linux-kernel - Re: [PATCH bpf-next v2] bpf: btf: limit logging of ignored BTF mismatches

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Date:   Tue, 20 Dec 2022 14:11:28 -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 v2] bpf: btf: limit logging of ignored BTF mismatches

On Thu, Dec 15, 2022 at 9:06 PM Yonghong Song <yhs@...a.com> wrote:
>
>
>
> On 12/14/22 4:53 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, handle this case with a single log warning that BTF
> > info may be unavailable.
> >
> > Mismatches also result in calls to __btf_verifier_log() via
> > __btf_verifier_log_type() or btf_verifier_log_member(), adding several
> > additional lines of logging per mismatched module. Add checks to these
> > paths to skip logging for module BTF mismatches in the "allow
> > mismatch" case.
> >
> > All existing logging behavior is preserved in the default
> > CONFIG_MODULE_ALLOW_BTF_MISMATCH=n case.
> >
> > Signed-off-by: Connor O'Brien <connoro@...gle.com>
>
> Ack with a few nits below.
>
> Acked-by: Yonghong Song <yhs@...com>
>
> > ---
> > v2:
> > - Use pr_warn_once instead of skipping logging entirely
> > - Also skip btf verifier logs for ignored mismatches
> >
> > v1: https://lore.kernel.org/bpf/20221109024155.2810410-1-connoro@google.com/
> > ---
> >   kernel/bpf/btf.c | 24 +++++++++++++++++++++---
> >   1 file changed, 21 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index f7dd8af06413..16b959b49595 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -1404,6 +1404,13 @@ __printf(4, 5) static void __btf_verifier_log_type(struct btf_verifier_env *env,
> >       if (log->level == BPF_LOG_KERNEL && !fmt)
> >               return;
> >
> > +     /*
> > +      * Skip logging when loading module BTF with mismatches permitted
> > +      */
>
> Just use one line for the above comment.
>
> > +     if (env->btf->base_btf && env->btf->kernel_btf &&
> > +         IS_ENABLED(CONFIG_MODULE_ALLOW_BTF_MISMATCH))
> > +             return;
>
> I believe env->btf->base_btf alone is enough to test it should be a
> module btf. If env->btf->base_btf is true, env->btf->kernel_btf should
> also be true. The other way is not true, env->btf->kernel_btf is true,
> the btf could be vmlinux (env->btf->base_btf == NULL) or be a module.
>

Seems like we are also using log->level == BPF_LOG_KERNEL check for
when working with kernel BTFs, so let's stick to the same pattern?


> > +
> >       __btf_verifier_log(log, "[%u] %s %s%s",
> >                          env->log_type_id,
> >                          btf_type_str(t),
> > @@ -1443,6 +1450,14 @@ static void btf_verifier_log_member(struct btf_verifier_env *env,
> >
> >       if (log->level == BPF_LOG_KERNEL && !fmt)
> >               return;
> > +
> > +     /*
> > +      * Skip logging when loading module BTF with mismatches permitted
> > +      */
>
> Same, just use one line for the above comments.
>
> > +     if (env->btf->base_btf && env->btf->kernel_btf &&
> > +         IS_ENABLED(CONFIG_MODULE_ALLOW_BTF_MISMATCH))
> > +             return;
> > +
> >       /* The CHECK_META phase already did a btf dump.
> >        *
> >        * If member is logged again, it must hit an error in
> > @@ -7260,11 +7275,14 @@ 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));
> >                       kfree(btf_mod);
> > -                     if (!IS_ENABLED(CONFIG_MODULE_ALLOW_BTF_MISMATCH))
> > +                     if (!IS_ENABLED(CONFIG_MODULE_ALLOW_BTF_MISMATCH)) {
> > +                             pr_warn("failed to validate module [%s] BTF: %ld\n",
> > +                                     mod->name, PTR_ERR(btf));
> >                               err = PTR_ERR(btf);
> > +                     } else {
> > +                             pr_warn_once("Kernel module BTF mismatch detected, BTF debug info may be unavailable for some modules\n");
> > +                     }
> >                       goto out;
> >               }
> >               err = btf_alloc_id(btf);