From 401cb7dae8130fd34eb84648e02ab4c506df7d5e Mon Sep 17 00:00:00 2001 From: Sebastian Andrzej Siewior Date: Thu, 20 Jun 2024 15:22:04 +0200 Subject: net: Reference bpf_redirect_info via task_struct on PREEMPT_RT. MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit The XDP redirect process is two staged: - bpf_prog_run_xdp() is invoked to run a eBPF program which inspects the packet and makes decisions. While doing that, the per-CPU variable bpf_redirect_info is used. - Afterwards xdp_do_redirect() is invoked and accesses bpf_redirect_info and it may also access other per-CPU variables like xskmap_flush_list. At the very end of the NAPI callback, xdp_do_flush() is invoked which does not access bpf_redirect_info but will touch the individual per-CPU lists. The per-CPU variables are only used in the NAPI callback hence disabling bottom halves is the only protection mechanism. Users from preemptible context (like cpu_map_kthread_run()) explicitly disable bottom halves for protections reasons. Without locking in local_bh_disable() on PREEMPT_RT this data structure requires explicit locking. PREEMPT_RT has forced-threaded interrupts enabled and every NAPI-callback runs in a thread. If each thread has its own data structure then locking can be avoided. Create a struct bpf_net_context which contains struct bpf_redirect_info. Define the variable on stack, use bpf_net_ctx_set() to save a pointer to it, bpf_net_ctx_clear() removes it again. The bpf_net_ctx_set() may nest. For instance a function can be used from within NET_RX_SOFTIRQ/ net_rx_action which uses bpf_net_ctx_set() and NET_TX_SOFTIRQ which does not. Therefore only the first invocations updates the pointer. Use bpf_net_ctx_get_ri() as a wrapper to retrieve the current struct bpf_redirect_info. The returned data structure is zero initialized to ensure nothing is leaked from stack. This is done on first usage of the struct. bpf_net_ctx_set() sets bpf_redirect_info::kern_flags to 0 to note that initialisation is required. First invocation of bpf_net_ctx_get_ri() will memset() the data structure and update bpf_redirect_info::kern_flags. bpf_redirect_info::nh is excluded from memset because it is only used once BPF_F_NEIGH is set which also sets the nh member. The kern_flags is moved past nh to exclude it from memset. The pointer to bpf_net_context is saved task's task_struct. Using always the bpf_net_context approach has the advantage that there is almost zero differences between PREEMPT_RT and non-PREEMPT_RT builds. Cc: Andrii Nakryiko Cc: Eduard Zingerman Cc: Hao Luo Cc: Jiri Olsa Cc: John Fastabend Cc: KP Singh Cc: Martin KaFai Lau Cc: Song Liu Cc: Stanislav Fomichev Cc: Yonghong Song Acked-by: Alexei Starovoitov Acked-by: Jesper Dangaard Brouer Reviewed-by: Toke Høiland-Jørgensen Signed-off-by: Sebastian Andrzej Siewior Link: https://patch.msgid.link/20240620132727.660738-15-bigeasy@linutronix.de Signed-off-by: Jakub Kicinski --- kernel/bpf/cpumap.c | 3 +++ kernel/bpf/devmap.c | 9 ++++++++- kernel/fork.c | 1 + 3 files changed, 12 insertions(+), 1 deletion(-) (limited to 'kernel') diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c index a8e34416e960..66974bd02710 100644 --- a/kernel/bpf/cpumap.c +++ b/kernel/bpf/cpumap.c @@ -240,12 +240,14 @@ static int cpu_map_bpf_prog_run(struct bpf_cpu_map_entry *rcpu, void **frames, int xdp_n, struct xdp_cpumap_stats *stats, struct list_head *list) { + struct bpf_net_context __bpf_net_ctx, *bpf_net_ctx; int nframes; if (!rcpu->prog) return xdp_n; rcu_read_lock_bh(); + bpf_net_ctx = bpf_net_ctx_set(&__bpf_net_ctx); nframes = cpu_map_bpf_prog_run_xdp(rcpu, frames, xdp_n, stats); @@ -255,6 +257,7 @@ static int cpu_map_bpf_prog_run(struct bpf_cpu_map_entry *rcpu, void **frames, if (unlikely(!list_empty(list))) cpu_map_bpf_prog_run_skb(rcpu, list, stats); + bpf_net_ctx_clear(bpf_net_ctx); rcu_read_unlock_bh(); /* resched point, may call do_softirq() */ return nframes; diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c index 7f3b34452243..fbfdfb60db8d 100644 --- a/kernel/bpf/devmap.c +++ b/kernel/bpf/devmap.c @@ -196,7 +196,14 @@ static void dev_map_free(struct bpf_map *map) list_del_rcu(&dtab->list); spin_unlock(&dev_map_lock); - bpf_clear_redirect_map(map); + /* bpf_redirect_info->map is assigned in __bpf_xdp_redirect_map() + * during NAPI callback and cleared after the XDP redirect. There is no + * explicit RCU read section which protects bpf_redirect_info->map but + * local_bh_disable() also marks the beginning an RCU section. This + * makes the complete softirq callback RCU protected. Thus after + * following synchronize_rcu() there no bpf_redirect_info->map == map + * assignment. + */ synchronize_rcu(); /* Make sure prior __dev_map_entry_free() have completed. */ diff --git a/kernel/fork.c b/kernel/fork.c index 99076dbe27d8..f314bdd7e610 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -2355,6 +2355,7 @@ __latent_entropy struct task_struct *copy_process( RCU_INIT_POINTER(p->bpf_storage, NULL); p->bpf_ctx = NULL; #endif + p->bpf_net_context = NULL; /* Perform scheduler related setup. Assign this task to a CPU. */ retval = sched_fork(clone_flags, p); -- cgit v1.3