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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: linux-kernel@vger.kernel.org,
	netdev@vger.kernel.org
Cc: "David S. Miller" <davem@davemloft.net>,
	Boqun Feng <boqun.feng@gmail.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Eric Dumazet <edumazet@google.com>,
	Frederic Weisbecker <frederic@kernel.org>,
	Ingo Molnar <mingo@redhat.com>,
	Jakub Kicinski <kuba@kernel.org>,
	Paolo Abeni <pabeni@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Waiman Long <longman@redhat.com>,
	Will Deacon <will@kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Alexei Starovoitov <ast@kernel.org>,
	Andrii Nakryiko <andrii@kernel.org>,
	Hao Luo <haoluo@google.com>,
	Jesper Dangaard Brouer <hawk@kernel.org>,
	Jiri Olsa <jolsa@kernel.org>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	Martin KaFai Lau <martin.lau@linux.dev>,
	Song Liu <song@kernel.org>,
	Stanislav Fomichev <sdf@google.com>,
	Yonghong Song <yonghong.song@linux.dev>,
	bpf@vger.kernel.org
Subject: [PATCH net-next 14/24] net: Add a lock which held during the redirect process.
Date: Fri, 15 Dec 2023 18:07:33 +0100
Message-ID: <20231215171020.687342-15-bigeasy@linutronix.de>
In-Reply-To: <20231215171020.687342-1-bigeasy@linutronix.de>
References: <20231215171020.687342-1-bigeasy@linutronix.de>
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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.

Introduce redirect_lock as a lock to be acquired when access to these
per-CPU variables is performed. Usually the lock is part of the per-CPU
variable which is about to be protected but since there are a few
different per-CPU variables which need to be protected at the same
time (and some of the variables depend on a CONFIG setting) a new
per-CPU data structure with variable bpf_run_lock is used for this.

The lock is a nested-BH lock meaning that on non-PREEMPT_RT kernels this
simply results in a lockdep check and ensuring that bottom halves are
disabled. On PREEMPT_RT kernels this will provide the needed
synchronisation once local_bh_disable() does not act as per-CPU lock.

This patch introduces the bpf_run_lock.redirect_lock lock. It will be
used by drivers in the following patches.

A follow-up step could be to keep bpf_prog_run_xdp() and the
XDP_REDIRECT switch case (with xdp_do_redirect()) close together. That
would allow a single scoped_guard() macro to cover the two required
instaces that require locking instead the whole switch case.

Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Hao Luo <haoluo@google.com>
Cc: Jesper Dangaard Brouer <hawk@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: KP Singh <kpsingh@kernel.org>
Cc: Martin KaFai Lau <martin.lau@linux.dev>
Cc: Song Liu <song@kernel.org>
Cc: Stanislav Fomichev <sdf@google.com>
Cc: Yonghong Song <yonghong.song@linux.dev>
Cc: bpf@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/bpf.h | 6 ++++++
 net/core/filter.c   | 5 +++++
 2 files changed, 11 insertions(+)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index cff5bb08820ec..6912b85209b12 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -291,6 +291,12 @@ struct bpf_map {
 	s64 __percpu *elem_count;
 };
=20
+struct bpf_run_lock {
+	local_lock_t redirect_lock;
+};
+
+DECLARE_PER_CPU(struct bpf_run_lock, bpf_run_lock);
+
 static inline const char *btf_field_type_name(enum btf_field_type type)
 {
 	switch (type) {
diff --git a/net/core/filter.c b/net/core/filter.c
index 896aa3fa699f9..7c9653734fb60 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -89,6 +89,11 @@
 static const struct bpf_func_proto *
 bpf_sk_base_func_proto(enum bpf_func_id func_id);
=20
+DEFINE_PER_CPU(struct bpf_run_lock, bpf_run_lock) =3D {
+	.redirect_lock =3D INIT_LOCAL_LOCK(redirect_lock),
+};
+EXPORT_PER_CPU_SYMBOL_GPL(bpf_run_lock);
+
 int copy_bpf_fprog_from_user(struct sock_fprog *dst, sockptr_t src, int le=
n)
 {
 	if (in_compat_syscall()) {
--=20
2.43.0