netdev - Re: [PATCH net-next 07/33] xdp: xdp_frame add member frame_sz and handle in convert_to_xdp

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Message-ID: <878sil3rqj.fsf@toke.dk>
Date:   Fri, 24 Apr 2020 16:04:04 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>, sameehj@...zon.com
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, zorik@...zon.com, akiyano@...zon.com,
        gtzalik@...zon.com, Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        John Fastabend <john.fastabend@...il.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        David Ahern <dsahern@...il.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Saeed Mahameed <saeedm@...lanox.com>,
        steffen.klassert@...unet.com
Subject: Re: [PATCH net-next 07/33] xdp: xdp_frame add member frame_sz and handle in convert_to_xdp_frame

Jesper Dangaard Brouer <brouer@...hat.com> writes:

> Use hole in struct xdp_frame, when adding member frame_sz, which keeps
> same sizeof struct (32 bytes)
>
> Drivers ixgbe and sfc had bug cases where the necessary/expected
> tailroom was not reserved. This can lead to some hard to catch memory
> corruption issues. Having the drivers frame_sz this can be detected when
> packet length/end via xdp->data_end exceed the xdp_data_hard_end
> pointer, which accounts for the reserved the tailroom.
>
> When detecting this driver issue, simply fail the conversion with NULL,
> which results in feedback to driver (failing xdp_do_redirect()) causing
> driver to drop packet. Given the lack of consistent XDP stats, this can
> be hard to troubleshoot. And given this is a driver bug, we want to
> generate some more noise in form of a WARN stack dump (to ID the driver
> code that inlined convert_to_xdp_frame).
>
> Inlining the WARN macro is problematic, because it adds an asm
> instruction (on Intel CPUs ud2) what influence instruction cache
> prefetching. Thus, introduce xdp_warn and macro XDP_WARN, to avoid this
> and at the same time make identifying the function and line of this
> inlined function easier.
>
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>

Acked-by: Toke Høiland-Jørgensen <toke@...hat.com>