netdev - Re: [PATCH net 1/4] virtchnl: Fix layout of RSS structures

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Date:   Sat, 27 Mar 2021 10:53:23 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
Cc:     anthony.l.nguyen@...el.com,
        Norbert Ciosek <norbertx.ciosek@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>, sassmann@...hat.com,
        Konrad Jankowski <konrad0.jankowski@...el.com>,
        Mateusz Palczewski <mateusz.palczewski@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net 1/4] virtchnl: Fix layout of RSS structures

Hi Samudrala,

On Fri, Mar 26, 2021 at 11:45 PM Samudrala, Sridhar
<sridhar.samudrala@...el.com> wrote:
> On 3/26/2021 1:06 AM, Geert Uytterhoeven wrote:
> > On Thu, Mar 25, 2021 at 11:29 PM Tony Nguyen <anthony.l.nguyen@...el.com> wrote:
> > From: Norbert Ciosek <norbertx.ciosek@...el.com>
> >
> > Remove padding from RSS structures. Previous layout
> > could lead to unwanted compiler optimizations
> > in loops when iterating over key and lut arrays.
> >
> > From an earlier private conversation with Mateusz, I understand the real
> > explanation is that key[] and lut[] must be at the end of the
> > structures, because they are used as flexible array members?
> >
> > Fixes: 65ece6de0114 ("virtchnl: Add missing explicit padding to structures")
> > Signed-off-by: Norbert Ciosek <norbertx.ciosek@...el.com>
> > Tested-by: Konrad Jankowski <konrad0.jankowski@...el.com>
> > Signed-off-by: Tony Nguyen <anthony.l.nguyen@...el.com>
> >
> > --- a/include/linux/avf/virtchnl.h
> > +++ b/include/linux/avf/virtchnl.h
> > @@ -476,7 +476,6 @@ struct virtchnl_rss_key {
> >         u16 vsi_id;
> >         u16 key_len;
> >         u8 key[1];         /* RSS hash key, packed bytes */
> > -       u8 pad[1];
> >  };
> >
> >  VIRTCHNL_CHECK_STRUCT_LEN(6, virtchnl_rss_key);
> > @@ -485,7 +484,6 @@ struct virtchnl_rss_lut {
> >         u16 vsi_id;
> >         u16 lut_entries;
> >         u8 lut[1];        /* RSS lookup table */
> > -       u8 pad[1];
> >  };
> >
> > If you use a flexible array member, it should be declared without a size,
> > i.e.
> >
> >     u8 key[];
> >
> > Everything else is (trying to) fool the compiler, and leading to undefined
> > behavior, and people (re)adding explicit padding.
>
> This header file is shared across other OSes that use C++ that doesn't support
> flexible arrays. So the structures in this file use an array of size 1 as a last
> element to enable variable sized arrays.

I don't think it is accepted practice to have non-Linux-isms in
include/*linux*/avf/virtchnl.h header files.  Moreover, using a size
of 1 is counter-intuitive for people used to Linux kernel development,
and may lead to off-by-one errors in calculation of sizes.

If you insist on ignoring the above, this definitely deserves a
comment next to the member's declaration.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds