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

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Date:   Wed, 17 Oct 2018 17:25:21 +0000
From:   Yonghong Song <yhs@...com>
To:     Edward Cree <ecree@...arflare.com>,
        Alexei Starovoitov <ast@...com>, Martin Lau <kafai@...com>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     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 10/17/18 9:13 AM, Edward Cree wrote:
> On 17/10/18 08:23, Yonghong Song wrote:
>> This patch adds BTF_KIND_FUNC and BTF_KIND_FUNC_PROTO
>> support to the type section. BTF_KIND_FUNC_PROTO is used
>> to specify the type of a function pointer. With this,
>> BTF has a complete set of C types (except float).
>>
>> BTF_KIND_FUNC is used to specify the signature of a
>> defined subprogram. BTF_KIND_FUNC_PROTO can be referenced
>> by another type, e.g., a pointer type, and BTF_KIND_FUNC
>> type cannot be referenced by another type.
> Why are distinct kinds created for these?  A function body is
>   a value of function type, and since there's no way (in C) to
>   declare a variable of function type (only pointer-to-
>   function), any declaration of function type must necessarily
>   be a BTF_KIND_FUNC, whereas any other reference to a function
>   type (e.g. a declaration of type pointer to function type)
>   must, as you state above, be a BTF_KIND_FUNC_PROTO.
> In fact, you can tell the difference just from name_off, since
>   a (C-legal) BTF_KIND_FUNC_PROTO will always be anonymous (as
>   the pointee of a pointer type), while a BTF_KIND_FUNC will
>   have the name of the subprogram.

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?

> 
> -Ed
>