linux-kernel - [PATCH] tracing: Fix printing ptrs in preempt/irq enable/disable events

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Message-ID: <20191127154428.191095-1-antonio.borneo@st.com>
Date:   Wed, 27 Nov 2019 16:44:28 +0100
From:   Antonio Borneo <antonio.borneo@...com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>
CC:     Antonio Borneo <antonio.borneo@...com>,
        Joel Fernandes <joelaf@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-stm32@...md-mailman.stormreply.com>
Subject: [PATCH] tracing: Fix printing ptrs in preempt/irq enable/disable events

This tracing event class is the only instance in kernel that logs
in the trace buffer the instruction pointer as offset to _stext,
instead of logging the full pointer.
This looks like a nice optimization for 64 bits platforms, where a
32 bit offset can take less space than a full 64 bits pointer. But
the symbol _stext is incorrectly resolved as zero in the expansion
of TP_printk(), which then prints only the hex offset instead of
the name of the caller function. Plus, on arm arch the kernel
modules are loaded at address lower than _stext, causing the u32
offset arithmetics to overflow and wrap at 32 bits.
I did not identified a 64 bit arch where the modules are loaded at
offset from _stext that exceed u32 range, but I also did not
identified any constraint to feel safe with a u32 offset.

Log directly the instruction pointer instead of the offset to
_stext.

Signed-off-by: Antonio Borneo <antonio.borneo@...com>
Fixes: d59158162e03 ("tracing: Add support for preempt and irq enable/disable events")
---
 include/trace/events/preemptirq.h | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/include/trace/events/preemptirq.h b/include/trace/events/preemptirq.h
index 95fba0471e5b..d548a6aafa18 100644
--- a/include/trace/events/preemptirq.h
+++ b/include/trace/events/preemptirq.h
@@ -18,18 +18,18 @@ DECLARE_EVENT_CLASS(preemptirq_template,
 	TP_ARGS(ip, parent_ip),

 	TP_STRUCT__entry(
-		__field(u32, caller_offs)
-		__field(u32, parent_offs)
+		__field(unsigned long, caller_ip)
+		__field(unsigned long, parent_ip)
 	),

 	TP_fast_assign(
-		__entry->caller_offs = (u32)(ip - (unsigned long)_stext);
-		__entry->parent_offs = (u32)(parent_ip - (unsigned long)_stext);
+		__entry->caller_ip = ip;
+		__entry->parent_ip = parent_ip;
 	),

 	TP_printk("caller=%pS parent=%pS",
-		  (void *)((unsigned long)(_stext) + __entry->caller_offs),
-		  (void *)((unsigned long)(_stext) + __entry->parent_offs))
+		  (void *)__entry->caller_ip,
+		  (void *)__entry->parent_ip)
 );

 #ifdef CONFIG_TRACE_IRQFLAGS
-- 
2.24.0