netdev - Re: [PATCH bpf-next v7 03/10] bpf: verifier: refactor check_attach_btf

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Message-ID: <CAEf4BzY4UR+KjZ3bY6ykyW5CPNwAzwgKVhYHGdgDuMT2nntmTg@mail.gmail.com>
Date:   Mon, 21 Sep 2020 16:05:53 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>,
        KP Singh <kpsingh@...omium.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v7 03/10] bpf: verifier: refactor check_attach_btf_id()

On Sat, Sep 19, 2020 at 4:50 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> From: Toke Høiland-Jørgensen <toke@...hat.com>
>
> The check_attach_btf_id() function really does three things:
>
> 1. It performs a bunch of checks on the program to ensure that the
>    attachment is valid.
>
> 2. It stores a bunch of state about the attachment being requested in
>    the verifier environment and struct bpf_prog objects.
>
> 3. It allocates a trampoline for the attachment.
>
> This patch splits out (1.) and (3.) into separate functions in preparation
> for reusing them when the actual attachment is happening (in the
> raw_tracepoint_open syscall operation), which will allow tracing programs
> to have multiple (compatible) attachments.
>
> No functional change is intended with this patch.
>
> Acked-by: Andrii Nakryiko <andriin@...com>
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---

Ok, so bad news: you broke another selftest (test_overhead). Please,
do run test_progs and make sure everything succeeds, every time before
you post a new version.

Good news, though, is that this refactoring actually fixed a pretty
nasty bug in check_attach_btf_id logic: whenever bpf_trampoline
already existed (e.g., due to successful fentry to function X), all
subsequent fentry/fexit/fmod_ret and all their sleepable variants
would skip a bunch of check. So please attach the following Fixes
tags:

Fixes: 6ba43b761c41 ("bpf: Attachment verification for BPF_MODIFY_RETURN")
Fixes: 1e6c62a88215 ("bpf: Introduce sleepable BPF programs")

As for selftests, feel free to just drop the fmod_ret program, it was
never supposed to be possible, I just never realized that.

>  include/linux/bpf.h          |    7 +
>  include/linux/bpf_verifier.h |    9 ++
>  kernel/bpf/trampoline.c      |   20 ++++
>  kernel/bpf/verifier.c        |  197 ++++++++++++++++++++++++------------------
>  4 files changed, 149 insertions(+), 84 deletions(-)
>

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