Difference between revisions of "Boot debugging"

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[[Category:Boot process]]
#REDIRECT: [[General troubleshooting]]
[[Category:System recovery]]
[[it:Boot Debugging]]
{{Note|Content moved from [[GRUB#Advanced_Debugging]].}}
The kernel provides for a convenient way to configure all sorts of advanced settings to enable you to quickly and conveniently boot into your existing system with varying levels of debugging output [http://www.kernel.org/doc/Documentation/kernel-parameters.txt extended kernel parameters]. It is very easy and useful to create several levels of debugging just by adding additional entries to your bootloader configuration. And if you ever have issues or problems down the road due to a power-failure or hardware failure, it can save you hours of trouble, and of course nothing can beat debugging output when it comes to learning about your system.
== Useful Entries ==
If you are interested in debugging, then you deserve some grub entries for powerusers, here are a few examples that you can add to your bootloader configuration (grub-legacy config {{ic|/boot/grub/menu.lst}} used as an example).
title Shutdown the Computer
title Reboot the Computer
title Command Line
title Install GRUB to hd0 MBR
root (hd0,0)
setup (hd0)
title Matrix
color green/black light-green/green
title Scan for /boot/grub/menu.lst
find --set-root --ignore-floppies /boot/grub/menu.lst
configfile /boot/grub/menu.lst
title Scan for /boot/menu.lst
find --set-root --ignore-floppies /menu.lst
configfile /boot/menu.lst
#make shure, that you place x86test_zImage.bin to /boot/x86test_zImage.bin first
#before uncommenting the following lines, assuming that you have a separate
#boot-partition /boot on (hd0,0)
#title    Run x86test (CPU Info)
#root (hd0,0)
#kernel /x86test_zImage.bin
# http://www.memtest.org/
# make shure you install memtest86+ first before uncommenting
#one of the following lines, assuming that you have a separate
#boot-partition /boot on (hd0,0):
#the easiest way to get memtest86+ is to install the memtest86+
#package from the extra repositories and then edit your /boot/grub/menu.lst
#as pacman tells you to: pacman creates a directory /boot/memtest86+
#with the memtest binary memtest.bin in it. (/boot/memtest86+/memtest.bin).
#After installing you should be fine uncommenting the following three lines.
#title    Run memtest86+ (Memory Testing)
#root (hd0,0)
#kernel /memtest86+/memtest.bin
#altenatively you can download the latest copy 'latest_copy.bin.gz'
#from http://www.memtest.org/, unzip it with gunzip, move it to
#/boot and uncomment the following three lines
#title    Run memtest86+ (Memory Testing)
#root (hd0,0)
#kernel /latest_copy.bin
== Light Debug ==
A quick way to see more verbose messages on your console is to boot up your bootloader entry after appending '''verbose''' to the kernel line.  This simple word added to your kernel line turns on more logging thanks to the {{ic|/etc/rc.sysinit}} file, which at the top of the file runs:
if /bin/grep -q " verbose" /proc/cmdline; then /bin/dmesg -n 8; fi
Very simple way to get a bit more messages and debug output in your logs.
title  Arch Linux DEBUG Light
kernel /vmlinuz-linux root=/dev/disk/by-label/ROOT ro rootwait verbose
initrd /initramfs-linux.img
== Medium Debug ==
This example {{ic|menu.lst}} entry turns on real logging that is set by the kernel and not in an init script.  Adding the '''debug''' kernel parameter to your kernel line is recognized by a lot of linux internals and enables quite a bit of debugging compared to the default.
title Arch Linux DEBUG Medium
kernel /vmlinuz-linux root=/dev/disk/by-label/ROOT ro rootdelay=5 panic=10 debug
initrd /initramfs-linux.img
== Heavy Debug ==
An even more impressive kernel parameter is the '''ignore_loglevel''', which causes the system to ignore any loglevel and keeps the internal loglevel at the maximum debugging level, basically rendering dmesg unable to lower the debug level.
title Arch Linux DEBUG Heavy
kernel /vmlinuz-linux root=/dev/disk/by-label/ROOT ro rootdelay=5 panic=10 debug ignore_loglevel
initrd /initramfs-linux.img
== Extreme Debug ==
If the "Heavy Debug" seemed like a lot of output, that's about 1/2 of the logging that occurs with this example. This does a couple things, it uses the '''earlyprintk''' parameter to setup your kernel for "early" "printing" of messages to your "vga" screen.  The '''keep''' just lets it stay on the screen longer. This will let you see logs that normally are hidden due to the boot-up process.
This also changes the log buffer length to 10MB, and also instructs that any fatal signals be printed with '''print_fatal_signals'''.  The last one, '''sched_debug''', you can look up in the very excellent kernel documentation on [http://www.kernel.org/doc/Documentation/kernel-parameters.txt kernel parameters].
title Arch Linux DEBUG Extreme
kernel /vmlinuz-linux root=/dev/disk/by-label/ROOT ro debug ignore_loglevel log_buf_len=10M print_fatal_signals=1 LOGLEVEL=8 earlyprintk=vga,keep sched_debug
initrd /initramfs-linux.img
== Insane Debug ==
The first few debugging examples showed some really nice kernel parameters to turn on really verbose debugging.  This kind of debugging is absolutely critical if you want to max out your system or just learn more about what is going on behind the scenes.  But there is a final trick that is my favorite, it is the ability to set both environment variables, and more importantly, module parameters at boot.
As an example, here is the boot line that I am using at the moment on an older Dell Desktop, just to illustrate module parameters and environment vars. 
title  Arch Linux X-256
kernel /vmlinuz-linux root=/dev/disk/by-label/ROOT ro rootwait pause_on_oops=5 panic=60 i915.modeset=1 no_console_suspend ipv6.disable=1 TERM=xterm-256color quiet 5
initrd /initramfs-linux.img
Since it is low on both memory and CPU, I disable ipv6. I also turn on kernel modesetting for the i915 video card, set my terminal to be xterm-256color, and boot straight into [[Xorg|X]].  This lets me use a very optimized arch-linux configuration, amazing how fast thanks to using [[SLiM|slim]] as the login manager, [[Ratpoison|ratpoison]] as my [[Display Manager|window manager]], and terminal with [[Tmux|tmux]] as my login shell, all from boot, as the pstree shows (plus [[Synergy]]!).
  |  |-X,3098 -nolisten tcp vt07 -auth /var/run/slim.auth
  |  `-ratpoison,3107,askapache
  |      |-terminal,5341 -x sh -c exec /usr/bin/tmux -2 -l -u -q attach -d -t tmux-askapache
  |      |  |-bash,11165
  |      |  |-tmux,5345 -2 -l -u -q attach -d -t tmux-askapache
  |      |  `-{terminal},5346
  |      `-xscreensaver,3113 -no-splash
  |-synergyc,6121,galileo -f --name galileo-fire --restart
  `-tmux,5348,askapache -2 -l -u -q attach -d -t tmux-askapache
      |  `-ssh,9969 lug@askapache.com
        `-vim,11149 -p sda1/grub/menu.lst /boot/grub/menu.lst
That kind of optimized system is only possible if you first can figure out your system, by debugging both the kernel as previously illustrated, debugging the init process, and most importantly, by debugging the modules enabled for your system's hardware/firmware/software.  Debugging modules is challenging but worth the effort, and then you are able to do some truly insane debugging from grub like the following example, note that the actual grub entry is all on one line, but I split it into 4 lines so you could see it all.  This basically turns on every module on this little Dell desktop to be at the absolute max debug level.  There is so much logging when I boot this that the system grinds to a halt and is slower than a TI-89 calculator (See [[Improve Boot Performance]]).
title  Arch Linux DEBUG INSANE
kernel /vmlinuz-linux root=/dev/disk/by-label/ROOT ro rootwait ignore_loglevel debug debug_locks_verbose=1 sched_debug initcall_debug mminit_loglevel=4 udev.log_priority=8
        loglevel=8 earlyprintk=vga,keep log_buf_len=10M print_fatal_signals=1 apm.debug=Y i8042.debug=Y drm.debug=1 scsi_logging_level=1 usbserial.debug=Y
        option.debug=Y pl2303.debug=Y firewire_ohci.debug=1 hid.debug=1 pci_hotplug.debug=Y pci_hotplug.debug_acpi=Y shpchp.shpchp_debug=Y apic=debug
        show_lapic=all hpet=verbose lmb=debug pause_on_oops=5 panic=10 sysrq_always_enabled
initrd /initramfs-linux.img
A couple key items from that grub entry are '''sysrq_always_enabled''' which forces on the sysrq magic, which really is a lifesaver when debugging at this level as your machine will freeze/stop-responding sometimes and it is nice to use sysrq to kill all tasks, change the loglevel, unmount all filesystems, or do a hard reboot.  Another key parameter is the '''initcall_debug''', which debugs the init process in excruciating detail.  Very useful at times.  The last parametery I find very useful is the '''udev.log_priority=8''' to turn on [[Udev|udev]] logging. 
=== Module Parameters ===
In [[Kernel modules#Parameters]] you can find a nice bash function to be run as root that will show a list of all the loaded modules and all of their parameters, including the current value of the parameter.
=== Break Into Init ===
For instance, If you add '''break=y''' to your kernel cmdline, init will pause early in the [[Arch Boot Process|boot process]] (after loading modules) and launch an interactive sh shell which can be used for troubleshooting purposes. (Normal boot continues after logout.)  This is very similar to the shell that shows up if your computer gets turned off before it is able to shutdown properly.  But using this parameter lets you enter into this mode differently at will.
title  Arch Linux Init Break
kernel /vmlinuz-linux root=/dev/disk/by-label/ROOT ro rootwait break=y
initrd /initramfs-linux.img
=== Debugging init ===
This awesome parameter '''udev.log_priority=8''' does the same thing as editing the file {{ic|/etc/udev/udev.conf}} except it executes earlier, turning on debugging output for [[Udev|udev]].  If you want to know your hardware, that is the key parameter right there.  Another trick is if you change the {{ic|/etc/udev/udev.conf}} to be verbose, then you can make your initrd image include that file to turn on verbose udeb debugging by adding it to your {{ic|/etc/mkinitcpio.conf}} like:
FILES="/etc/modprobe.d/modprobe.conf /etc/udev/udev.conf"
, which on arch is as easy as
# mkinitcpio -p linux
Debugging [[Udev|udev]] is key because the [[Initrd|initrd]] performs a [[Change Root|root change]] at the end of its run to usually launch a program like /sbin/init as part of a chroot, and unless the new file system has a valid /dev directory, udev must be initialized before invoking chroot in order to provide {{ic|/dev/console}}. 
exec chroot . /sbin/init <dev/console >dev/console 2>&1
So basically, you are not able to view the logs that are generated before /dev/console is initialized by udev or by a special initrd you compiled yourself.  One method the kernel developers use to be able to still get the log messages generated before /dev/console is available is to provide an alternative console that you can enable or disable from grub.
=== Net Console ===
If you read through the [http://lxr.linux.no/linux+v2.6.32/Documentation/networking/netconsole.txt kernel documentation regarding debugging], you will hear about [[Netconsole|Netconsole]], which can be loaded from the kernel line in GRUB, compiled into your kernel, or loaded at runtime as a module.  Having a netconsole entry in your {{ic|menu.lst}} is most excellent for debugging slower computers like old laptops or thin-clients.  It is easy to use.  Just setup a 2nd computer (running arch) to accept syslog requests on a remote port, very fast and quick to do on arch-linux, 1 line to syslog.conf.  Then you could use a log-color-parser like ccze to view all syslog logs, or just tail your everything.log. Then on your laptop, boot up and select the netconsole entry from the grub menu, and you will start seeing as much logging as you want on your syslog system.  This logging lets you view even earlier log output than is available with the earlyprintk=vga kernel parameter, as netconsole is used by kernel hackers and developers, so it is very powerful.
title  Arch Linux DEBUG Netconsole
kernel /vmlinuz-linux root=/dev/disk/by-label/ROOT ro netconsole=514@ debug ignore_loglevel
initrd /initramfs-linux.img
=== Hijacking cmdline ===
If you do not have access to GRUB or the kernel boottime cmdline, like on a server or virtual machine, as long as you have root permissions you can still enable this kind of simplistic verbose logging using a neat hack.  While you cannot modify the {{ic|/proc/cmdline}} even as root, you can place your own cmdline file on top of /proc/cmdline, so that accessing /proc/cmdline actually accesses your file.
For example if I '''cat /proc/cmdline''', I have the following:
root=/dev/disk/by-label/ROOT ro console=tty1 logo.nologo quiet
So I use a simple sed command to replace '''quiet''' with '''verbose''' like:
sed 's/ quiet/ verbose/' /proc/cmdline > /root/cmdline
Then I bind mount /root/cmdline so that it becomes /proc/cmdline, using the '''-n''' option to mount so that this mount will not be recorded in the systems mtab.
mount -n --bind -o ro /root/cmdline /proc/cmdline
Now if I '''cat /proc/cmdline''', I have the following:
root=/dev/disk/by-label/ROOT ro console=tty1 logo.nologo verbose
== Troubleshooting ==
=== Repairing with Arch live-cd ===
In case grub is unable to boot your kernel, or if your initramfs is broken, you can boot into a safe system using an [https://www.archlinux.org/download/ Arch live-cd].  Once finished with repairs, unmount the broken system and reboot.
==== Mounting and Chrooting broken system ====
Once booted and at a console prompt, use the following to mount and repair your broken system (where {{ic|/dev/sda3}} is {{ic|/}} and {{ic|/dev/sda1}} is {{ic|/boot}}):
First create the mount-point and mount your root {{ic|/}} filesystem to it, then cd into it.
# mkdir /mnt/arch
# mount /dev/sda3 /mnt/arch
# cd /mnt/arch
Now create the proc, sysfs, and dev filesystems
# mount -t proc proc proc/
# mount -t sysfs sys sys/
# mount -o bind /dev dev/
Next mount the boot partition if you use one.
# mount /dev/sda1 boot/
Finally chroot into {{ic|/mnt/arch}} which will become {{ic|/}}.
# chroot .
Turn on networking
==== Reinstalling with Pacman ====
The author uses pacman in a chrooted broken system to reinstall the kernel, grub, initramfs, udev, and any other packages that may be broken and/or needed to get the system up and running.
This will reinstall the kernel and initramfs so check that {{ic|/etc/mkinitcpio.conf}} is correct or remove the file entirely and re-install mkinitcpio.
# pacman -Syyu mkinitcpio linux udev
Afterwards, unmount and reboot.
==See Also==
* [[Netconsole]]
* [[Syslinux]]
* [[GRUB2]]
* [[GRUB Legacy]]
* [[Kernel modules]]
* [[mkinitcpio]]
* [[Fstab]]
* [[Kernel Mode Setting]]
* [[LILO]]
* [[GUID Partition Table]]
* [[Systemd]]
==External Links==
* [http://www.memtest.org/ Memtest ]
* [http://wiki.ultimatebootcd.com/index.php?title=Tools List of Tools for UBCD] - Can be added to custom menu.lst like memtest
* Official GRUB2 Manual - https://www.gnu.org/software/grub/manual/grub.html
* Ubuntu wiki page for GRUB2 - https://help.ubuntu.com/community/Grub2
* GRUB2 wiki page describing steps to compile for UEFI systems - https://help.ubuntu.com/community/UEFIBooting
* Wikipedia's page on [[Wikipedia:BIOS Boot partition|BIOS Boot partition]]

Latest revision as of 21:35, 8 August 2016