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: <CAGw6cBuF8Dj-22bH=ryL+17N48pwMD5hN49sH4AHYYyMm2xgtg@mail.gmail.com>
Date:   Tue, 2 Jun 2020 16:21:00 -0700
From:   Michael Forney <mforney@...rney.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
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 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
/usr/include/linux/bpf.h:76:7: warning: ISO C forbids zero-size array
'data' [-Wpedantic]
   76 |  __u8 data[0]; /* Arbitrary size */
      |       ^~~~
/usr/include/linux/bpf.h:3220:22: warning: ISO C restricts enumerator
values to range of 'int' [-Wpedantic]
 3220 |  BPF_F_INDEX_MASK  = 0xffffffffULL,
      |                      ^~~~~~~~~~~~~
/usr/include/linux/bpf.h:3221:23: warning: ISO C restricts enumerator
values to range of 'int' [-Wpedantic]
 3221 |  BPF_F_CURRENT_CPU  = BPF_F_INDEX_MASK,
      |                       ^~~~~~~~~~~~~~~~
/usr/include/linux/bpf.h:3223:23: warning: ISO C restricts enumerator
values to range of 'int' [-Wpedantic]
 3223 |  BPF_F_CTXLEN_MASK  = (0xfffffULL << 32),
      |                       ^
/usr/include/linux/bpf.h:3797:8: warning: ISO C forbids zero-size
array 'args' [-Wpedantic]
 3797 |  __u64 args[0];
      |        ^~~~
$

-Michael