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

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Date:   Fri, 19 Oct 2018 22:26:53 +0100
From:   Edward Cree <ecree@...arflare.com>
To:     Martin Lau <kafai@...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 19/10/18 20:36, Martin Lau wrote:
> On Fri, Oct 19, 2018 at 06:04:11PM +0100, Edward Cree wrote:
>> But you *do* have such a new section.
>> The patch comment talks about a 'FuncInfo Table' which appears to
> Note that the new section, which contains the FuncInfo Table,
> is in a new ELF section ".BTF.ext" instead of the ".BTF".
> It is not in the ".BTF" section because it is only useful during
> bpf_prog_load().
I thought it was because it needed to be munged by the loader/linker?

> IIUC, I think what you are suggesting here is to use (type_id, name)
> to describe DW_TAG_subprogram "int foo1(int) {}", "int foo2(int) {}",
> "int foo3(int) {}" where type_id here is referring to the same
> DW_TAG_subroutine_type, and only define that _one_
> DW_TAG_subroutine_type in the BTF "type" section.
Yes, something like that.

> If the concern is having both FUNC and FUNC_PROTO is confusing,
The concern is that you're conflating different entities (types
 and instances); FUNC_PROTO is just a symptom/canary of that.

> we could go back to the CTF way which adds a new function section
> in ".BTF" and it is only for DW_TAG_subprogram.
> BTF_KIND_FUNC_PROTO is then no longer necessary.
> Some of new BTF verifier checkings may actually go away also.
> The down side is there will be two id spaces.
Two id spaces... one for types and the other for subprograms.
These are different things, so why would you _want_ them to share
 an id space?  I don't, for instance, see any situation in which
 you'd want some other record to have a field that could reference
 either.
And the 'subprogram id' doesn't have to be just for subprograms;
 it could be for instances generally — like I've been saying, a
 variable declaration is to an object type what a subprogram is to
 a function type, just with a few complications like "subprograms
 can only appear at file scope, not nested in other functions" and
 "variables of function type are immutable".
(I'm assuming that at some point we're going to want to be able to
 have BTF information for e.g. variables stored on a subprogram's
 stack, if only for stuff like single-stepping in a debugger in
 userspace with some sort of mock.  At that point, the variable
 has to have its own record — you can't just have some sort of
 magic type record because e.g. "struct foo bar;" has two names,
 one for the type and one for the variable.)

> Discussed a bit offline with folks about the two id spaces
> situation and it is not good for debugging purpose.
Could you unpack this a bit more?

-Ed