Re: [PATCH bpf 2/2] selftests/bpf: Add XDP dispatcher FineIBT regression test

[Leon Hwang]

On 8/7/26 10:57, David Windsor wrote:
> Add a regression test for the x86 BPF dispatcher's indirect-jump fallback
> under FineIBT.
> 
> This test can crash the kernel on an unfixed FineIBT build and so should
> be run in a VM.
> 
> Signed-off-by: David Windsor <dwindsor@xxxxxxxxx>
> ---
>  .../bpf/prog_tests/xdp_dispatcher_fineibt.c   | 47 +++++++++++++++++++
>  .../bpf/progs/xdp_dispatcher_fineibt.c        | 18 +++++++
>  2 files changed, 65 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/xdp_dispatcher_fineibt.c
>  create mode 100644 tools/testing/selftests/bpf/progs/xdp_dispatcher_fineibt.c
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_dispatcher_fineibt.c b/tools/testing/selftests/bpf/prog_tests/xdp_dispatcher_fineibt.c
> new file mode 100644
> index 000000000000..2d3fc4034eb9
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/xdp_dispatcher_fineibt.c
> @@ -0,0 +1,47 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2026 David Windsor */
> +
> +/*
> + * Regression test for the x86 BPF dispatcher's indirect-jump fallback
> + * under FineIBT.
> + *
> + * WARNING
> + * -------
> + *  This test can crash the kernel, thus should be run in a VM.
> + */
> +#include <uapi/linux/if_link.h>
> +#include <test_progs.h>
> +#include <network_helpers.h>
> +#include "xdp_dispatcher_fineibt.skel.h"
> +
> +#define IFINDEX_LO		1
> +
> +void test_xdp_dispatcher_fineibt(void)
> +{
> +	struct xdp_dispatcher_fineibt *skel;
> +	int err, attached_fd, unattached_fd;
> +
> +	LIBBPF_OPTS(bpf_test_run_opts, topts,
> +		.data_in = &pkt_v4,
> +		.data_size_in = sizeof(pkt_v4),
> +	);
> +
> +	skel = xdp_dispatcher_fineibt__open_and_load();
> +	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
> +		return;
> +
> +	attached_fd = bpf_program__fd(skel->progs.xdp_dispatcher_attached);
> +	unattached_fd = bpf_program__fd(skel->progs.xdp_dispatcher_unattached);
> +
> +	err = bpf_xdp_attach(IFINDEX_LO, attached_fd, XDP_FLAGS_SKB_MODE, NULL);
> +	if (!ASSERT_OK(err, "attach_prog"))
> +		goto out;
> +
> +	err = bpf_prog_test_run_opts(unattached_fd, &topts);
> +	ASSERT_OK(err, "unattached_test_run");
> +	ASSERT_EQ(topts.retval, XDP_PASS, "unattached_retval");
> +
> +	bpf_xdp_detach(IFINDEX_LO, XDP_FLAGS_SKB_MODE, NULL);
> +out:
> +	xdp_dispatcher_fineibt__destroy(skel);
> +}
> diff --git a/tools/testing/selftests/bpf/progs/xdp_dispatcher_fineibt.c b/tools/testing/selftests/bpf/progs/xdp_dispatcher_fineibt.c
> new file mode 100644
> index 000000000000..b49b84f01c8e
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/xdp_dispatcher_fineibt.c
> @@ -0,0 +1,18 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2026 David Windsor */
> +#include <linux/bpf.h>
> +#include <bpf/bpf_helpers.h>
> +
> +SEC("xdp")
> +int xdp_dispatcher_attached(struct xdp_md *ctx)
> +{
> +	return XDP_PASS;
> +}
> +
> +SEC("xdp")
> +int xdp_dispatcher_unattached(struct xdp_md *ctx)
> +{
> +	return XDP_PASS;
> +}
> +
> +char _license[] SEC("license") = "GPL";


Instead of creating xdp_dispatcher_fineibt.c, better to reuse
xdp_dummy.c. Then, in xdp_dispatcher_fineibt.c, you can create two
xdp_dummy skels, one for attaching, one for running.

And, add the test to xdp_attach.c instead of new file
prog_tests/xdp_dispatcher_fineibt.c?

Thanks,
Leon