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From: Lorenz Bauer <lmb@cloudflare.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: "David S . Miller" <davem@davemloft.net>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	John Fastabend <john.fastabend@gmail.com>,
	mcroce@microsoft.com, bpf <bpf@vger.kernel.org>,
	Kernel Team <kernel-team@fb.com>
Subject: Re: [PATCH RFC bpf-next 00/10] bpf: CO-RE support in the kernel.
Date: Thu, 23 Sep 2021 12:33:58 +0100	[thread overview]
Message-ID: <CACAyw98puHhO7f=OmEACNaje0DvVdpS7FosLY9aM8z46hy=7ww@mail.gmail.com> (raw)
In-Reply-To: <20210917215721.43491-1-alexei.starovoitov@gmail.com>

On Fri, 17 Sept 2021 at 22:57, Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> From: Alexei Starovoitov <ast@kernel.org>
>
> Hi All,
>
> This is very early RFC that introduces CO-RE support in the kernel.
> There are several reasons to add such support:
> 1. It's a step toward signed BPF programs.
> 2. It allows golang like languages that struggle to adopt libbpf
>    to take advantage of CO-RE powers.
> 3. Currently the field accessed by 'ldx [R1 + 10]' insn is recognized
>    by the verifier purely based on +10 offset. If R1 points to a union
>    the verifier picks one of the fields at this offset.
>    With CO-RE the kernel can disambiguate the field access.
>
> This set wires all relevant pieces and passes two selftests with CO-RE
> in the kernel.
>
> The main goal of RFC is to get feedback on patch 3.
> It's passing CO-RE relocation into the kernel via bpf_core_apply_relo()
> helper that is called by the loader program.
> It works, but there is no clean way to add error reporting here.
> So I'm thinking that the better approach would be to pass an array
> of 'struct bpf_core_relo_desc' into PROG_LOAD command similar to
> how func_info and line_info are passed.
> Such approach would allow for the use case 3 above (which
> current approach in patch 3 doesn't support).

+1 to having good error reporting, it's hard to debug CO-RE failures
as they are. PROG_LOAD seems nice, provided that relocation happens
before verification.

Some questions:
* How can this handle kernels that don't have built-in BTF? Not a
problem for myself, but some people have to deal with BTF-less distro
kernels by using pahole to generate external BTF from debug symbols.
Can we accommodate that?
* Does in-kernel CO-RE need to account for packed structs w/ bitfields
in them? If relocation happens after verification this could be a
problem: [1].
* Tangent: libbpf CO-RE has this res->validate flag, which turns off
some checks for bitfields. I've never fully understood why that is,
maybe Andrii can explain it?

Lorenz

1: https://lore.kernel.org/bpf/CACAyw9_R4_ib0KvcuQC4nSOy5+Hn8-Xq-G8geDdLsNztX=0Fsw@mail.gmail.com/

-- 
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

www.cloudflare.com

  parent reply	other threads:[~2021-09-23 11:34 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-17 21:57 [PATCH RFC bpf-next 00/10] bpf: CO-RE support in the kernel Alexei Starovoitov
2021-09-17 21:57 ` [PATCH RFC bpf-next 01/10] bpf: Prepare relo_core.c for kernel duty Alexei Starovoitov
2021-09-21 21:25   ` Andrii Nakryiko
2021-09-28 14:45   ` Matteo Croce
2021-09-28 16:37     ` Alexei Starovoitov
2021-09-28 17:11       ` Matteo Croce
2021-09-28 20:34         ` Alexei Starovoitov
2021-09-29 12:32           ` Matteo Croce
2021-09-29 17:38             ` Matteo Croce
2021-09-29 23:00               ` Alexei Starovoitov
2021-09-29 23:49                 ` Matteo Croce
2021-10-22  0:48                   ` Matteo Croce
2021-10-22  0:51                     ` Alexei Starovoitov
2021-09-17 21:57 ` [PATCH RFC bpf-next 02/10] bpf: Define enum bpf_core_relo_kind as uapi Alexei Starovoitov
2021-09-21 21:27   ` Andrii Nakryiko
2021-09-17 21:57 ` [PATCH RFC bpf-next 03/10] bpf: Add proto of bpf_core_apply_relo() Alexei Starovoitov
2021-09-23 11:21   ` Lorenz Bauer
2021-09-23 18:48     ` Andrii Nakryiko
2021-09-17 21:57 ` [PATCH RFC bpf-next 04/10] bpf: Add bpf_core_add_cands() and wire it into bpf_core_apply_relo_insn() Alexei Starovoitov
2021-09-21 21:34   ` Andrii Nakryiko
2021-09-27 18:04   ` Matteo Croce
2021-09-17 21:57 ` [PATCH RFC bpf-next 05/10] libbpf: Use CO-RE in the kernel in light skeleton Alexei Starovoitov
2021-09-17 21:57 ` [PATCH RFC bpf-next 06/10] libbpf: Make gen_loader data aligned Alexei Starovoitov
2021-09-17 21:57 ` [PATCH RFC bpf-next 07/10] libbpf: Support init of inner maps in light skeleton Alexei Starovoitov
2021-09-17 21:57 ` [PATCH RFC bpf-next 08/10] selftests/bpf: Convert kfunc test with CO-RE to lskel Alexei Starovoitov
2021-09-17 21:57 ` [PATCH RFC bpf-next 09/10] selftests/bpf: Improve inner_map test coverage Alexei Starovoitov
2021-09-17 21:57 ` [PATCH RFC bpf-next 10/10] selftests/bpf: Convert map_ptr_kern test to use light skeleton Alexei Starovoitov
2021-09-23 11:33 ` Lorenz Bauer [this message]
2021-09-23 18:52   ` [PATCH RFC bpf-next 00/10] bpf: CO-RE support in the kernel Andrii Nakryiko
2021-09-24 23:13   ` Alexei Starovoitov
2021-09-27 16:12     ` Lorenz Bauer
2021-09-27 16:50       ` Alexei Starovoitov
2021-09-28  8:30         ` Lorenz Bauer
2021-09-28 16:35           ` Alexei Starovoitov

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