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 17:13:21 +0100
From:   Edward Cree <ecree@...arflare.com>
To:     Yonghong Song <yhs@...com>, <ast@...com>, <kafai@...com>,
        <daniel@...earbox.net>, <netdev@...r.kernel.org>
CC:     <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 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.

-Ed