# SystemTap

SystemTap provides free software (GPL) infrastructure to simplify the gathering of information about the running Linux system.

## SystemTap

Simply install SystemTap from AUR: systemtapAUR, all done. Compare it to the most recent upstream release at [1].

Consider also building it from sources at [2], where support for newer kernels or distros makes first appearance.

## Standard kernel

You will need at least the linux-headers package installed.

Because Arch permanently strips debugging data from its distributed binaries (including the kernel), many normal/fancier systemtap capabilities are simply not available, so many examples at /usr/share/doc/systemtap/examples will not work. However, see the stapprobes man page for the NON-DWARF and AUTO-DWARF probe types for what should still work, for example:

- kernel tracepoints: kernel.trace("*")
- user-space probes: process("...").function("...") (for programs you build yourself with -g)
- user-space markers: process("...").mark("...") (if they were configured with the <sys/sdt.h> markers)
- perfctr-based probes: perf.*
- non-dwarf kernel probes: kprobe.function("...") and nd_syscall.* tapset (if a /boot/System.map* file is available, see below).

## Kernel rebuild

You may consider to build a linux-custom package to run SystemTap, but rebuilding the original linux package is easy and efficient. See also Kernels/Traditional compilation.

### Prepare

First, run `ABSROOT=. abs core/linux; cd core/linux` to get the original kernel build files. Then use `makepkg --verifysource` to get the additional files. By performing the verification, you can safely skip the steps on "Update checksum".

### modify config

Edit config (for 32-bit systems) or config.x86_64 (for 64-bit systems), turn on these options:

- CONFIG_KPROBES=y
- CONFIG_KPROBES_SANITY_TEST=n
- CONFIG_KPROBE_EVENT=y
- CONFIG_NET_DCCPPROBE=m
- CONFIG_NET_SCTPPROBE=m
- CONFIG_NET_TCPPROBE=y
- CONFIG_DEBUG_INFO=y
- CONFIG_DEBUG_INFO_REDUCED=n
- CONFIG_X86_DECODER_SELFTEST=n

By default only CONFIG_DEBUG_INFO and CONFIG_DEBUG_INFO_REDUCED are not set.

With current core/linux (tested with 3.15.2) you can simply append these lines into config.[x86_64]:

    x86_64

    echo '
    CONFIG_DEBUG_INFO=y
    CONFIG_DEBUG_INFO_REDUCED=n
    ' >> config.x86_64

Note that if you want to put these lines into a self-maintained script, do not insert any space before CONFIG_* lines.

### Update checksum

You can safely skip this step if you believe the source files are correct.

Run `md5sum config[.x86_64]` to get a new md5sum.

In PKGBUILD file, the `md5sums=('sum-of-first' ... 'sum-of-last')` has the same order with `source=('first-source' ... 'last-source')`, put your new md5sum in the right place.

### Build and Install

Optional: It is recommended to set `MAKEFLAGS="-j16"` in `/etc/makepkg.conf` to speed up the compilation.

You will need about 12 GB disk space for this build. Consider using an in-memory tmpfs if you have large DRAM. Run `makepkg` or `makepkg --skipchecksums` to compile, then simply `sudo pacman -U *.pkg.tar.gz` to install the packages. pacman will tell you reinstall, and you should say y.

linux and linux-headers should be reinstalled, linux-docs does not matter.

Via this method, external modules (e.g. nvidia and virtualbox) do not need to be rebuilt.

## Build custom kernel

Please reference this README

## Troubleshooting

### Pass 4 fails when launching

If you have:

       /usr/share/systemtap/runtime/stat.c:214:2: error: 'cpu_possible_map' undeclared (first use in this function)

Try to install systemtap-git package

### System.map is missing

You can recover it where you build your linux kernel with DEBUG_INFO enabled

       cp src/linux-3.6/System.map /boot/System.map-3.6.7-1-ARCH

Alternately,

       sudo cp /proc/kallsyms /boot/System.map-`uname -r`

### Process return probes not available

If you are sure that your kernel configuration is correct, but on launching `stap` you get both of the following messages:

       WARNING: Kernel function symbol table missing [man warning::symbols]

       semantic error: process return probes not available [man error::inode-uprobes]

then SystemTap may have failed to verify support for this feature. You can fix this by following the steps in System.map is missing.

Category:

- Kernel