netdev - Re: [PATCH bpf-next v2 02/13] bpf: btf: Add BTF_KIND_FUNC and BTF_KIND_FUNC

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Date:   Thu, 18 Oct 2018 17:47:11 +0100
From:   Edward Cree <ecree@...arflare.com>
To:     Martin Lau <kafai@...com>, Yonghong Song <yhs@...com>
CC:     Alexei Starovoitov <ast@...com>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next v2 02/13] bpf: btf: Add BTF_KIND_FUNC and
 BTF_KIND_FUNC_PROTO

On 17/10/18 18:50, Martin Lau wrote:
> On Wed, Oct 17, 2018 at 10:25:21AM -0700, Yonghong Song wrote:
>> What you stated is true, BTF_KIND_FUNC_PROTO corresponds to
>> dwarf subroutine tag which has no name while BTF_KIND_FUNC
>> must have a valid name. The original design is to have both
>> since they are corresponding to different dwarf constructs.
>>
>> Martin, what do you think?
> I prefer to have separate kinds.  We need a way to distinguish them.
> For example, the BTF verifier is checking it.  Having two kinds is
> cleaner instead of resorting to other hints from 'struct btf_type'.
> We don't lack of bits for kind.
But my point is that (a) they can be distinguished by how they are
 used, and (b) the _only_ difference between them is how they are
 used.  In this C code:
    int a = 5;
    int foo(int x) { return a; }
    int *b = &a;
    int (*bar)(int) = &foo;
 foo and *bar are _the same type_, just as a and *b are.  It's just
 that C has a slightly odd way of writing
    int foo(int) = lambda x: a;
 and foo itself is implicitly sorta-const.
What am I missing?

-Ed