Kernel panic

Kernel panic inUbuntu 4.10,this one due to a VFS error.

A kernel panic message from aLinuxsystem

AnOpenSolariskernel panic.

Kernel panic inUbuntu 13.04 "Raring Ringtail"(Linux kernel 3.8) inOracle VM VirtualBox

Akernel panic(sometimes abbreviated asKP) is a safety measure taken by anoperating system'skernelupon detecting an internalfatal errorin which either it is unable to safely recover or continuing to run the system would have a higher risk of major data loss. The term is largely specific toUnixandUnix-likesystems. The equivalent onMicrosoft Windowsoperating systems is astop error,often called a "blue screen of death".

Thekernelroutines that handle panics, known aspanic()inAT&T-derived andBSDUnix source code, are generally designed to output anerror messageto theconsole,dump an image of kernel memory to disk forpost-mortem debugging,and then either wait for the system to be manually rebooted, or initiate an automaticreboot.The information provided is of a highly technical nature and aims to assist asystem administratororsoftware developerin diagnosing the problem. Kernel panics can also be caused by errors originating outsidekernel space.For example, many Unix operating systems panic if theinitprocess, which runs inuser space,terminates.

History

TheUnixkernel maintains internal consistency andruntimecorrectness withassertionsas thefault detectionmechanism. The basic assumption is that the hardware and the software should perform correctly and a failure of an assertion results in apanic,i.e. a voluntary halt to all system activity.The kernel panic was introduced in an early version of Unix and demonstrated a major difference between the design philosophies of Unix and its predecessorMultics.Multics developerTom van Vleckrecalls a discussion of this change with Unix developerDennis Ritchie:

I remarked to Dennis that easily half the code I was writing in Multics was error recovery code. He said, "We left all that stuff out. If there's an error, we have this routine called panic, and when it is called, the machine crashes, and you holler down the hall, 'Hey, reboot it.'"

The originalpanic()function was essentially unchanged from Fifth Edition UNIX to theVAX-based UNIX 32V and output only an error message with no other information, then dropped the system into an endless idle loop.

Source code ofpanic()function inV6 UNIX:

/*
* In case console is off,
* panicstr contains argument to last
* call to panic.
*/
char*panicstr;

/*
* Panic is called on unresolvable
* fatal errors.
* It syncs, prints "panic: mesg" and
* then loops.
*/
panic(s)
char*s;
{
panicstr=s;
update();
printf("panic: %s\n",s);
for(;;)
idle();
}

As the Unixcodebasewas enhanced, thepanic()function was also enhanced to dump various forms of debugging information to the console.

Causes

A panic may occur as a result of a hardware failure or asoftware bugin the operating system. In many cases, the operating system is capable of continued operation after an error has occurred. If the system is in an unstable state, rather than risking security breaches and data corruption, the operating system stops in order to prevent further damage, which helps to facilitate diagnosis of the error and may restart automatically.

After recompiling a kernel binary image fromsource code,a kernel panic whilebootingthe resulting kernel is a common problem if the kernel was not correctly configured, compiled or installed.Add-on hardware or malfunctioningRAMcould also be sources of fatal kernel errors during start up, due to incompatibility with the OS or a missingdevice driver.A kernel may also go intopanic()if it is unable to locate aroot file system.During the final stages of kerneluserspaceinitialization, a panic is typically triggered if the spawning ofinitfails. A panic might also be triggered if the init process terminates, as the system would then be unusable.

The following is an implementation of the Linux kernel final initialization inkernel_init():

staticint__refkernel_init(void*unused)
{

...

/*
* We try each of these until one succeeds.
*
* The Bourne shell can be used instead of init if we are
* trying to recover a really broken machine.
*/
if(execute_command){
if(!run_init_process(execute_command))
return0;
pr_err("Failed to execute %s. Attempting defaults...\n",
execute_command);
}
if(!run_init_process("/sbin/init")||
!run_init_process("/etc/init")||
!run_init_process("/bin/init")||
!run_init_process("/bin/sh"))
return0;

panic("No init found. Try passing init= option to kernel."
"See Linux Documentation/init.txt for guidance.");
}

Operating system specifics

Linux

See also:kdump (Linux)

Kernel panic as seen on an iKVM console

Kernel panics appear inLinuxlike in otherUnix-likesystems; however, serious but non-fatal errors can generate another kind of error condition, known as akernel oops.In this case, the kernel normally continues to run afterkillingthe offendingprocess.As an oops could cause some subsystems or resources to become unavailable, they can later lead to a full kernel panic.

On Linux, a kernel panic causes keyboard LEDs to blink as a visual indication of a critical condition.

macOS

When a kernel panic occurs inMac OS X10.2 through 10.7, the computer displays a multilingual message informing the user that they need to reboot the system.Prior to 10.2, a more traditional Unix-style panic message was displayed; in 10.8 and later, the computer automatically reboots and the message is only displayed as a skippable warning afterward. The format of the message varies from version to version:

• 10.0–10.1: The system displays text on the screen, giving details about the error, and becomes unresponsive.
• 10.2: Rolls down a black transparent curtain then displays a message on a white background informing the user that they should restart the computer. The message is shown inEnglish,French,GermanandJapanese.
• 10.3–10.5: Similar to 10.2, but the background of the error message is dark grey.
• 10.6–10.7: The text has been revised and now includes aSpanishtranslation.
• 10.8 and later: The computer becomes unresponsive before it immediately reboots. After restarting, it shows a message for a few seconds informing the user that a problem caused the computer to restart, before continuing to boot. The message now includes aChinesetranslation.

If five new kernel panics occur within three minutes of the first one, the Mac will display aprohibitory signfor thirty seconds, and then shut down; this is known as a "recurring kernel panic".

In all versions above 10.2, the text is superimposed on astandby symboland is not full screen. Debugging information is saved inNVRAMand written to a log file on reboot. In 10.7 there is a feature to automatically restart after a kernel panic. In some cases, on 10.2 and later, white text detailing the error may appear in addition to the standby symbol.

• 
  Mac OS X 10.0–10.1 kernel panic
• 
  Mac OS X 10.2 kernel panic
• 
  Mac OS X 10.3–10.5 kernel panic
• 
  Mac OS X 10.6and 10.7 kernel panic
• 
  Message shown after a system restart due to a kernel panic inOS X 10.8and later versions

See also

• Core dump
• Blue screen of death
• Fatal system error
• Screen of death
• Machine-check exception(MCE)
• Reliability, availability and serviceability(RAS)