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

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Message-ID: <5d9ce369d6e5_17cc2aba94c845b415@john-XPS-13-9370.notmuch>
Date:   Tue, 08 Oct 2019 12:28:41 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Magnus Karlsson <magnus.karlsson@...el.com>,
        magnus.karlsson@...el.com, bjorn.topel@...el.com, ast@...nel.org,
        daniel@...earbox.net, netdev@...r.kernel.org
Cc:     jonathan.lemon@...il.com, bpf@...r.kernel.org
Subject: RE: [PATCH bpf] libbpf: fix compatibility for kernels without
 need_wakeup

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.

Thanks,
John