netdev - [PATCH net 2/2] bpf: Make sure bpf_disable_instrumentation() is safe vs preemption.

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Date:   Sat, 27 Nov 2021 17:32:00 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-doc@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Jonathan Corbet <corbet@....net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH net 2/2] bpf: Make sure bpf_disable_instrumentation() is safe vs preemption.

The initial implementation of migrate_disable() for mainline was a
wrapper around preempt_disable(). RT kernels substituted this with
a real migrate disable implementation.

Later on mainline gained true migrate disable support, but neither
documentation nor affected code were updated.

Remove stale comments claiming that migrate_disable() is PREEMPT_RT
only.
Don't use __this_cpu_inc() in the !PREEMPT_RT path because preemption is
not disabled and the RMW operation can be preempted.

Fixes: 74d862b682f51 ("sched: Make migrate_disable/enable() independent of RT")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 include/linux/bpf.h    | 16 ++--------------
 include/linux/filter.h |  3 ---
 2 files changed, 2 insertions(+), 17 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index e7a163a3146b6..327a2bec06ca0 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1352,28 +1352,16 @@ extern struct mutex bpf_stats_enabled_mutex;
  * kprobes, tracepoints) to prevent deadlocks on map operations as any of
  * these events can happen inside a region which holds a map bucket lock
  * and can deadlock on it.
- *
- * Use the preemption safe inc/dec variants on RT because migrate disable
- * is preemptible on RT and preemption in the middle of the RMW operation
- * might lead to inconsistent state. Use the raw variants for non RT
- * kernels as migrate_disable() maps to preempt_disable() so the slightly
- * more expensive save operation can be avoided.
  */
 static inline void bpf_disable_instrumentation(void)
 {
 	migrate_disable();
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		this_cpu_inc(bpf_prog_active);
-	else
-		__this_cpu_inc(bpf_prog_active);
+	this_cpu_inc(bpf_prog_active);
 }
 
 static inline void bpf_enable_instrumentation(void)
 {
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		this_cpu_dec(bpf_prog_active);
-	else
-		__this_cpu_dec(bpf_prog_active);
+	this_cpu_dec(bpf_prog_active);
 	migrate_enable();
 }
 
diff --git a/include/linux/filter.h b/include/linux/filter.h
index 24b7ed2677afd..534f678ca50fa 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -640,9 +640,6 @@ static __always_inline u32 bpf_prog_run(const struct bpf_prog *prog, const void
  * This uses migrate_disable/enable() explicitly to document that the
  * invocation of a BPF program does not require reentrancy protection
  * against a BPF program which is invoked from a preempting task.
- *
- * For non RT enabled kernels migrate_disable/enable() maps to
- * preempt_disable/enable(), i.e. it disables also preemption.
  */
 static inline u32 bpf_prog_run_pin_on_cpu(const struct bpf_prog *prog,
 					  const void *ctx)
-- 
2.34.0