[PATCH 2/3] samples/fprobe: Use trace_printk() and trace_dump_stack()

[Masami Hiramatsu (Google)]

From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

Fprobe entry and exit callbacks execute in atomic context with
preemption disabled under ftrace recursion protection. Calling
printk() directly inside handlers can lead to deadlocks if console
or scheduler locks are already held by the probed or interrupted context.

Drop the use_trace module parameter and always use trace_printk()
for handler logging. In addition, replace stack_trace_print() (which
uses printk()) with trace_dump_stack(0) to output backtraces safely
to the ftrace buffer. Also update the module header comment to
reference /sys/kernel/tracing/trace.

Additionally, update CONFIG_SAMPLE_FPROBE in samples/Kconfig to
depend on EVENT_TRACING so that the tracing subsystem is enabled
when building the sample.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
---
 samples/Kconfig                 |    2 +-
 samples/fprobe/fprobe_example.c |   49 ++++++++-------------------------------
 2 files changed, 11 insertions(+), 40 deletions(-)

diff --git a/samples/Kconfig b/samples/Kconfig
index 54414e57ff75..744d991ea620 100644
--- a/samples/Kconfig
+++ b/samples/Kconfig
@@ -88,7 +88,7 @@ config SAMPLE_HW_BREAKPOINT
 
 config SAMPLE_FPROBE
 	tristate "Build fprobe examples -- loadable modules only"
-	depends on FPROBE && m
+	depends on FPROBE && EVENT_TRACING && m
 	help
 	  This builds a fprobe example module. This module has an option 'symbol'.
 	  You can specify a probed symbol or symbols separated with ','.
diff --git a/samples/fprobe/fprobe_example.c b/samples/fprobe/fprobe_example.c
index bfe98ce826f3..3b94839b3199 100644
--- a/samples/fprobe/fprobe_example.c
+++ b/samples/fprobe/fprobe_example.c
@@ -1,12 +1,12 @@
 // SPDX-License-Identifier: GPL-2.0-only
 /*
  * Here's a sample kernel module showing the use of fprobe to dump a
- * stack trace and selected registers when kernel_clone() is called.
+ * stack trace and function entry/exit when kernel_clone() is called.
  *
- * For more information on theory of operation of kprobes, see
- * Documentation/trace/kprobes.rst
+ * For more information on theory of operation of fprobe, see
+ * Documentation/trace/fprobe.rst
  *
- * You will see the trace data in /var/log/messages and on the console
+ * You will see the trace data in /sys/kernel/tracing/trace
  * whenever kernel_clone() is invoked to create a new process.
  */
 
@@ -15,10 +15,9 @@
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/fprobe.h>
-#include <linux/sched/debug.h>
 #include <linux/slab.h>
+#include <linux/trace_printk.h>
 
-#define BACKTRACE_DEPTH 16
 #define MAX_SYMBOL_LEN 4096
 static struct fprobe sample_probe;
 static unsigned long nhit;
@@ -35,34 +34,14 @@ static bool stackdump = true;
 module_param(stackdump, bool, 0644);
 MODULE_PARM_DESC(stackdump, "Enable stackdump.");
 
-static bool use_trace = false;
-module_param(use_trace, bool, 0644);
-MODULE_PARM_DESC(use_trace, "Use trace_printk instead of printk. This is only for debugging.");
-
-static void show_backtrace(void)
-{
-	unsigned long stacks[BACKTRACE_DEPTH];
-	unsigned int len;
-
-	len = stack_trace_save(stacks, BACKTRACE_DEPTH, 2);
-	stack_trace_print(stacks, len, 24);
-}
-
 static int sample_entry_handler(struct fprobe *fp, unsigned long ip,
 				unsigned long ret_ip,
 				struct ftrace_regs *fregs, void *data)
 {
-	if (use_trace)
-		/*
-		 * This is just an example, no kernel code should call
-		 * trace_printk() except when actively debugging.
-		 */
-		trace_printk("Enter <%pS> ip = 0x%p\n", (void *)ip, (void *)ip);
-	else
-		pr_info("Enter <%pS> ip = 0x%p\n", (void *)ip, (void *)ip);
+	trace_printk("Enter <%pS> ip = 0x%p\n", (void *)ip, (void *)ip);
 	nhit++;
 	if (stackdump)
-		show_backtrace();
+		trace_dump_stack(0);
 	return 0;
 }
 
@@ -72,19 +51,11 @@ static void sample_exit_handler(struct fprobe *fp, unsigned long ip,
 {
 	unsigned long rip = ret_ip;
 
-	if (use_trace)
-		/*
-		 * This is just an example, no kernel code should call
-		 * trace_printk() except when actively debugging.
-		 */
-		trace_printk("Return from <%pS> ip = 0x%p to rip = 0x%p (%pS)\n",
-			(void *)ip, (void *)ip, (void *)rip, (void *)rip);
-	else
-		pr_info("Return from <%pS> ip = 0x%p to rip = 0x%p (%pS)\n",
-			(void *)ip, (void *)ip, (void *)rip, (void *)rip);
+	trace_printk("Return from <%pS> ip = 0x%p to rip = 0x%p (%pS)\n",
+		     (void *)ip, (void *)ip, (void *)rip, (void *)rip);
 	nhit++;
 	if (stackdump)
-		show_backtrace();
+		trace_dump_stack(0);
 }
 
 static int __init fprobe_init(void)