netdev - Re: [PATCH v6 bpf-next 10/14] bpf: allow to specify kernel module BTFs when attaching BPF programs

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Date:   Thu, 3 Dec 2020 17:53:58 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andrii@...nel.org>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org, ast@...com,
        daniel@...earbox.net, kernel-team@...com
Subject: Re: [PATCH v6 bpf-next 10/14] bpf: allow to specify kernel module
 BTFs when attaching BPF programs

On Thu, Dec 03, 2020 at 12:46:30PM -0800, Andrii Nakryiko wrote:
> +	if (attr->attach_prog_fd) {
> +		dst_prog = bpf_prog_get(attr->attach_prog_fd);
> +		if (IS_ERR(dst_prog)) {
> +			dst_prog = NULL;
> +			attach_btf = btf_get_by_fd(attr->attach_btf_obj_fd);
> +			if (IS_ERR(attach_btf))
> +				return -EINVAL;
> +			if (!btf_is_kernel(attach_btf)) {
> +				btf_put(attach_btf);
> +				return -EINVAL;

Applied, but please consider follow up with different err code here.
I think we might support this case in the future.
Specifying prog's BTF as a base and attach_btf_id within it might make
user space simpler in some cases. prog's btf covers the whole elf file.
Where prog_fd is a specific prog. That narrow scope isn't really necessary.
So may be return ENOTSUPP here for now? With a hint that this might
work in the future?