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

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Message-ID: <0fc1e9bd-e70b-28a2-bc09-629414a619b0@meta.com>
Date:   Thu, 15 Dec 2022 21:06:16 -0800
From:   Yonghong Song <yhs@...a.com>
To:     Connor O'Brien <connoro@...gle.com>, bpf@...r.kernel.org
Cc:     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 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.

> +
>   	__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);