[lttng-dev] Instrumenting a module?

[Thibault, Daniel]

   I'm trying to find out the recipe for instrumenting a kernel module with some custom LTTng tracepoints.  Oddly, writing the LTTng tracepoint provider kernel module turned out not too bad (the READMEs help), but I have seen no documentation of the process of instrumenting a module.

   Here is a simple module and its Makefile.  The module works as expected, adding events to the dmesg kernel log buffer upon being loaded and unloaded.

######
hello.c
######

#include <linux/module.h>  // included for all kernel modules
#include <linux/kernel.h>  // included for KERN_INFO
#include <linux/init.h>    // included for __init and __exit macros

//Module meta-data
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Your name here");
MODULE_DESCRIPTION("A simple module");

//Module initialization
static int __init hello_init(void)
{
   printk(KERN_INFO "Hello!\n");
   return 0;
   // Any other return value means the module couldn't be loaded.
}

//Module finalization
static void __exit hello_cleanup(void)
{
   printk(KERN_INFO "Cleaning up hello module.\n");
}

//Identify the initialization and finalization handlers
module_init(hello_init);
module_exit(hello_cleanup);

######
Makefile
######

obj-m += hello.o

all:
	@echo "~~~~~~Copying hello.h to linux-headers:"
	gksudo cp hello.h /usr/src/linux-headers-3.2.0-53/include/trace/events/hello.h
	@echo "~~~~~~Copying hello.h to lttng-modules/instrumentation/events/lttng-module:"
	sudo cp hello-module.h /usr/src/lttng-modules-2.3.0/instrumentation/events/lttng-module/hello.h
	@echo "~~~~~~Copying lttng-probe-hello.c to lttng-modules/probes:"
	sudo cp lttng-probe-hello.c /usr/src/lttng-modules-2.3.0/probes/lttng-probe-hello.c
	@echo "~~~~~~Making hello.ko:"
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

hello.h defines the tracepoints for my custom LTTng module (lttng-probe-hello).  hello-module.h is hello.h "translated" for the lttng-module directory.  lttng-probe-hello was added to the probes Makefile.  lttng-modules makes and installs it just fine, and once I manually insmod lttng-probe-hello.ko, LTTng reports the new kernel tracepoints in 'lttng list -k'.

I instrument hello.c by adding:

######
hello.c
######

#include <linux/module.h>  // included for all kernel modules
#include <linux/kernel.h>  // included for KERN_INFO
#include <linux/init.h>    // included for __init and __exit macros
#include <trace/events/hello.h>

...

static int __init hello_init(void)
{
   trace_hello_init("Hello!");
   printk(KERN_INFO "Hello!\n");
...

static void __exit hello_cleanup(void)
{
   trace_hello_exit("Bye!");
   printk(KERN_INFO "Cleaning up hello module.\n");
...

I get a couple of warnings when I make the instrumented hello module:

WARNING: "__tracepoint_hello_exit" [/home/daniel/module/hello.ko] undefined!
WARNING: "__tracepoint_hello_init" [/home/daniel/module/hello.ko] undefined!

And hello.ko refuses to insmod as a consequence:

$ sudo insmod hello.ko
insmod: error inserting 'hello.ko': -1 Unknown symbol in module

Assuming I got the LTTng tracepoint provider kernel module right, what am I doing wrong with the instrumented module?  I suspect a missing define flag.
 
Daniel U. Thibault
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