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Message-ID: <20241115102248.60710e5d@gandalf.local.home>
Date: Fri, 15 Nov 2024 10:22:48 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com>
Subject: [for-linus][PATCH] tracing/ring-buffer: Clear all memory mapped CPU
ring buffers on first recording
Clear all CPU buffers when starting tracing in a boot mapped buffer
To properly process events from a previous boot, the address space needs to
be accounted for due to KASLR and the events in the buffer are updated
accordingly when read. This also requires that when the buffer has tracing
enabled again in the current boot that the buffers are reset so that events
from the previous boot do not interact with the events of the current boot
and cause confusing due to not having the proper meta data.
It was found that if a CPU is taken offline, that its per CPU buffer is not
reset when tracing starts. This allows for events to be from both the
previous boot and the current boot to be in the buffer at the same time.
Clear all CPU buffers when tracing is started in a boot mapped buffer.
git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace.git
ring-buffer/fixes
Head SHA1: 09663753bb7c50b33f8e5fa562c20ce275b88237
Steven Rostedt (1):
tracing/ring-buffer: Clear all memory mapped CPU ring buffers on first recording
----
kernel/trace/trace.c | 28 ++++++++++++++++++++++++++--
1 file changed, 26 insertions(+), 2 deletions(-)
---------------------------
commit 09663753bb7c50b33f8e5fa562c20ce275b88237
Author: Steven Rostedt <rostedt@...dmis.org>
Date: Thu Nov 14 11:28:25 2024 -0500
tracing/ring-buffer: Clear all memory mapped CPU ring buffers on first recording
The events of a memory mapped ring buffer from the previous boot should
not be mixed in with events from the current boot. There's meta data that
is used to handle KASLR so that function names can be shown properly.
Also, since the timestamps of the previous boot have no meaning to the
timestamps of the current boot, having them intermingled in a buffer can
also cause confusion because there could possibly be events in the future.
When a trace is activated the meta data is reset so that the pointers of
are now processed for the new address space. The trace buffers are reset
when tracing starts for the first time. The problem here is that the reset
only happens on online CPUs. If a CPU is offline, it does not get reset.
To demonstrate the issue, a previous boot had tracing enabled in the boot
mapped ring buffer on reboot. On the following boot, tracing has not been
started yet so the function trace from the previous boot is still visible.
# trace-cmd show -B boot_mapped -c 3 | tail
<idle>-0 [003] d.h2. 156.462395: __rcu_read_lock <-cpu_emergency_disable_virtualization
<idle>-0 [003] d.h2. 156.462396: vmx_emergency_disable_virtualization_cpu <-cpu_emergency_disable_virtualization
<idle>-0 [003] d.h2. 156.462396: __rcu_read_unlock <-__sysvec_reboot
<idle>-0 [003] d.h2. 156.462397: stop_this_cpu <-__sysvec_reboot
<idle>-0 [003] d.h2. 156.462397: set_cpu_online <-stop_this_cpu
<idle>-0 [003] d.h2. 156.462397: disable_local_APIC <-stop_this_cpu
<idle>-0 [003] d.h2. 156.462398: clear_local_APIC <-disable_local_APIC
<idle>-0 [003] d.h2. 156.462574: mcheck_cpu_clear <-stop_this_cpu
<idle>-0 [003] d.h2. 156.462575: mce_intel_feature_clear <-stop_this_cpu
<idle>-0 [003] d.h2. 156.462575: lmce_supported <-mce_intel_feature_clear
Now, if CPU 3 is taken offline, and tracing is started on the memory
mapped ring buffer, the events from the previous boot in the CPU 3 ring
buffer is not reset. Now those events are using the meta data from the
current boot and produces just hex values.
# echo 0 > /sys/devices/system/cpu/cpu3/online
# trace-cmd start -B boot_mapped -p function
# trace-cmd show -B boot_mapped -c 3 | tail
<idle>-0 [003] d.h2. 156.462395: 0xffffffff9a1e3194 <-0xffffffff9a0f655e
<idle>-0 [003] d.h2. 156.462396: 0xffffffff9a0a1d24 <-0xffffffff9a0f656f
<idle>-0 [003] d.h2. 156.462396: 0xffffffff9a1e6bc4 <-0xffffffff9a0f7323
<idle>-0 [003] d.h2. 156.462397: 0xffffffff9a0d12b4 <-0xffffffff9a0f732a
<idle>-0 [003] d.h2. 156.462397: 0xffffffff9a1458d4 <-0xffffffff9a0d12e2
<idle>-0 [003] d.h2. 156.462397: 0xffffffff9a0faed4 <-0xffffffff9a0d12e7
<idle>-0 [003] d.h2. 156.462398: 0xffffffff9a0faaf4 <-0xffffffff9a0faef2
<idle>-0 [003] d.h2. 156.462574: 0xffffffff9a0e3444 <-0xffffffff9a0d12ef
<idle>-0 [003] d.h2. 156.462575: 0xffffffff9a0e4964 <-0xffffffff9a0d12ef
<idle>-0 [003] d.h2. 156.462575: 0xffffffff9a0e3fb0 <-0xffffffff9a0e496f
Reset all CPUs when starting a boot mapped ring buffer for the first time,
and not just the online CPUs.
Fixes: 7a1d1e4b9639f ("tracing/ring-buffer: Add last_boot_info file to boot instance")
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index a8f52b6527ca..619e9aa62201 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2386,6 +2386,25 @@ void tracing_reset_online_cpus(struct array_buffer *buf)
ring_buffer_record_enable(buffer);
}
+static void tracing_reset_all_cpus(struct array_buffer *buf)
+{
+ struct trace_buffer *buffer = buf->buffer;
+
+ if (!buffer)
+ return;
+
+ ring_buffer_record_disable(buffer);
+
+ /* Make sure all commits have finished */
+ synchronize_rcu();
+
+ buf->time_start = buffer_ftrace_now(buf, buf->cpu);
+
+ ring_buffer_reset(buffer);
+
+ ring_buffer_record_enable(buffer);
+}
+
/* Must have trace_types_lock held */
void tracing_reset_all_online_cpus_unlocked(void)
{
@@ -6141,8 +6160,13 @@ static void update_last_data(struct trace_array *tr)
if (!tr->text_delta && !tr->data_delta)
return;
- /* Clear old data */
- tracing_reset_online_cpus(&tr->array_buffer);
+ /*
+ * Need to clear all CPU buffers as there cannot be events
+ * from the previous boot mixed with events with this boot
+ * as that will cause a confusing trace. Need to clear all
+ * CPU buffers, even for those that may currently be offline.
+ */
+ tracing_reset_all_cpus(&tr->array_buffer);
/* Using current data now */
tr->text_delta = 0;
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