netdev - Re: [PATCH bpf 2/2] bpf/selftests: test that kernel rejects a TCP CC with an invalid license

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Message-ID: <20210325183200.jzk7dvxmkn2bj5q3@kafai-mbp.dhcp.thefacebook.com>
Date:   Thu, 25 Mar 2021 11:32:00 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Clark Williams <williams@...hat.com>, <bpf@...r.kernel.org>,
        <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf 2/2] bpf/selftests: test that kernel rejects a TCP CC
 with an invalid license

On Thu, Mar 25, 2021 at 04:40:34PM +0100, Toke Høiland-Jørgensen wrote:
> This adds a selftest to check that the verifier rejects a TCP CC struct_ops
> with a non-GPL license. To save having to add a whole new BPF object just
> for this, reuse the dctcp CC, but rewrite the license field before loading.
> 
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---
>  .../selftests/bpf/prog_tests/bpf_tcp_ca.c     | 31 +++++++++++++++++++
>  1 file changed, 31 insertions(+)
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/bpf_tcp_ca.c b/tools/testing/selftests/bpf/prog_tests/bpf_tcp_ca.c
> index 37c5494a0381..613cf8a00b22 100644
> --- a/tools/testing/selftests/bpf/prog_tests/bpf_tcp_ca.c
> +++ b/tools/testing/selftests/bpf/prog_tests/bpf_tcp_ca.c
> @@ -227,10 +227,41 @@ static void test_dctcp(void)
>  	bpf_dctcp__destroy(dctcp_skel);
>  }
>  
> +static void test_invalid_license(void)
> +{
> +	/* We want to check that the verifier refuses to load a non-GPL TCP CC.
> +	 * Rather than create a whole new file+skeleton, just reuse an existing
> +	 * object and rewrite the license in memory after loading. Sine libbpf
> +	 * doesn't expose this, we define a struct that includes the first couple
> +	 * of internal fields for struct bpf_object so we can overwrite the right
> +	 * bits. Yes, this is a bit of a hack, but it makes the test a lot simpler.
> +	 */
> +	struct bpf_object_fragment {
> +		char name[BPF_OBJ_NAME_LEN];
> +		char license[64];
> +	} *obj;
It is fragile.  A new bpf_nogpltcp.c should be created and it does
not have to be a full tcp-cc.  A very minimal implementation with
only .init. Something like this (uncompiled code):

char _license[] SEC("license") = "X";

void BPF_STRUCT_OPS(nogpltcp_init, struct sock *sk)
{
}

SEC(".struct_ops")
struct tcp_congestion_ops bpf_nogpltcp = {
	.init           = (void *)nogpltcp_init,
	.name           = "bpf_nogpltcp",
};

libbpf_set_print() can also be used to look for the
the verifier log "struct ops programs must have a GPL compatible license".