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

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Date:   Tue, 22 Sep 2020 09:28:44 -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 Tue, Sep 22, 2020 at 4:16 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Andrii Nakryiko <andrii.nakryiko@...il.com> writes:
>
> > 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.
>
> Right, so I looked into this, and it seems the only reason it was
> succeeding before were those skipped checks you pointed out that are now
> fixed. I.e., __set_task_comm() is not actually supposed to be
> fmod_ret'able according to check_attach_modify_return(). So I'm not sure
> what the right way to fix this is?

You have to remove the fmod_ret part from test_overhead, it was never
supposed to work.

>
> The fmod_ret bit was added to test_overhead by:
>
> 4eaf0b5c5e04 ("selftest/bpf: Fmod_ret prog and implement test_overhead as part of bench")
>
> so the obvious thing is to just do a (partial) revert of that? WDYT?
>
> -Toke
>