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From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date:   Tue, 1 Mar 2022 09:51:45 -0500
Message-ID: <CA+FuTSfi1aXiBr-fOQ+8XJPjCCTnqTicW2A3OUVfNHurfDL3jA@mail.gmail.com>
Subject: Re: [PATCH] SO_ZEROCOPY should rather return -ENOPROTOOPT
To:     Samuel Thibault <samuel.thibault@labri.fr>, willemb@google.com,
        davem@davemloft.net, kuba@kernel.org, linux-kernel@vger.kernel.org
Cc:     Network Development <netdev@vger.kernel.org>
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On Tue, Mar 1, 2022 at 9:44 AM Samuel Thibault <samuel.thibault@labri.fr> wrote:
>
> ENOTSUPP is documented as "should never be seen by user programs", and
> is not exposed in <errno.h>, so applications cannot safely check against
> it. We should rather return the well-known -ENOPROTOOPT.
>
> Signed-off-by: Samuel Thibault <samuel.thibault@labri.fr>
>
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 4ff806d71921..6e5b84194d56 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -1377,9 +1377,9 @@ int sock_setsockopt(struct socket *sock, int level, int optname,
>                         if (!(sk_is_tcp(sk) ||
>                               (sk->sk_type == SOCK_DGRAM &&
>                                sk->sk_protocol == IPPROTO_UDP)))
> -                               ret = -ENOTSUPP;
> +                               ret = -ENOPROTOOPT;
>                 } else if (sk->sk_family != PF_RDS) {
> -                       ret = -ENOTSUPP;
> +                       ret = -ENOPROTOOPT;
>                 }
>                 if (!ret) {
>                         if (val < 0 || val > 1)

That should have been a public error code. Perhaps rather EOPNOTSUPP.

The problem with a change now is that it will confuse existing
applications that check for -524 (ENOTSUPP).