From eb18b49ea758ec052ac2a12c6bb204e1e877ec31 Mon Sep 17 00:00:00 2001 From: Martin KaFai Lau Date: Tue, 24 Aug 2021 10:30:07 -0700 Subject: 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. Signed-off-by: Martin KaFai Lau Signed-off-by: Alexei Starovoitov Link: https://lore.kernel.org/bpf/20210824173007.3976921-1-kafai@fb.com --- kernel/bpf/bpf_struct_ops.c | 22 +++++++++++++++++++++- 1 file changed, 21 insertions(+), 1 deletion(-) (limited to 'kernel/bpf/bpf_struct_ops.c') diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c index 70f6fd4fa305..d6731c32864e 100644 --- a/kernel/bpf/bpf_struct_ops.c +++ b/kernel/bpf/bpf_struct_ops.c @@ -28,6 +28,7 @@ struct bpf_struct_ops_value { struct bpf_struct_ops_map { struct bpf_map map; + struct rcu_head rcu; const struct bpf_struct_ops *st_ops; /* protect map_update */ struct mutex lock; @@ -622,6 +623,14 @@ bool bpf_struct_ops_get(const void *kdata) return refcount_inc_not_zero(&kvalue->refcnt); } +static void bpf_struct_ops_put_rcu(struct rcu_head *head) +{ + struct bpf_struct_ops_map *st_map; + + st_map = container_of(head, struct bpf_struct_ops_map, rcu); + bpf_map_put(&st_map->map); +} + void bpf_struct_ops_put(const void *kdata) { struct bpf_struct_ops_value *kvalue; @@ -632,6 +641,17 @@ void bpf_struct_ops_put(const void *kdata) st_map = container_of(kvalue, struct bpf_struct_ops_map, kvalue); - bpf_map_put(&st_map->map); + /* The struct_ops's function may switch to another struct_ops. + * + * For example, bpf_tcp_cc_x->init() may switch to + * another tcp_cc_y by calling + * setsockopt(TCP_CONGESTION, "tcp_cc_y"). + * During the switch, bpf_struct_ops_put(tcp_cc_x) is called + * and its map->refcnt may reach 0 which then free its + * trampoline image while tcp_cc_x is still running. + * + * Thus, a rcu grace period is needed here. + */ + call_rcu(&st_map->rcu, bpf_struct_ops_put_rcu); } } -- cgit v1.2.3