Message-ID: <20181019232704.j45f7fbhreyinjxk@kafai-mbp.dhcp.thefacebook.com>
Date:   Fri, 19 Oct 2018 23:27:07 +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 Fri, Oct 19, 2018 at 10:26:53PM +0100, Edward Cree wrote:
> 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.)
> 
btf_type is not exactly a C type.

btf_type is a debug-info.  Each btf_type carries specific
debug information.  Name is part of the debug-info/btf_type.
If something carries different debug-info, it is another btf_type.
Like struct, the member's names of struct is part of the btf_type.
A struct with the same member's types but different member's names
is a different btf_type.

The same go for function.  The function with different function
names and arg names is a different btf_type.

> > 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?
Having two id spaces for debug-info is confusing.  They are
all debug-info at the end.

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