mercredi 13 juin 2012

Can't trace userspace processes with SystemTap

What is System Tap ?

It is an infrastructuer coming with a scripting language that allows collecting information on a running linux kernel. It can be used by System Administrators to see for example which process are hitting hardly the disks, or by Kernel/UserSpace developers to instrument their code and gather information about disk access,  network usage... A great tool to fine tune and optimize application.
It is similar to DTrace available for Solaris and BDS Oses.

A good documentation exists for System Tap:

User Space debugging

For Debian Squeeze System Tap works out of the box for probing kernel code. But when I wanted to trace userspace code I was facing the following error:

# cat test.stp
probe process("./test").function("main").return { printf ("Exit from the main\n"); }
#stap -v test.stp -c ./test
Pass 1: parsed user script and 68 library script(s) using 49964virt/20928res/1768shr kb, in 10usr/20sys/424real ms. semantic error: process probes not available without kernel CONFIG_UTRACE while resolving probe point process("./test").function("main").return

So it seems like my Debian kernel is lacking of utrace supports. Let's recompile it :). First of all we have to get a vanilla kernel (because with the debian I can't apply utrace patches properly):

Then the utrace patches:

The trick here is that tracehook.patch is a gzip archive and not a plain text file:
#mv tracehook.patch tracehook.gz
#gunzip tracehook.gz
#mv tracehook tracehook.patch

Untar linux sources and apply the patches:
#tar -xvf linux-2.6.32.tar.bz2
#cd linux-2.6.32
#patch -p1 < ../tracehook.patch
#patch -p1 < ../utrace.patch
#patch -p1 < ../utrace-ptrace.patch

Compile the kernel:
#cp /boot/config-`uname -r` .config
#make oldconfig

Enable the option CONFIG_UTRACE
#make menuconfig (->General Setup -> Infrastructure for debugging and tracing user processes ) 

#sudo make
#sudo make modules_install install
#mkinitramfs -o /boot/initrd-img.2.6.32 2.6.32

Also modify grub to add an entry pointing to your new kernel and initrd image.

Reboot and that's it !

Aucun commentaire:

Enregistrer un commentaire