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

[Michael Forney <mforney@mforney.org>]

On 2020-06-02, Alexei Starovoitov <alexei.starovoitov@gmail.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