netdev - Re: [PATCH bpf 2/2] libbpf: allow loading empty BTFs

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Message-ID: <e621981d-5c3d-6d92-871b-a98520778363@fb.com>
Date:   Mon, 11 Jan 2021 10:13:36 -0800
From:   Yonghong Song <yhs@...com>
To:     Andrii Nakryiko <andrii@...nel.org>, <bpf@...r.kernel.org>,
        <netdev@...r.kernel.org>, <ast@...com>, <daniel@...earbox.net>
CC:     <kernel-team@...com>,
        Christopher William Snowhill <chris@...e54.net>
Subject: Re: [PATCH bpf 2/2] libbpf: allow loading empty BTFs



On 1/9/21 11:03 PM, Andrii Nakryiko wrote:
> Empty BTFs do come up (e.g., simple kernel modules with no new types and
> strings, compared to the vmlinux BTF) and there is nothing technically wrong
> with them. So remove unnecessary check preventing loading empty BTFs.
> 
> Reported-by: Christopher William Snowhill <chris@...e54.net>
> Fixes: ("d8123624506c libbpf: Fix BTF data layout checks and allow empty BTF")
> Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
> ---
>   tools/lib/bpf/btf.c | 5 -----
>   1 file changed, 5 deletions(-)
> 
> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index 3c3f2bc6c652..9970a288dda5 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -240,11 +240,6 @@ static int btf_parse_hdr(struct btf *btf)
>   	}
>   
>   	meta_left = btf->raw_size - sizeof(*hdr);
> -	if (!meta_left) {
> -		pr_debug("BTF has no data\n");
> -		return -EINVAL;
> -	}

Previous kernel patch allows empty btf only if that btf is module (not 
base/vmlinux) btf. Here it seems we allow any empty non-module btf to be 
loaded into the kernel. In such cases, loading may fail? Maybe we should
detect such cases in libbpf and error out instead of going to kernel and
get error back?

> -
>   	if (meta_left < hdr->str_off + hdr->str_len) {
>   		pr_debug("Invalid BTF total size:%u\n", btf->raw_size);
>   		return -EINVAL;
>