the last one there, get_flags_off(). I think it would
> > > be a bit cleaner to just use the macros where its actually
> > > needed IMO.
> >
> > The flag is indeed added to the end of struct xdp_ring_offsets, but
> > this struct is replicated four times in the struct xdp_mmap_offsets,
> > so the added flags are present four time there at different offsets.
> > This means that 3 out of the 4 prod, cons and desc variables are
> > located at different offsets from the original. Do not know how I can
> > get rid of these macros in this case. But it might just be me not
> > seeing it, of course :-).
>
> Not sure I like it but not seeing a cleaner solution that doesn't cause
> larger changes so...
>
> Acked-by: John Fastabend <john.fastabend.gmail.com>

Frankly above hack looks awful.
What is _v1 ?! Is it going to be _v2?
What was _v0?
I also don't see how this is a fix. imo bpf-next is more appropriate
and if "large changes" are necessary then go ahead and do them.
We're not doing fixes-branches in libbpf.
The library always moves forward and compatible with all older kernels.