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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Sun, 18 Apr 2021 15:55:52 -0700
Message-ID: <CAADnVQLJDsnQ1YO9a_pQ-1aTJ1hNKYJXcSHypfzCare-c4HO1A@mail.gmail.com>
Subject: Re: [PATCH] bpf: fix errno code for unsupported batch ops
To:     Pedro Tammela <pctammela@gmail.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Pedro Tammela <pctammela@mojatatu.com>
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On Sun, Apr 18, 2021 at 1:03 PM Pedro Tammela <pctammela@gmail.com> wrote:
>
> ENOTSUPP is not a valid userland errno[1], which is annoying for
> userland applications that implement a fallback to iterative, report
> errors via 'strerror()' or both.
>
> The batched ops return this errno whenever an operation
> is not implemented for kernels that implement batched ops.
>
> In older kernels, pre batched ops, it returns EINVAL as the arguments
> are not supported in the syscall.
>
> [1] https://lore.kernel.org/netdev/20200511165319.2251678-1-kuba@kernel.org/
>
> Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
> ---
>  kernel/bpf/syscall.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index fd495190115e..88fe19c0aeb1 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -3961,7 +3961,7 @@ static int bpf_task_fd_query(const union bpf_attr *attr,
>  #define BPF_DO_BATCH(fn)                       \
>         do {                                    \
>                 if (!fn) {                      \
> -                       err = -ENOTSUPP;        \
> +                       err = -EOPNOTSUPP;      \

$ git grep EOPNOTSUPP kernel/bpf/|wc -l
11
$ git grep ENOTSUPP kernel/bpf/|wc -l
51

For new code EOPNOTSUPP is better, but I don't think changing all 51 case
is a good idea. Something might depend on it already.