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Make sure a new flow dissector program can be attached to replace the old one with a single syscall. Also check that attaching the same program twice is prohibited. Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Reviewed-by: Stanislav Fomichev <sdf@google.com> Acked-by: Martin KaFai Lau <kafai@fb.com> Link: https://lore.kernel.org/bpf/20191011082946.22695-3-jakub@cloudflare.com