A quick overview of Linux kernel crash dump analysis

The Red Hat Crash Utility is a kernel-specific debugger. It is usually used for performing postmortem system analysis when the system panicked, locked up, or appears unresponsive. In this short article, Eugene Teo will give a quick overview of how you can install crash and how you can use it to get important information from the crash dump files for debugging and root-cause analysis purposes.

Prerequisites

The crash utility has the following three prerequisites:

  • Kernel object file: A vmlinux kernel object file. The vmlinux file associated with the running kernel is typically found in the /boot directory for Red Hat Enterprise Linux 3 and /usr/lib/debug/lib/modules/ directory for both Red Hat Enterprise Linux 4 and 5.
  • Kernel crash dump: This may consist of a kernel crash dump file generated from any of the three crash dump facilities (Diskdump, Netdump, or Kdump). The filename is called vmcore or vmcore.incomplete (if it is not generated completely), and is typically found in /var/crash/ by default. Diskdump will be discussed in the next section.
  • Linux kernel versions: The crash utility is backwards-compatible to at least Red Hat Linux 6.0, up to Red Hat Enterprise Linux 5.

Install crash

Starting with the Red Hat Enterprise Linux 3 release, the crash utility is automatically installed during the system installation if the Development Tools package set is selected. If the crash utility is not installed, download and install the binary RPM as follows:

# rpm -ivh crash-4.0-2.30.i386.rpm

Preparing…                ########################################### [100%]

1:crash                  ########################################### [100%]

The crash executable will be installed in the /usr/bin directory.

Also, before you can invoke crash on a vmcore, you need to install the associated kernel debuginfo package. The vmlinux kernel debug information is stored in a separate debuginfo file. The debuginfo package needs to match the kernel version, variant (like “smp” or “hugemem”) and architecture. You can download the packages at ftp://ftp.redhat.com/pub. See the comments in the following example:

# file ./vmcore <– will show you the kernel architecture

./vmcore: ELF 32-bit LSB core file Intel 80386, version 1 (SYSV), SVR4-style, from ‘vmlinux’

# strings vmcore | fgrep -m1 ‘Linux ‘ <– will show you the kernel variant

Linux version 2.6.9-22.EL (bhcompile@porky.build.redhat.com) (gcc version 3.4.4

20050721 (Red Hat 3.4.4-2)) #1 Mon Sep 19 18:20:28 EDT 2005

# rpm -qa kernel-debuginfo <– will show you all the versions installed

# rpm -ivh kernel-debuginfo-2.6.9-22.EL.i686.rpm

Preparing…                ########################################### [100%]

1:kernel-debuginfo       ########################################### [100%]

# ls /usr/lib/debug/lib/modules/ -l

total 24

drwxr-xr-x 3 root root 4096 May 2 10:41 2.6.9-22.EL

drwxr-xr-x 3 root root 4096 May 2 10:41 2.6.9-22.ELhugemem

drwxr-xr-x 3 root root 4096 May 2 10:41 2.6.9-22.ELsmp

# ls /usr/lib/debug/lib/modules/2.6.9-22.EL -l

total 32848

drwxr-xr-x 9 root root     4096 May 1 19:50 kernel

-rwxr-xr-x 1 root root 33583473 Sep 20 2005 vmlinux

Take note that:

  • You should use -ivh rather than -Uvh when installing the kernel package. This will preserve the older version of the kernel installed so that you can revert back to a known working version of the kernel should you encounter any problems with the new version.
  • The kernel-debuginfo package for an older kernel can safely remain installed when installing a newer version. The kernel-debuginfo must match the kernel version, variant, and architecture that created the vmcore. See the file ./vmcore and strings vmcore | fgrep -m1 ‘Linux ‘ commands in the above output.
  • In Red Hat Enterprise Linux 5, the vmlinux kernel-debuginfo package is divided into two packages: kernel-debuginfo-version.arch.rpm and kernel-debuginfo-common-version.arch.rpm. Both are required in order to perform crash dump analysis on the Red Hat Enterprise Linux 5 kernels.

Run crash

When crash is run on a vmcore, at least two arguments are always required:
<ul

  • The vmlinux file associated with the running kernel, typically found in /usr/lib/debug/lib/modules/ directory.
  • The kernel crash dump name, vmcore.

For example:

# crash /usr/lib/debug/lib/module/vmlinux

/var/crash/127.0.0.1-2007-04-30-21:38/vmcore

[…]

KERNEL: /usr/lib/debug/lib/modules/2.6.9-22.EL/vmlinux

DUMPFILE: /home/eteo/crash/127.0.0.1-2007-04-30-21:38/vmcore

CPUS: 1

DATE: Mon Apr 30 21:38:40 2007

UPTIME: 00:04:04

LOAD AVERAGE: 0.36, 0.23, 0.08

TASKS: 36

NODENAME: localhost.localdomain

RELEASE: 2.6.9-22.EL

VERSION: #1 Mon Sep 19 18:20:28 EDT 2005

MACHINE: i686 (1862 Mhz)

MEMORY: 1 GB

PANIC: “Oops: 0002 [#1]” (check log for details)

PID: 2857

COMMAND: “bash”

TASK: f7b677f0 [THREAD_INFO: f7191000]

CPU: 0

STATE: TASK_RUNNING (SYSRQ)

crash>

Setup Diskdump

Diskdump is one of the two different crash dump facilities that we shipped with Red Hat Enterprise Linux 3 and 4. This article will not cover Netdump or Kdump.

Before you beginning setting up Diskdump on your machine, do read /usr/share/doc/diskdumputils-version/README to make sure that your machine has a supported storage adapter before proceeding.

Assign a disk device to dump memory. It may be:

  • a full disk device (for Red Hat Enterprise Linux 3 only), e.g. /dev/sda
  • a partition of a disk device, e.g. /dev/sda2
  • a swap partition (for Red Hat Enterprise Linux 4 only), e.g. /dev/sda2

Define the disk device in /etc/sysconfig/diskdump. In this example, we will use /dev/sda2:

# vi /etc/sysconfig/diskdump

add the line “DEVICE=/dev/sda2”

Load the kernel module:

# tail -f /var/log/message &

# modprobe diskdump

Apr 30 21:29:20 kerndev kernel: disk_dump: Maximum block size: 16384

Apr 30 21:29:20 kerndev kernel: disk_dump: total blocks required: 261770

(header 3 + bitmap 8 + memory 261759)

See /proc/diskdump after loading diskdump kernel module:

# cat /proc/diskdump

# sample_rate: 8

# block_order: 2

# fallback_on_err: 1

# allow_risky_dumps: 1

# dump_level: 0

# total_blocks: 261770

#

Format the diskdump device:

# service diskdump initialformat

/dev/sda2: [100.0%]

See /proc/diskdump after formatting:

# tail -n2 /proc/diskdump

sda2 102398310 10233405

Enable Diskdump service:

# chkconfig diskdump on

# service diskdump start

Starting diskdump:                            [ OK ]

# Apr 30 21:31:19 kerndev diskdump: activating succeeded

Test that Diskdump works. The following commands will crash your machine:

# echo 1 > /proc/sys/kernel/sysrq

# echo c > /proc/sysrq-trigger

Make sure that you run the above two commands in console (press Ctrl + Alt + F1), so that we can see what is happening when your system crashes. You have to perform this so that you can have a vmcore file to follow the rest of the paper. It will be located at /var/crash.

Commonly Used Crash Commands

There are many commands in crash. It is also possible to extend crash by adding new commands, by writing new code and compiling it into the crash executable, or creating a shared object library that can be dynamically loaded by using the extend command. The following are some commonly used crash commands that you will likely use:

  • help – get help
    crash has a readily available help information built into the utility, by typing help. Each command has its own man-like page, which can be viewed by typing help command-name.
  • crash> help
  • *      files   mod    runq   union
  • alias  foreach mount  search vm
  • ascii  fuser   net    set    vtop
  • bt     gdb     p      sig    waitq
  • btop   help    ps     struct whatis
  • dev    irq     pte    swap   wr
  • dis    kmem    ptob   sym    q
  • eval   list    ptov   sys
  • exit   log     rd     task
  • extend mach    repeat timer
  • crash version: 4.0-3.3   gdb version: 6.1
  • For help on any command above, enter “help “.
  • For help on input options, enter “help input”.

For help on output options, enter “help output”.

Tip: all the crash commands can be piped to external programs or redirected to files:

crash> log > log.txt

This will send the in-kernel log to a local file called log.txt.

crash> ps | fgrep bash | wc -l

This will count the number of bash tasks that were running.

  • sys – system data
  • crash> sys
  •        KERNEL: /usr/lib/debug/lib/modules/2.6.9-22.EL/vmlinux
  •      DUMPFILE: /home/eteo/crash/127.0.0.1-2007-04-30-21:38/vmcore
  •          CPUS: 1
  •          DATE: Mon Apr 30 21:38:40 2007
  •        UPTIME: 00:04:04
  •  LOAD AVERAGE:  0.36, 0.23, 0.08
  •         TASKS: 36
  •      NODENAME: localhost.localdomain
  •       RELEASE: 2.6.9-22.EL
  •       VERSION: #1 Mon Sep 19 18:20:28 EDT 2005
  •       MACHINE: i686 (1862 Mhz)
  •        MEMORY: 1 GB

PANIC: “Oops: 0002 [#1]” (check log for details)

The sys messages have information of the system (e.g. kernel release, kernel version, number of CPUs, amount of memory, etc), the time of vmcore taken, the operating period, and the panic (e.g. oops type, panic task/PID/command, etc).

  • bt – backtrace
  • crash> bt
  • PID: 2857   TASK: f7b677f0 CPU: 0    COMMAND: “bash”
  •  #0 [f7191e04] start_disk_dump at f89d7bb3
  •  #1 [f7191e18] die at c010682e
  •  #2 [f7191e48] do_page_fault at c011ab00
  • […]
  •  #9 [f7191fc0] system_call at c030f918
  •     EAX: 00000004 EBX: 00000001 ECX: b7de7000 EDX: 00000002
  •     DS: 007b       ESI: 00000002 ES: 007b        EDI: b7de7000
  •     SS: 007b       ESP: bfe01650 EBP: bfe01670

CS: 0073       EIP: 003297a2 ERR: 00000004 EFLAGS: 00000246

  • log – dump system message buffer
  • crash> log
  • […]
  • SysRq : Crashing the kernel by request
  • Unable to handle kernel NULL pointer dereference at virtual address 00000000
  •  printing eip:
  • c0233fa7
  • *pde = 3e9f3067
  • Oops: 0002 [#1]
  • Modules linked in: md5 ipv6 autofs4 i2c_dev i2c_core sunrpc scsi_dump diskdump dm_mirror dm_mod button battery ac yenta_socket pcmcia_core uhci_hcd ehci_hcd shpchp snd_intel8x0 snd_ac97_codec snd_pcm_oss snd_mixer_oss snd_pcm snd_timer snd_page_alloc snd_mpu401_uart snd_rawmidi snd_seq_device snd soundcore ipw2200 ieee80211 ieee80211_crypt tg3 floppy ext3 jbd ata_piix libata sd_mod scsi_mod
  • CPU:    0
  • EIP:    0060:[]    Not tainted VLI
  • EFLAGS: 00010246    (2.6.9-22.EL)
  • EIP is at sysrq_handle_crash+0x0/0x8
  • eax: 00000063   ebx: c0370db4    ecx: 00000000  edx: 00000000
  • esi: 00000063   edi: 00000000    ebp: 00000000  esp: f7191f60
  • ds: 007b   es: 007b    ss: 0068
  • Process bash (pid: 2857, threadinfo=f7191000 task=f7b677f0)
  • Stack: c02342d8 c032dc4e c032f105 00000003 00000002 f7b6adc0 00000002
  •        f7191fac c01a8a13 c0362740 c0168205 f7191fac b7de7000 f7b6adc0
  •        fffffff7 b7de7000 f7191000 c01682cf f7191fac 00000000 00000000
  •        00000000 00000001 00000002
  • Call Trace:
  •  [] __handle_sysrq+0x58/0xc6
  •  [] write_sysrq_trigger+0x23/0x29
  •  [] vfs_write+0xb6/0xe2
  •  [] sys_write+0x3c/0x62
  •  [] syscall_call+0x7/0xb

Code: 4c 11 42 c0 05 00 00 00 c7 05 50 11 42 c0 2f cc 31 c0 c7 05 54 11 42 c0 00 00 00 00 c7 05 58 11 42 c0 00 00 00 00 e9 e5 0b f0 ff  05 00 00 00 00 00 c3 e9 e1 59 f3 ff e9 1e bc f3 ff 85 d2 89

The log command dumps the kernel log buffer contents in chronological order. This is similar to what you would see when you type dmesg on a running machine. This is useful when you want to look at the panic or oops message. An oops is triggered by some exception. It is a dump of the CPU register’s state and kernel stack at that instant. From the panic message, we can find hints as to how the panic was triggered (e.g. the function or process or pid or command or address that triggered the panic), the register’s information, kernel module list, whether the kernel is tainted with proprietary kernel modules loaded, and so on. Let’s walk through the panic message to see what we can learn from it. See the comments below each section within the log:

crash> log

[…]

SysRq : Crashing the kernel by request <– this panic is intentional

Unable to handle kernel NULL pointer dereference at virtual address 00000000

This is the address to which reference was attempted.

printing eip:

c0233fa7

This is the address at which the failure occurred.

*pde = 3e9f3067

Oops: 0002 [#1]

Often one oops will trigger more; only the first is reliable.

Modules linked in: md5 ipv6 autofs4 i2c_dev i2c_core sunrpc scsi_dump diskdump dm_mirror dm_mod button battery ac yenta_socket pcmcia_core uhci_hcd ehci_hcd shpchp snd_intel8x0 snd_ac97_codec snd_pcm_oss snd_mixer_oss snd_pcm snd_timer snd_page_alloc snd_mpu401_uart snd_rawmidi snd_seq_device snd soundcore ipw2200 ieee80211 ieee80211_crypt tg3 floppy ext3 jbd ata_piix libata sd_mod scsi_mod

CPU: 0

EIP: 0060:[]    Not tainted VLI

The first part is the code segment and instruction address. If tainted, it will be followed by:

G – All modules loaded have a GPL

or compatible license

P – Proprietary modules loaded

F – Module forcibly loaded

S – Oops on hardware that are not

SMP capable

R – Module forcibly unloaded

M – Machine Check Exception (MCE)

occurred

etc (see Further readings section).

EFLAGS: 00010246    (2.6.9-22.EL)

This line denotes program status, registers information.
<preMsoNormal” style=”mso-margin-top-alt:auto;mso-margin-bottom-alt:auto; margin-left:.5in;line-height:normal”>These are the stack, operations, return addresses.

f7191fac f7191000 c01682cf f7191fac 00000000 00000000 00000000

00000001 00000002

Call Trace:

This is the backtrace of function calls.

[] __handle_sysrq+0x58/0xc6

[] write_sysrq_trigger+0x23/0x29

[] vfs_write+0xb6/0xe2

[] sys_write+0x3c/0x62

[] syscall_call+0x7/0xb

Code: 4c 11 42 c0 05 00 00 00 c7 05 50 11 42 c0 2f cc 31 c0 c7 05 54 11 42 c0 00 00 00 00 c7 05 58 11 42 c0 00 00 00 00 e9 e5 0b f0 ff  05 00 00 00 00 00 c3 e9 e1 59 f3 ff e9 1e bc f3 ff 85 d2 89

From the line c0233fa7, we can see that this is the address at which the failure occurred. Issuing the following command can give us more hints as to which function or source code or assembly statement in the kernel triggered that:

crash> dis -lr c0233fa7

/usr/src/build/614745-i686/BUILD/kernel-2.6.9/linux-

2.6.9/drivers/char/sysrq.c: 115

0xc0233fa7 :        movb   $0x0,0x0

  • ps – display process status information
  • crash> ps
  •    PID    PPID CPU   TASK   ST %MEM  VSZ  RSS COMM
  •       0      0  0 c0358be0  RU  0.0    0    0 [swapper]
  •       1      0  0 f7e01770  IN  0.1 1680  684 init
  • […]
  •    2380      1  0  f7ac2800 IN  0.0 1604  504 mingetty
  •    2769   2371  0  f7ac3970 IN  0.2 5740 1636 bash
  •    2852      1  0  f7b1a880 IN  0.2 4240 2012 sshd
  •   2855 2852 0 f7b66680 IN 0.3 8316 2756 sshd
  • > 2857 2855 0 f7b677f0 RU 0.2 6260 1628 bash
  • Sometimes it is useful to know which process belongs to which parent or vice versa. ps has -c and -p to show the child and parent processes.
  • crash> ps -p 2857
  • PID: 0       TASK: c0358be0 CPU: 0    COMMAND: “swapper”
  •  PID: 1       TASK: f7e01770 CPU: 0    COMMAND: “init”
  •   PID: 2852    TASK: f7b1a880 CPU: 0    COMMAND: “sshd”
  •    PID: 2855    TASK: f7b66680 CPU: 0    COMMAND: “sshd”

PID: 2857    TASK: f7b677f0 CPU: 0    COMMAND: “bash”

 

  • files – open files
  • crash> files
  • PID: 2857    TASK: f7b677f0 CPU: 0   COMMAND: “bash”
  • ROOT: /     CWD: /root
  •  FD    FILE      DENTRY    INODE   TYPE  PATH
  •   0 f7a6e7c0 f7790198 f7b0fdcc     CHR   /dev/pts/0
  •   1 f7b6adc0 f7190130 f7b9ca4c     REG   /proc/sysrq-trigger
  •   2 f7a6e7c0 f7790198 f7b0fdcc     CHR   /dev/pts/0
  •  10 f7a6e7c0 f7790198 f7b0fdcc     CHR   /dev/pts/0
  • 255 f7a6e7c0 f7790198 f7b0fdcc     CHR   /dev/pts/0
  • crash> files 2852
  • PID: 2852    TASK: f7b1a880 CPU: 0   COMMAND: “sshd”
  • ROOT: /     CWD: /
  •  FD    FILE      DENTRY    INODE   TYPE  PATH
  •   0 f7b336c0 f78001d8 f7cb1ba4     CHR   /dev/null
  •   1 f7b336c0 f78001d8 f7cb1ba4     CHR   /dev/null
  •   2 f7b336c0 f78001d8 f7cb1ba4     CHR   /dev/null

3 f7b69600 f7bf5280 f7aadafc     SOCK  socket:/[6277]

  • dev – device data
  • crash> help dev
  • […]
  •  If no argument is entered, this command dumps the contents of the
  •  chrdevs and blkdevs arrays.
  • crash> dev
  • CHRDEV    NAME         OPERATIONS
  •  1      mem              (none)
  •  4      /dev/vc/0        (none)
  •  4      tty              (none)
  • […]
  • BLKDEV     NAME        OPERATIONS
  •  1      ramdisk        c0376d08
  •  2      fd            (unknown)

8      sd             f880e070

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