netdev - Re: [PATCH] net: Shrink sock.sk_err sk_err

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Date:   Wed, 6 Jul 2022 16:04:51 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Leonard Crestez <cdleonard@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Wei Wang <weiwan@...gle.com>,
        Joanne Koong <joannelkoong@...il.com>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: Shrink sock.sk_err sk_err_soft to u16 from int

On Sun, Jul 3, 2022 at 10:07 PM Leonard Crestez <cdleonard@...il.com> wrote:
>
> These fields hold positive errno values which are limited by
> ERRNO_MAX=4095 so 16 bits is more than enough.
>
> They are also always positive; setting them to a negative errno value
> can result in falsely reporting a successful read/write of incorrect
> size.
>
> Signed-off-by: Leonard Crestez <cdleonard@...il.com>
> ---

We can not do this safely.

sk->sk_err_soft can be written without lock, this needs to be a full integer,
otherwise this might pollute adjacent bytes.

>  include/net/sock.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> I ran some relatively complex tests without noticing issues but some corner
> case where this breaks might exist.
>
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 0dd43c3df49b..acd85d1702d9 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -480,11 +480,11 @@ struct sock {
>         u16                     sk_protocol;
>         u16                     sk_gso_max_segs;
>         unsigned long           sk_lingertime;
>         struct proto            *sk_prot_creator;
>         rwlock_t                sk_callback_lock;
> -       int                     sk_err,
> +       u16                     sk_err,
>                                 sk_err_soft;
>         u32                     sk_ack_backlog;
>         u32                     sk_max_ack_backlog;
>         kuid_t                  sk_uid;
>         u8                      sk_txrehash;
> --
> 2.25.1
>