From: Sebastian Andrzej Siewior Date: Thu, 26 Oct 2023 12:10:40 +0200 Subject: [PATCH 12/15] seg6: Use nested-BH locking for seg6_bpf_srh_states. Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.10/older/patches-6.10.2-rt14.tar.xz The access to seg6_bpf_srh_states is protected by disabling preemption. Based on the code, the entry point is input_action_end_bpf() and every other function (the bpf helper functions bpf_lwt_seg6_*()), that is accessing seg6_bpf_srh_states, should be called from within input_action_end_bpf(). input_action_end_bpf() accesses seg6_bpf_srh_states first at the top of the function and then disables preemption. This looks wrong because if preemption needs to be disabled as part of the locking mechanism then the variable shouldn't be accessed beforehand. Looking at how it is used via test_lwt_seg6local.sh then input_action_end_bpf() is always invoked from softirq context. If this is always the case then the preempt_disable() statement is superfluous. If this is not always invoked from softirq then disabling only preemption is not sufficient. Replace the preempt_disable() statement with nested-BH locking. This is not an equivalent replacement as it assumes that the invocation of input_action_end_bpf() always occurs in softirq context and thus the preempt_disable() is superfluous. Add a local_lock_t the data structure and use local_lock_nested_bh() for locking. Add lockdep_assert_held() to ensure the lock is held while the per-CPU variable is referenced in the helper functions. Cc: Alexei Starovoitov Cc: Andrii Nakryiko Cc: David Ahern 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 Cc: bpf@vger.kernel.org Signed-off-by: Sebastian Andrzej Siewior --- include/net/seg6_local.h | 1 + net/core/filter.c | 3 +++ net/ipv6/seg6_local.c | 22 ++++++++++++++-------- 3 files changed, 18 insertions(+), 8 deletions(-) --- a/include/net/seg6_local.h +++ b/include/net/seg6_local.h @@ -19,6 +19,7 @@ extern int seg6_lookup_nexthop(struct sk extern bool seg6_bpf_has_valid_srh(struct sk_buff *skb); struct seg6_bpf_srh_state { + local_lock_t bh_lock; struct ipv6_sr_hdr *srh; u16 hdrlen; bool valid; --- a/net/core/filter.c +++ b/net/core/filter.c @@ -6455,6 +6455,7 @@ BPF_CALL_4(bpf_lwt_seg6_store_bytes, str void *srh_tlvs, *srh_end, *ptr; int srhoff = 0; + lockdep_assert_held(&srh_state->bh_lock); if (srh == NULL) return -EINVAL; @@ -6511,6 +6512,7 @@ BPF_CALL_4(bpf_lwt_seg6_action, struct s int hdroff = 0; int err; + lockdep_assert_held(&srh_state->bh_lock); switch (action) { case SEG6_LOCAL_ACTION_END_X: if (!seg6_bpf_has_valid_srh(skb)) @@ -6587,6 +6589,7 @@ BPF_CALL_3(bpf_lwt_seg6_adjust_srh, stru int srhoff = 0; int ret; + lockdep_assert_held(&srh_state->bh_lock); if (unlikely(srh == NULL)) return -EINVAL; --- a/net/ipv6/seg6_local.c +++ b/net/ipv6/seg6_local.c @@ -1380,7 +1380,9 @@ static int input_action_end_b6_encap(str return err; } -DEFINE_PER_CPU(struct seg6_bpf_srh_state, seg6_bpf_srh_states); +DEFINE_PER_CPU(struct seg6_bpf_srh_state, seg6_bpf_srh_states) = { + .bh_lock = INIT_LOCAL_LOCK(bh_lock), +}; bool seg6_bpf_has_valid_srh(struct sk_buff *skb) { @@ -1388,6 +1390,7 @@ bool seg6_bpf_has_valid_srh(struct sk_bu this_cpu_ptr(&seg6_bpf_srh_states); struct ipv6_sr_hdr *srh = srh_state->srh; + lockdep_assert_held(&srh_state->bh_lock); if (unlikely(srh == NULL)) return false; @@ -1408,8 +1411,7 @@ bool seg6_bpf_has_valid_srh(struct sk_bu static int input_action_end_bpf(struct sk_buff *skb, struct seg6_local_lwt *slwt) { - struct seg6_bpf_srh_state *srh_state = - this_cpu_ptr(&seg6_bpf_srh_states); + struct seg6_bpf_srh_state *srh_state; struct ipv6_sr_hdr *srh; int ret; @@ -1420,10 +1422,14 @@ static int input_action_end_bpf(struct s } advance_nextseg(srh, &ipv6_hdr(skb)->daddr); - /* preempt_disable is needed to protect the per-CPU buffer srh_state, - * which is also accessed by the bpf_lwt_seg6_* helpers + /* The access to the per-CPU buffer srh_state is protected by running + * always in softirq context (with disabled BH). On PREEMPT_RT the + * required locking is provided by the following local_lock_nested_bh() + * statement. It is also accessed by the bpf_lwt_seg6_* helpers via + * bpf_prog_run_save_cb(). */ - preempt_disable(); + local_lock_nested_bh(&seg6_bpf_srh_states.bh_lock); + srh_state = this_cpu_ptr(&seg6_bpf_srh_states); srh_state->srh = srh; srh_state->hdrlen = srh->hdrlen << 3; srh_state->valid = true; @@ -1446,15 +1452,15 @@ static int input_action_end_bpf(struct s if (srh_state->srh && !seg6_bpf_has_valid_srh(skb)) goto drop; + local_unlock_nested_bh(&seg6_bpf_srh_states.bh_lock); - preempt_enable(); if (ret != BPF_REDIRECT) seg6_lookup_nexthop(skb, NULL, 0); return dst_input(skb); drop: - preempt_enable(); + local_unlock_nested_bh(&seg6_bpf_srh_states.bh_lock); kfree_skb(skb); return -EINVAL; }