netdevsim: Add debugfs toggle to reject BPF programs in verifier

This adds a new debugfs toggle ('bpf_bind_verifier_accept') that can be
used to make netdevsim reject BPF programs from being accepted by the
verifier. If this toggle (which defaults to true) is set to false,
nsim_bpf_verify_insn() will return EOPNOTSUPP on the last
instruction (after outputting the 'Hello from netdevsim' verifier message).

This makes it possible to check the verification callback in the driver
from test_offload.py in selftests, since the verifier now clears the
verifier log on a successful load, hiding the message from the driver.

Fixes: 6f8a57ccf8 ("bpf: Make verifier log more relevant by default")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/bpf/160752225964.110217.12584017165318065332.stgit@toke.dk

drivers/net/netdevsim/netdevsim.h

@@ -189,6 +189,7 @@ struct nsim_dev {
 	struct dentry *take_snapshot;
 	struct bpf_offload_dev *bpf_dev;
 	bool bpf_bind_accept;
+	bool bpf_bind_verifier_accept;
 	u32 bpf_bind_verifier_delay;
 	struct dentry *ddir_bpf_bound_progs;
 	u32 prog_id_gen;