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

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Message-ID: <20181018181121.ekhdkb4r4sg2ryli@kafai-mbp.dhcp.thefacebook.com>
Date:   Thu, 18 Oct 2018 18:12:51 +0000
From:   Martin Lau <kafai@...com>
To:     Edward Cree <ecree@...arflare.com>
CC:     Yonghong Song <yhs@...com>, 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 Thu, Oct 18, 2018 at 05:47:11PM +0100, Edward Cree wrote:
> 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?
The BTF verification and bpf_prog_load() has to treat
them differently.

Are you suggesting they can be treated the same for
the kernel verification purpose?

or

The concern is, having two kind, BTF_KIND_FUNC_PROTO and
BTF_KIND_FUNC, is confusing because they have almost the
same BTF metadata?

>From the BTF perspective, they are different because
they are allowed to contain different information.
For example, only "foo" can have func_info in patch 5
(the to-be-added line_info can only belong to "foo" also).

This patch 2 is also doing checks on func_proto.
e.g. in btf_ptr_resolve().

The early idea is to have "foo_a", "foo_b", "foo_c"...
in a separate BTF section called "func" section (on top of
the current "type" and "string" section).  By doing this,
only one BTF_KIND_FUNC would be needed as you suggested
because "foo" could be clearly distinguished from "bar" by
observing they belong to different BTF sections.  The down
side is the "func" section will have its own id space separated
from the current "type" section id space and it is less ideal
for debugging purpose.

Hence, "foo" is contained in the "type" section also
and added FUNC_PROTO to distinguish them.