netdev - Re: [PATCH v3 bpf-next 1/3] bpf: switch BPF UAPI #define constants used from BPF program side to enums

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Message-ID: <CAADnVQLzQcUyi3Trtr0iT7gEhpSQYAH8WD5q+X8EmRwMYMzhbQ@mail.gmail.com>
Date:   Tue, 2 Jun 2020 16:36:46 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Michael Forney <mforney@...rney.org>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Kernel Team <kernel-team@...com>, Yonghong Song <yhs@...com>
Subject: Re: [PATCH v3 bpf-next 1/3] bpf: switch BPF UAPI #define constants
 used from BPF program side to enums

On Tue, Jun 2, 2020 at 4:21 PM Michael Forney <mforney@...rney.org> wrote:
>
> On 2020-06-02, Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> > the enum definition of BPF_F_CTXLEN_MASK is certainly within standard.
> > I don't think kernel should adjust its headers because some compiler
> > is failing to understand C standard.
>
> This is not true. See C11 6.7.2.2p2: "The expression that defines the
> value of an enumeration constant shall be an integer constant
> expression that has a value representable as an int."
>
> You can also see this with gcc if you turn on -Wpedantic and include
> it in a way such that warnings are not silenced:
>
> $ gcc -Wpedantic -x c -c -o /dev/null /usr/include/linux/bpf.h

ISO C forbids zero-size arrays, unnamed struct/union, gcc extensions,
empty unions, etc
So ?

warning: ISO C forbids zero-size array ‘args’ [-Wpedantic]
 4095 |  __u64 args[0];
 warning: ISO C90 doesn’t support unnamed structs/unions [-Wpedantic]
 3795 |  __bpf_md_ptr(void *, data_end);

#define BPF_F_CTXLEN_MASK BPF_F_CTXLEN_MASK
will only work as workaround for _your_ compiler.
We are not gonna add hacks like this for every compiler.