bpf: tcp: Allow bpf-tcp-cc to call bpf_(get|set)sockopt

This patch allows the bpf-tcp-cc to call bpf_setsockopt. One use case is to allow a bpf-tcp-cc switching to another cc during init(). For example, when the tcp flow is not ecn ready, the bpf_dctcp can switch to another cc by calling setsockopt(TCP_CONGESTION). During setsockopt(TCP_CONGESTION), the new tcp-cc's init() will be called and this could cause a recursion but it is stopped by the current trampoline's logic (in the prog->active counter). While retiring a bpf-tcp-cc (e.g. in tcp_v[46]_destroy_sock()), the tcp stack calls bpf-tcp-cc's release(). To avoid the retiring bpf-tcp-cc making further changes to the sk, bpf_setsockopt is not available to the bpf-tcp-cc's release(). This will avoid release() making setsockopt() call that will potentially allocate new resources. Although the bpf-tcp-cc already has a more powerful way to read tcp_sock from the PTR_TO_BTF_ID, it is usually expected that bpf_getsockopt and bpf_setsockopt are available together. Thus, bpf_getsockopt() is also added to all tcp_congestion_ops except release(). When the old bpf-tcp-cc is calling setsockopt(TCP_CONGESTION) to switch to a new cc, the old bpf-tcp-cc will be released by bpf_struct_ops_put(). Thus, this patch also puts the bpf_struct_ops_map after a rcu grace period because the trampoline's image cannot be freed while the old bpf-tcp-cc is still running. bpf-tcp-cc can only access icsk_ca_priv as SCALAR. All kernel's tcp-cc is also accessing the icsk_ca_priv as SCALAR. The size of icsk_ca_priv has already been raised a few times to avoid extra kmalloc and memory referencing. The only exception is the kernel's tcp_cdg.c that stores a kmalloc()-ed pointer in icsk_ca_priv. To avoid the old bpf-tcp-cc accidentally overriding this tcp_cdg's pointer value stored in icsk_ca_priv after switching and without over-complicating the bpf's verifier for this one exception in tcp_cdg, this patch does not allow switching to tcp_cdg. If there is a need, bpf_tcp_cdg can be implemented and then use the bpf_sk_storage as the extended storage. bpf_sk_setsockopt proto has only been recently added and used in bpf-sockopt and bpf-iter-tcp, so impose the tcp_cdg limitation in the same proto instead of adding a new proto specifically for bpf-tcp-cc.

MKMartin KaFai Lau committed 5 years ago