linux-kernel - Re: [PATCH bpf-next] bpf: btf: don't log ignored BTF mismatches

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Message-ID: <CAEf4BzbTeJWkFhF-bW3mp94THeYEMM7R-ZX3C_c1aazY0Togsw@mail.gmail.com>
Date:   Thu, 8 Dec 2022 16:03:37 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...a.com>
Cc:     "Connor O'Brien" <connoro@...gle.com>, bpf@...r.kernel.org,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next] bpf: btf: don't log ignored BTF mismatches

On Thu, Dec 8, 2022 at 10:01 AM Yonghong Song <yhs@...a.com> wrote:
>
>
>
> On 12/7/22 6:19 PM, Connor O'Brien wrote:
> > On Wed, Nov 9, 2022 at 8:45 AM Yonghong Song <yhs@...a.com> wrote:
> >>
> >>
> >>
> >> On 11/8/22 6:41 PM, Connor O'Brien wrote:
> >>> Enabling CONFIG_MODULE_ALLOW_BTF_MISMATCH is an indication that BTF
> >>> mismatches are expected and module loading should proceed
> >>> anyway. Logging with pr_warn() on every one of these "benign"
> >>> mismatches creates unnecessary noise when many such modules are
> >>> loaded. Instead, limit logging to the case where a BTF mismatch
> >>> actually prevents a module form loading.
> >>>
> >>> Signed-off-by: Connor O'Brien <connoro@...gle.com>
> >>> ---
> >>>    kernel/bpf/btf.c | 7 ++++---
> >>>    1 file changed, 4 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> >>> index 5579ff3a5b54..406370487413 100644
> >>> --- a/kernel/bpf/btf.c
> >>> +++ b/kernel/bpf/btf.c
> >>> @@ -7190,11 +7190,12 @@ static int btf_module_notify(struct notifier_block *nb, unsigned long op,
> >>>                }
> >>>                btf = btf_parse_module(mod->name, mod->btf_data, mod->btf_data_size);
> >>>                if (IS_ERR(btf)) {
> >>> -                     pr_warn("failed to validate module [%s] BTF: %ld\n",
> >>> -                             mod->name, PTR_ERR(btf));
> >>
> >> I think such warning still useful even with
> >> CONFIG_MODULE_ALLOW_BTF_MISMATCH.
> >> Can we use pr_warn_ratelimited instead of pr_warn in the above to
> >> avoid excessive warnings?
> >
> > I gave this a try on a Pixel 6 but I'm not sure it quite addresses the
> > issue. The amount of logging doesn't seem to decrease much, I think
> > because the interval between loading one mismatched module and the
> > next can be greater than the default rate limit. To my mind, the issue
> > is the total volume of these messages more so than their rate.
> >
> > For context, Android devices using the GKI may load hundreds of
> > separately-built modules, and BTF mismatches are possible for any/all
> > of these. It was pointed out to me that btf_verifier_log_type can also
> > print several more lines per mismatched module. ~5 lines of logging
> > for each mismatched module can start to add up, in terms of both
> > overhead and the noise added to the kernel logs.
> >
> > This is more subjective but I think the warnings also read as though
> > this is a more serious failure that might prevent affected modules
> > from working correctly; anecdotally, I've gotten multiple questions
> > about them asking if something is broken. This can be a red herring
> > for anyone unfamiliar with BTF who is reading the logs to debug
> > unrelated issues. In the CONFIG_MODULE_ALLOW_BTF_MISMATCH=y case the
> > flood of warnings seems out of proportion to the actual result
> > (modules still load successfully, just without debug info) especially
> > since the user has explicitly enabled a config saying they expect
> > mismatches.
> >
> > If there needs to be some logging in the "mismatch allowed" case,
> > could an acceptable middle ground be to use pr_warn_once to send a
>
> So it looks like pr_warn_ratelimited still produces a lot of warning.
> In this case, I guess pr_warn_once should be okay.

Maybe pr_warn_once generic "some kernel module BTF mismatched". And
also warn per-module message with details if
CONFIG_MODULE_ALLOW_BTF_MISMATCH is not set?

>
> > single message reporting that mismatches were detected & module BTF
> > debug info might be unavailable? Alternatively, if we could opt out of
> > module BTF loading then that would also avoid this issue, but that's
> > already been proposed before ([1], [2]) so I thought working with the
> > existing config option might be preferred.
> >
> > [1] https://lore.kernel.org/bpf/20220209052141.140063-1-connoro@google.com/
> > [2] https://lore.kernel.org/bpf/20221004222725.2813510-1-sdf@google.com/