crash man page

crash — Analyze Linux crash dump data or a live system

Synopsis

crash [OPTION]... NAMELIST MEMORY-IMAGE[@ADDRESS]    (dumpfile form)
crash [OPTION]... [NAMELIST]                         (live system form)

Description

Crash is a tool for interactively analyzing the state of the Linux system while it is running, or after a kernel crash has occurred and a  core dump has been created by the netdump, diskdump, LKCD, kdump, xendump or kvmdump facilities.  It is loosely based on the SVR4 UNIX crash  command, but has been significantly enhanced by completely merging it with the gdb(1) debugger. The marriage of the two effectively combines the  kernel-specific nature of the traditional UNIX crash utility with the  source code level debugging capabilities of gdb(1).

In the dumpfile form, both a NAMELIST and a MEMORY-IMAGE argument must be entered. In the live system form, the NAMELIST argument must be entered if  the kernel's vmlinux file is not located in a known location, such as the /usr/lib/debug/lib/modules/<kernel-version> directory.  

The crash utility has also been extended to support the analysis of dumpfiles generated by a crash of the Xen hypervisor.  In that case, the NAMELIST argument must be that of the xen-syms binary. Live system analysis is not supported for the Xen hypervisor.

The crash utility command set consists of common kernel core analysis tools  such as kernel stack back traces of all processes, source code disassembly, formatted kernel structure and variable displays, virtual memory data,  dumps of linked-lists, etc., along with several commands that delve  deeper into specific kernel subsystems.  Appropriate gdb commands may also be entered, which in turn are passed on to the gdb module for execution.  If desired, commands may be placed in either a $HOME/.crashrc file and/or in a .crashrc file in the current directory. During initialization, the commands in $HOME/.crashrc are executed first, followed by those in the ./.crashrc file.

The crash utility is designed to be independent of Linux version  dependencies. When new kernel source code impacts the correct functionality of crash and its command set, the utility will  be updated to recognize new kernel code changes, while maintaining backwards compatibility with earlier releases.

Options

NAMELIST

This is a pathname to an uncompressed kernel image (a vmlinux file), or a Xen hypervisor image (a xen-syms file)  which has been compiled with the "-g" option. If using the dumpfile form, a vmlinux file may be compressed in either gzip or bzip2 formats.

MEMORY-IMAGE[@ADDRESS]

A kernel core dump file created by the netdump, diskdump, LKCD kdump, xendump or kvmdump facilities.  

If a MEMORY-IMAGE argument is not entered, the session will be invoked on the live system, which typically requires root privileges because of the device file used to access system RAM.  By default, /dev/crash will be used if it exists.  If it does not exist, then /dev/mem will be used; but if the kernel has been configured  with CONFIG_STRICT_DEVMEM, then /proc/kcore will be used. It is permissible to explicitly enter /dev/crash, /dev/mem or /proc/kcore.

An @ADDRESS value must be appended to the MEMORY-IMAGE if the dumpfile is a raw RAM dumpfile that has no header information describing the file contents.  Multiple MEMORY-IMAGE@ADDRESS ordered pairs may be entered, with each dumpfile containing a contiguous block of RAM, where the ADDRESS value is the physical start address of the block expressed in hexadecimal. The physical address value(s) will be used to create a temporary ELF header in /var/tmp, which will only exist during the crash session.  If a raw RAM dumpile represents a live memory source, such as that specified by the QEMU mem-path argument of a memory-backend-file object, then "live:" must be prepended to the MEMORY-IMAGE name.

mapfile

If the NAMELIST file is not the same kernel that is running (live system form), or the kernel that was running  when the system crashed (dumpfile form), then the System.map file of the original kernel should be entered on the command line.

-h [option]
--help [option]

Without an option argument, display a crash usage help message.  If the option argument is a crash command name, the help page for that command is displayed.  If it is the string "input", a page describing the various crash command line input options is displayed.  If it is the string "output", a page describing command line output options is displayed.   If it is the string "all", then all of the possible help messages are displayed.  After the help message is displayed, crash exits.

-s

Silently proceed directly to the "crash>" prompt without displaying any version, GPL, or crash initialization data during startup, and by default, runtime command output is not passed to any scrolling command.

-i file

Execute the command(s) contained in file prior to displaying the "crash>" prompt for interactive user input.

-d num

Set the internal debug level. The higher the number, the more debugging data will be printed when crash initializes and runs.

-S

Use /boot/System.map as the mapfile.

-e vi | emacs

Set the readline(3) command line editing mode to "vi" or "emacs".  The default editing mode is "vi".

-f

Force the usage of a compressed vmlinux file if its original name does not start with "vmlinux".

-k

Indicate that the NAMELIST file is an LKCD "Kerntypes" debuginfo file.

-g [namelist]

Determine if a   vmlinux or xen-syms namelist file contains debugging data.

-t

Display the system-crash timestamp and exit.

-L

Attempt to lock all of its virtual address space into memory by calling mlockall(MCL_CURRENT|MCL_FUTURE)  during initialization.  If the system call fails, an error message will be displayed, but the session continues.

-c tty-device

Open the tty-device as the console used for debug messages.

-p page-size

If a processor's page size cannot be determined by the dumpfile, and the processor default cannot be used, use page-size.

-o filename

Only used with the MEMORY-IMAGE@ADDRESS format for raw RAM dumpfiles,  specifies a filename of a new ELF vmcore that will be created and used as the dumpfile.  It will be saved to allow future use as a standalone vmcore, replacing the original raw RAM dumpfile.

-m option=value
--machdep option=value

Pass an option and value pair to machine-dependent code.  These architecture-specific option/pairs should only be required in very rare circumstances:

X86_64:
  phys_base=<physical-address>
  irq_eframe_link=<value>
  max_physmem_bits=<value>
  kernel_image_size=<value>
  vm=orig       (pre-2.6.11 virtual memory address ranges)
  vm=2.6.11     (2.6.11 and later virtual memory address ranges)
  vm=xen        (Xen kernel virtual memory address ranges)
  vm=xen-rhel4  (RHEL4 Xen kernel virtual address ranges)
PPC64:
  vm=orig
  vm=2.6.14     (4-level page tables)
IA64:
  phys_start=<physical-address>
  init_stack_size=<size>
  vm=4l         (4-level page tables)
ARM:  
  phys_base=<physical-address>
ARM64:  
  phys_offset=<physical-address>
  kimage_voffset=<kimage_voffset-value>
X86:
  page_offset=<CONFIG_PAGE_OFFSET-value>
-x

Automatically load extension modules from a particular directory.  If a directory is specified in the CRASH_EXTENSIONS shell environment variable, then that directory will be used.  Otherwise /usr/lib64/crash/extensions (64-bit architectures) or /usr/lib/crash/extensions (32-bit architectures) will be used; if they do not exist, then the ./extensions directory will be used.

--active

Track only the active task on each cpu.

--buildinfo

Display the crash binary's build date, the user ID of the builder, the hostname of the machine where the build was done, the target  architecture, the version number, and the compiler version.

--memory_module modname

Use the modname as an alternative kernel module to the crash.ko module that creates the /dev/crash device.

--memory_device device

Use device as an alternative device to the /dev/crash, /dev/mem or /proc/kcore devices.

--log dumpfile

Dump the contents of the kernel log buffer.  A kernel namelist argument is not necessary, but the dumpfile must contain the VMCOREINFO data taken from the original /proc/vmcore ELF header.

--no_kallsyms

Do not use kallsyms-generated symbol information contained within  kernel module object files.

--no_modules

Do not access or display any kernel module related information.

--no_ikconf

Do not attempt to read configuration data that was built into kernels configured with CONFIG_IKCONFIG.

--no_data_debug

Do not verify the validity of all structure member offsets and structure  sizes that it uses.

--no_kmem_cache

Do not initialize the kernel's slab cache infrastructure, and commands that use kmem_cache-related data will not work.

--no_elf_notes

Do not use the registers from the ELF NT_PRSTATUS notes saved in a compressed kdump header for backtraces.

--kmem_cache_delay

Delay the initialization of the kernel's slab cache infrastructure until it is required by a run-time command.

--readnow

Pass this flag to the embedded gdb module, which will override its two-stage strategy that it uses for reading symbol tables from the NAMELIST.

--smp

Specify that the system being analyzed is an SMP kernel.

-v
--version

Display the version of the crash utility, the version of the embedded gdb module, GPL information, and copyright notices.

--cpus number

Specify the number of cpus in the SMP system being analyzed.

--osrelease dumpfile

Display the OSRELEASE vmcoreinfo string from a kdump dumpfile header.

--hyper

Force the session to be that of a Xen hypervisor.

--p2m_mfn pfn

When a Xen Hypervisor or its dom0 kernel crashes, the dumpfile is typically analyzed with either the Xen hypervisor or the dom0 kernel. It is also possible to analyze any of the guest domU kernels if  the pfn_to_mfn_list_list pfn value of the guest kernel is passed on the command line along with its NAMELIST and the   dumpfile.

--xen_phys_start physical-address

Supply the base physical address of the Xen hypervisor's text and static data for older xendump dumpfiles that did not pass that information in the dumpfile header.

--zero_excluded

If the makedumpfile(8) facility has filtered a compressed kdump dumpfile to exclude various types of non-essential pages, or has marked a compressed or ELF kdump dumpfile as incomplete due to an ENOSPC or other error during its creation, any attempt to read missing pages will fail.  With this flag, reads from any of those pages will return zero-filled memory.

--no_panic

Do not attempt to find the task that was running when the kernel crashed. Set the initial context to that of the "swapper" task on cpu 0.

--more

Use /bin/more as the command output scroller, overriding the default of /usr/bin/less and any settings in either ./.crashrc or $HOME/.crashrc.

--less

Use /usr/bin/less as the command output scroller, overriding  any settings in either ./.crashrc or $HOME/.crashrc.

--hex

Set the default command output radix to 16, overriding the default radix of 10, and any radix settings in either ./.crashrc or $HOME/.crashrc.

--dec

Set the default command output radix to 10, overriding any  radix settings in either ./.crashrc or $HOME/.crashrc. This is the default radix setting.

--CRASHPAGER

Use the output paging command defined in the CRASHPAGER shell environment variable, overriding any settings in either ./.crashrc or $HOME/.crashrc.

--no_scroll

Do not pass run-time command output to any scrolling command.

--no_strip

Do not strip cloned kernel text symbol names.

--no_crashrc

Do not execute the commands in either $HOME/.crashrc or ./.crashrc.

--mod directory

When loading the debuginfo data of kernel modules with the mod -S command, search for their object files in directory instead of in the standard location.

--kaslr offset|auto

If an x86_64 kernel was configured with CONFIG_RANDOMIZE_BASE, the offset value is equal to the difference between the symbol values  compiled into the vmlinux file and their relocated KASLR values.  If set to auto, the KASLR offset value will be automatically calculated.

--reloc size

When analyzing live x86 kernels that were configured with a CONFIG_PHYSICAL_START value that is larger than its CONFIG_PHYSICAL_ALIGN value, then it will be necessary to enter a relocation size equal to the difference between the two values.

--hash count

Set the number of internal hash queue heads used for list gathering and verification.  The default count is 32768.

--minimal

Bring up a session that is restricted to the log, dis, rd, sym, eval, set and exit commands.  This option may provide a way to extract some minimal/quick information from a corrupted or truncated dumpfile, or in situations where one of the several kernel subsystem initialization routines would abort the crash session.

--kvmhost [32|64]

When examining an x86 KVM guest dumpfile, this option specifies that the KVM host that created the dumpfile was an x86 (32-bit)  or an x86_64 (64-bit) machine, overriding the automatically  determined value.

--kvmio <size>

override the automatically-calculated KVM guest I/O hole size.

--offline [show|hide]

Show or hide command output that is related to offline cpus.  The default setting is show.

Commands

Each crash command generally falls into one of the following categories:

Symbolic display

Displays of kernel text/data, which take full advantage of the power of gdb to format and display data structures symbolically.

System state

The majority of crash commands consist of a set of "kernel-aware"  commands, which delve into various kernel subsystems on a system-wide  or per-task basis.

Utility functions

A set of useful helper commands serving various purposes, some simple,  others quite powerful.

Session control

Commands that control the crash session itself.

The following alphabetical list consists of a very simple overview of each crash command. However, since individual commands often have several options resulting in  significantly different output, it is suggested that the full description of each command be viewed by executing crash -h <command>, or during a crash session by simply entering help command.

*

"pointer to" is shorthand for either the struct or union commands.  It displays the contents of a kernel structure or union.

alias

creates a single-word alias for a command.

ascii

displays an ascii chart or translates a numeric value into its ascii components.

bt

displays a task's kernel-stack backtrace.  If it is given the -a option, it displays the stack traces of the active tasks on all CPUs. It is often used with the foreach command to display the backtraces of all tasks with one command.

btop

translates a byte value (physical offset) to its page number.

dev

displays data concerning the character and block device assignments, I/O port usage, I/O memory usage, and PCI device data.

dis

disassembles memory, either entire kernel functions, from a location for a specified number of instructions, or from the start of a function up to a specified memory location.

eval

evaluates an expression or numeric type and displays the result in hexadecimal, decimal, octal and binary.

exit

causes crash to exit.

extend

dynamically loads or unloads crash shared object extension modules.

files

displays information about open files in a context.

foreach

repeats a specified command for the specified (or all) tasks in the system.

fuser

displays the tasks using the specified file or socket.

gdb

passes its argument to the embedded gdb module.  It is useful for executing gdb commands that have the same name as crash commands.

help

alone displays the command menu; if followed by a command name, a full description of a command, its options, and examples are displayed. Its output is far more complete and useful than this man page.

ipcs

displays data about the System V IPC facilities.

irq

displays data concerning interrupt request numbers and bottom-half interrupt handling.

kmem

displays information about the use of kernel memory.

list

displays the contents of a linked list.

log

displays the kernel log_buf contents in chronological order.

mach

displays data specific to the machine type.

mod

displays information about the currently installed kernel modules, or adds or deletes symbolic or debugging information about specified kernel modules.

mount

displays information about the currently-mounted filesystems.

net

display various network related data.

p

passes its arguments to the gdb "print" command for evaluation and display.

ps

displays process status for specified, or all, processes in the system.

pte

translates the hexadecimal contents of a PTE into its physical page address and page bit settings.

ptob

translates a page frame number to its byte value.

ptov

translates a hexadecimal physical address into a kernel  virtual address.

q

is an alias for the "exit" command.

rd

displays the contents of memory, with the output formatted in several different manners.

repeat

repeats a command indefinitely, optionally delaying a given number of seconds between each command execution.

runq

displays the tasks on the run queue.

search

searches a range of user or kernel memory space for given value.

set

either sets a new context, or gets the current context for display.

sig

displays signal-handling data of one or more tasks.

struct

displays either a structure definition or the contents of a kernel structure at a specified address.

swap

displays information about each configured swap device.

sym

translates a symbol to its virtual address, or a static  kernel virtual address to its symbol -- or to a symbol-plus-offset value, if appropriate.

sys

displays system-specific data.

task

displays the contents of a task_struct.

tree

displays the contents of a red-black tree or a radix tree.

timer

displays the timer queue entries, both old- and new-style, in chronological order.

union

is similar to the struct command, except that it works on kernel unions.

vm

displays basic virtual memory information of a context.

vtop

translates a user or kernel virtual address to its physical address.

waitq

walks the wait queue list displaying the tasks which  are blocked on the specified wait queue.

whatis

displays the definition of structures, unions, typedefs or text/data symbols.

wr

modifies the contents of memory on a live system.  It can only be used if /dev/mem is the device file being used to access system RAM, and should obviously be used with great care.

When crash is invoked with a Xen hypervisor binary as the NAMELIST, the command set is slightly modified.  The *, alias, ascii, bt, dis, eval, exit, extend, gdb, help, list, log, p, pte, rd, repeat, search, set, struct, sym, sys, union, whatis, wr and q commands are the same as above.  The following commands are specific to the Xen hypervisor:

domain

displays the contents of the domain structure for selected, or all, domains.

doms

displays domain status for selected, or all, domains.

dumpinfo

displays Xen dump information for selected, or all, cpus.

pcpus

displays physical cpu information for selected, or all, cpus.

vcpus

displays vcpu status for selected, or all, vcpus.

Files

.crashrc

Initialization commands.  The file can be located in the user's HOME directory and/or the current directory.  Commands found in the .crashrc file in the HOME directory are executed before those in the current directory's .crashrc file.

Environment

EDITOR

Command input is read using readline(3). If EDITOR is set to emacs or vi then suitable keybindings are used.  If EDITOR is not set, then vi is used.  This can be overridden by set vi or set emacs commands located in a .crashrc file, or by entering -e emacs on the crash command line.

CRASHPAGER

If CRASHPAGER is set, its value is used as the name of the program to which command output will be sent.  If not, then command output is sent to /usr/bin/less -E -X by default.

CRASH_MODULE_PATH

Specifies an alternative directory tree to search for kernel module object files.

CRASH_EXTENSIONS

Specifies a directory containing extension modules that will be loaded automatically if the -x command line option is used.

Notes

If crash does not work, look for a newer version: kernel evolution frequently makes crash updates necessary.

The command set scroll off will cause output to be sent directly to the terminal rather than through a paging program.  This is useful, for example, if you are running crash in a window of emacs.

Author

Dave Anderson <anderson@redhat.com> wrote crash.

Jay Fenlason <fenlason@redhat.com> and Dave Anderson <anderson@redhat.com> wrote this man page.

See Also

The help command within crash provides more complete and accurate documentation than this man page.

http://people.redhat.com/anderson - the home page of the crash utility.

netdump(8), gdb(1), makedumpfile(8)

Referenced By

makedumpfile(8).