Documentation for Kdump - The kexec-based Crash Dumping Solution — The Linux Kernel 5.18.5-gnu-1 documentation

# Documentation for Kdump - The kexec-based Crash Dumping Solution

This document includes overview, setup, installation, and analysis information.

## Overview

Kdump uses kexec to quickly boot to a dump-capture kernel whenever a dump of the system kernel’s memory needs to be taken (for example, when the system panics). The system kernel’s memory image is preserved across the reboot and is accessible to the dump-capture kernel.

You can use common commands, such as cp, scp or makedumpfile to copy the memory image to a dump file on the local disk, or across the network to a remote system.

Kdump and kexec are currently supported on the x86, x86_64, ppc64, ia64, s390x, arm and arm64 architectures.

When the system kernel boots, it reserves a small section of memory for the dump-capture kernel. This ensures that ongoing Direct Memory Access (DMA) from the system kernel does not corrupt the dump-capture kernel. The kexec -p command loads the dump-capture kernel into this reserved memory.

On x86 machines, the first 640 KB of physical memory is needed for boot, regardless of where the kernel loads. For simpler handling, the whole low 1M is reserved to avoid any later kernel or device driver writing data into this area. Like this, the low 1M can be reused as system RAM by kdump kernel without extra handling.

On PPC64 machines first 32KB of physical memory is needed for booting regardless of where the kernel is loaded and to support 64K page size kexec backs up the first 64KB memory.

For s390x, when kdump is triggered, the crashkernel region is exchanged with the region [0, crashkernel region size] and then the kdump kernel runs in [0, crashkernel region size]. Therefore no relocatable kernel is needed for s390x.

All of the necessary information about the system kernel’s core image is encoded in the ELF format, and stored in a reserved area of memory before a crash. The physical address of the start of the ELF header is passed to the dump-capture kernel through the elfcorehdr= boot parameter. Optionally the size of the ELF header can also be passed when using the elfcorehdr=[size[KMG]@]offset[KMG] syntax.

With the dump-capture kernel, you can access the memory image through /proc/vmcore. This exports the dump as an ELF-format file that you can write out using file copy commands such as cp or scp. You can also use makedumpfile utility to analyze and write out filtered contents with options, e.g with ‘-d 31’ it will only write out kernel data. Further, you can use analysis tools such as the GNU Debugger (GDB) and the Crash tool to debug the dump file. This method ensures that the dump pages are correctly ordered.

## Setup and Installation

### Install kexec-tools

1. Login as the root user.

2. Download the kexec-tools user-space package from the following URL:

http://kernel.org/pub/linux/utils/kernel/kexec/kexec-tools.tar.gz

This is a symlink to the latest version.

The latest kexec-tools git tree is available at:

- git://git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git

- http://www.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git

There is also a gitweb interface available at http://www.kernel.org/git/?p=utils/kernel/kexec/kexec-tools.git

More information about kexec-tools can be found at http://horms.net/projects/kexec/

3. Unpack the tarball with the tar command, as follows:

       tar xvpzf kexec-tools.tar.gz

4. Change to the kexec-tools directory, as follows:

       cd kexec-tools-VERSION

5. Configure the package, as follows:

       ./configure

6. Compile the package, as follows:

       make

7. Install the package, as follows:

       make install

### Build the system and dump-capture kernels

There are two possible methods of using Kdump.

1. Build a separate custom dump-capture kernel for capturing the kernel core dump.

2. Or use the system kernel binary itself as dump-capture kernel and there is no need to build a separate dump-capture kernel. This is possible only with the architectures which support a relocatable kernel. As of today, i386, x86_64, ppc64, ia64, arm and arm64 architectures support relocatable kernel.

Building a relocatable kernel is advantageous from the point of view that one does not have to build a second kernel for capturing the dump. But at the same time one might want to build a custom dump capture kernel suitable to his needs.

Following are the configuration setting required for system and dump-capture kernels for enabling kdump support.

### System kernel config options

1. Enable “kexec system call” or “kexec file based system call” in “Processor type and features.”:

       CONFIG_KEXEC=y or CONFIG_KEXEC_FILE=y

   And both of them will select KEXEC_CORE:

       CONFIG_KEXEC_CORE=y

   Subsequently, CRASH_CORE is selected by KEXEC_CORE:

       CONFIG_CRASH_CORE=y

2. Enable “sysfs file system support” in “Filesystem” -> “Pseudo filesystems.” This is usually enabled by default:

       CONFIG_SYSFS=y

   Note that “sysfs file system support” might not appear in the “Pseudo filesystems” menu if “Configure standard kernel features (expert users)” is not enabled in “General Setup.” In this case, check the .config file itself to ensure that sysfs is turned on, as follows:

       grep 'CONFIG_SYSFS' .config

3. Enable “Compile the kernel with debug info” in “Kernel hacking.”:

       CONFIG_DEBUG_INFO=Y

   This causes the kernel to be built with debug symbols. The dump analysis tools require a vmlinux with debug symbols in order to read and analyze a dump file.

### Dump-capture kernel config options (Arch Independent)

1. Enable “kernel crash dumps” support under “Processor type and features”:

       CONFIG_CRASH_DUMP=y

2. Enable “/proc/vmcore support” under “Filesystems” -> “Pseudo filesystems”:

       CONFIG_PROC_VMCORE=y

   (CONFIG_PROC_VMCORE is set by default when CONFIG_CRASH_DUMP is selected.)

### Dump-capture kernel config options (Arch Dependent, i386 and x86_64)

1. On i386, enable high memory support under “Processor type and features”:

       CONFIG_HIGHMEM64G=y

   or:

       CONFIG_HIGHMEM4G

2. With CONFIG_SMP=y, usually nr_cpus=1 need specified on the kernel command line when loading the dump-capture kernel because one CPU is enough for kdump kernel to dump vmcore on most of systems.

   However, you can also specify nr_cpus=X to enable multiple processors in kdump kernel. In this case, “disable_cpu_apicid=” is needed to tell kdump kernel which cpu is 1st kernel’s BSP. Please refer to admin-guide/kernel-parameters.txt for more details.

   With CONFIG_SMP=n, the above things are not related.

3. A relocatable kernel is suggested to be built by default. If not yet, enable “Build a relocatable kernel” support under “Processor type and features”:

       CONFIG_RELOCATABLE=y

4. Use a suitable value for “Physical address where the kernel is loaded” (under “Processor type and features”). This only appears when “kernel crash dumps” is enabled. A suitable value depends upon whether kernel is relocatable or not.

   If you are using a relocatable kernel use CONFIG_PHYSICAL_START=0x100000 This will compile the kernel for physical address 1MB, but given the fact kernel is relocatable, it can be run from any physical address hence kexec boot loader will load it in memory region reserved for dump-capture kernel.

   Otherwise it should be the start of memory region reserved for second kernel using boot parameter “crashkernel=Y@X”. Here X is start of memory region reserved for dump-capture kernel. Generally X is 16MB (0x1000000). So you can set CONFIG_PHYSICAL_START=0x1000000

5. Make and install the kernel and its modules. DO NOT add this kernel to the boot loader configuration files.

### Dump-capture kernel config options (Arch Dependent, ppc64)

1. Enable “Build a kdump crash kernel” support under “Kernel” options:

       CONFIG_CRASH_DUMP=y

2. Enable “Build a relocatable kernel” support:

       CONFIG_RELOCATABLE=y

> Make and install the kernel and its modules.

### Dump-capture kernel config options (Arch Dependent, ia64)

- No specific options are required to create a dump-capture kernel for ia64, other than those specified in the arch independent section above. This means that it is possible to use the system kernel as a dump-capture kernel if desired.

  The crashkernel region can be automatically placed by the system kernel at runtime. This is done by specifying the base address as 0, or omitting it all together:

      crashkernel=256M@0

  or:

      crashkernel=256M

### Dump-capture kernel config options (Arch Dependent, arm)

- To use a relocatable kernel, Enable “AUTO_ZRELADDR” support under “Boot” options:

      AUTO_ZRELADDR=y

### Dump-capture kernel config options (Arch Dependent, arm64)

- Please note that kvm of the dump-capture kernel will not be enabled on non-VHE systems even if it is configured. This is because the CPU will not be reset to EL2 on panic.

## crashkernel syntax

1. crashkernel=size@offset

   Here ‘size’ specifies how much memory to reserve for the dump-capture kernel and ‘offset’ specifies the beginning of this reserved memory. For example, “crashkernel=64M@16M” tells the system kernel to reserve 64 MB of memory starting at physical address 0x01000000 (16MB) for the dump-capture kernel.

   The crashkernel region can be automatically placed by the system kernel at run time. This is done by specifying the base address as 0, or omitting it all together:

       crashkernel=256M@0

   or:

       crashkernel=256M

   If the start address is specified, note that the start address of the kernel will be aligned to a value (which is Arch dependent), so if the start address is not then any space below the alignment point will be wasted.

2. range1:size1[,range2:size2,…][@offset]

   While the “crashkernel=size[@offset]” syntax is sufficient for most configurations, sometimes it’s handy to have the reserved memory dependent on the value of System RAM – that’s mostly for distributors that pre-setup the kernel command line to avoid a unbootable system after some memory has been removed from the machine.

   The syntax is:

       crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
       range=start-[end]

   For example:

       crashkernel=512M-2G:64M,2G-:128M

   This would mean:

   > 1. if the RAM is smaller than 512M, then don’t reserve anything (this is the “rescue” case)

   > 2. if the RAM size is between 512M and 2G (exclusive), then reserve 64M

   > 3. if the RAM size is larger than 2G, then reserve 128M

3. crashkernel=size,high and crashkernel=size,low

   If memory above 4G is preferred, crashkernel=size,high can be used to fulfill that. With it, physical memory is allowed to be allocated from top, so could be above 4G if system has more than 4G RAM installed. Otherwise, memory region will be allocated below 4G if available.

   When crashkernel=X,high is passed, kernel could allocate physical memory region above 4G, low memory under 4G is needed in this case. There are three ways to get low memory:

   > 1. Kernel will allocate at least 256M memory below 4G automatically if crashkernel=Y,low is not specified.

   > 2. Let user specify low memory size instead.

   > 3. Specified value 0 will disable low memory allocation:

   >        crashkernel=0,low

### Boot into System Kernel

1. Update the boot loader (such as grub, yaboot, or lilo) configuration files as necessary.

2. Boot the system kernel with the boot parameter “crashkernel=Y@X”.

   On x86 and x86_64, use “crashkernel=Y[@X]”. Most of the time, the start address ‘X’ is not necessary, kernel will search a suitable area. Unless an explicit start address is expected.

   On ppc64, use “crashkernel=128M@32M”.

   On ia64, 256M@256M is a generous value that typically works. The region may be automatically placed on ia64, see the dump-capture kernel config option notes above. If use sparse memory, the size should be rounded to GRANULE boundaries.

   On s390x, typically use “crashkernel=xxM”. The value of xx is dependent on the memory consumption of the kdump system. In general this is not dependent on the memory size of the production system.

   On arm, the use of “crashkernel=Y@X” is no longer necessary; the kernel will automatically locate the crash kernel image within the first 512MB of RAM if X is not given.

   On arm64, use “crashkernel=Y[@X]”. Note that the start address of the kernel, X if explicitly specified, must be aligned to 2MiB (0x200000).

## Load the Dump-capture Kernel

After booting to the system kernel, dump-capture kernel needs to be loaded.

Based on the architecture and type of image (relocatable or not), one can choose to load the uncompressed vmlinux or compressed bzImage/vmlinuz of dump-capture kernel. Following is the summary.

For i386 and x86_64:

> - Use bzImage/vmlinuz if kernel is relocatable.

> - Use vmlinux if kernel is not relocatable.

For ppc64:

> - Use vmlinux

For ia64:

> - Use vmlinux or vmlinuz.gz

For s390x:

> - Use image or bzImage

For arm:

> - Use zImage

For arm64:

> - Use vmlinux or Image

If you are using an uncompressed vmlinux image then use following command to load dump-capture kernel:

    kexec -p <dump-capture-kernel-vmlinux-image> \
    --initrd=<initrd-for-dump-capture-kernel> --args-linux \
    --append="root=<root-dev> <arch-specific-options>"

If you are using a compressed bzImage/vmlinuz, then use following command to load dump-capture kernel:

    kexec -p <dump-capture-kernel-bzImage> \
    --initrd=<initrd-for-dump-capture-kernel> \
    --append="root=<root-dev> <arch-specific-options>"

If you are using a compressed zImage, then use following command to load dump-capture kernel:

    kexec --type zImage -p <dump-capture-kernel-bzImage> \
    --initrd=<initrd-for-dump-capture-kernel> \
    --dtb=<dtb-for-dump-capture-kernel> \
    --append="root=<root-dev> <arch-specific-options>"

If you are using an uncompressed Image, then use following command to load dump-capture kernel:

    kexec -p <dump-capture-kernel-Image> \
    --initrd=<initrd-for-dump-capture-kernel> \
    --append="root=<root-dev> <arch-specific-options>"

Please note, that –args-linux does not need to be specified for ia64. It is planned to make this a no-op on that architecture, but for now it should be omitted

Following are the arch specific command line options to be used while loading dump-capture kernel.

For i386, x86_64 and ia64:

> “1 irqpoll nr_cpus=1 reset_devices”

For ppc64:

> “1 maxcpus=1 noirqdistrib reset_devices”

For s390x:

> “1 nr_cpus=1 cgroup_disable=memory”

For arm:

> “1 maxcpus=1 reset_devices”

For arm64:

> “1 nr_cpus=1 reset_devices”

Notes on loading the dump-capture kernel:

- By default, the ELF headers are stored in ELF64 format to support systems with more than 4GB memory. On i386, kexec automatically checks if the physical RAM size exceeds the 4 GB limit and if not, uses ELF32. So, on non-PAE systems, ELF32 is always used.

  The –elf32-core-headers option can be used to force the generation of ELF32 headers. This is necessary because GDB currently cannot open vmcore files with ELF64 headers on 32-bit systems.

- The “irqpoll” boot parameter reduces driver initialization failures due to shared interrupts in the dump-capture kernel.

- You must specify <root-dev> in the format corresponding to the root device name in the output of mount command.

- Boot parameter “1” boots the dump-capture kernel into single-user mode without networking. If you want networking, use “3”.

- We generally don’t have to bring up a SMP kernel just to capture the dump. Hence generally it is useful either to build a UP dump-capture kernel or specify maxcpus=1 option while loading dump-capture kernel. Note, though maxcpus always works, you had better replace it with nr_cpus to save memory if supported by the current ARCH, such as x86.

- You should enable multi-cpu support in dump-capture kernel if you intend to use multi-thread programs with it, such as parallel dump feature of makedumpfile. Otherwise, the multi-thread program may have a great performance degradation. To enable multi-cpu support, you should bring up an SMP dump-capture kernel and specify maxcpus/nr_cpus, disable_cpu_apicid=[X] options while loading it.

- For s390x there are two kdump modes: If a ELF header is specified with the elfcorehdr= kernel parameter, it is used by the kdump kernel as it is done on all other architectures. If no elfcorehdr= kernel parameter is specified, the s390x kdump kernel dynamically creates the header. The second mode has the advantage that for CPU and memory hotplug, kdump has not to be reloaded with kexec_load().

- For s390x systems with many attached devices the “cio_ignore” kernel parameter should be used for the kdump kernel in order to prevent allocation of kernel memory for devices that are not relevant for kdump. The same applies to systems that use SCSI/FCP devices. In that case the “allow_lun_scan” zfcp module parameter should be set to zero before setting FCP devices online.

## Kernel Panic

After successfully loading the dump-capture kernel as previously described, the system will reboot into the dump-capture kernel if a system crash is triggered. Trigger points are located in `panic()`, die(), die_nmi() and in the sysrq handler (ALT-SysRq-c).

The following conditions will execute a crash trigger point:

If a hard lockup is detected and “NMI watchdog” is configured, the system will boot into the dump-capture kernel ( die_nmi() ).

If die() is called, and it happens to be a thread with pid 0 or 1, or die() is called inside interrupt context or die() is called and panic_on_oops is set, the system will boot into the dump-capture kernel.

On powerpc systems when a soft-reset is generated, die() is called by all cpus and the system will boot into the dump-capture kernel.

For testing purposes, you can trigger a crash by using “ALT-SysRq-c”, “echo c > /proc/sysrq-trigger” or write a module to force the panic.

## Write Out the Dump File

After the dump-capture kernel is booted, write out the dump file with the following command:

    cp /proc/vmcore <dump-file>

or use scp to write out the dump file between hosts on a network, e.g:

    scp /proc/vmcore remote_username@remote_ip:<dump-file>

You can also use makedumpfile utility to write out the dump file with specified options to filter out unwanted contents, e.g:

    makedumpfile -l --message-level 1 -d 31 /proc/vmcore <dump-file>

## Analysis

Before analyzing the dump image, you should reboot into a stable kernel.

You can do limited analysis using GDB on the dump file copied out of /proc/vmcore. Use the debug vmlinux built with -g and run the following command:

    gdb vmlinux <dump-file>

Stack trace for the task on processor 0, register display, and memory display work fine.

Note: GDB cannot analyze core files generated in ELF64 format for x86. On systems with a maximum of 4GB of memory, you can generate ELF32-format headers using the –elf32-core-headers kernel option on the dump kernel.

You can also use the Crash utility to analyze dump files in Kdump format. Crash is available at the following URL:

> https://github.com/crash-utility/crash

Crash document can be found at:

  https://crash-utility.github.io/

## Trigger Kdump on WARN()

The kernel parameter, panic_on_warn, calls `panic()` in all WARN() paths. This will cause a kdump to occur at the `panic()` call. In cases where a user wants to specify this during runtime, /proc/sys/kernel/panic_on_warn can be set to 1 to achieve the same behaviour.

## Trigger Kdump on add_taint()

The kernel parameter panic_on_taint facilitates a conditional call to `panic()` from within `add_taint()` whenever the value set in this bitmask matches with the bit flag being set by `add_taint()`. This will cause a kdump to occur at the `add_taint()`->`panic()` call.

## Contact

- kexec@lists.infradead.org

## GDB macros

    #
    # This file contains a few gdb macros (user defined commands) to extract
    # useful information from kernel crashdump (kdump) like stack traces of
    # all the processes or a particular process and trapinfo.
    #
    # These macros can be used by copying this file in .gdbinit (put in home
    # directory or current directory) or by invoking gdb command with
    # --command=<command-file-name> option
    #
    # Credits:
    # Alexander Nyberg <alexn@telia.com>
    # V Srivatsa <vatsa@in.ibm.com>
    # Maneesh Soni <maneesh@in.ibm.com>
    #

    define bttnobp
            set $tasks_off=((size_t)&((struct task_struct *)0)->tasks)
            set $pid_off=((size_t)&((struct task_struct *)0)->thread_group.next)
            set $init_t=&init_task
            set $next_t=(((char *)($init_t->tasks).next) - $tasks_off)
            set var $stacksize = sizeof(union thread_union)
            while ($next_t != $init_t)
                    set $next_t=(struct task_struct *)$next_t
                    printf "\npid %d; comm %s:\n", $next_t.pid, $next_t.comm
                    printf "===================\n"
                    set var $stackp = $next_t.thread.sp
                    set var $stack_top = ($stackp & ~($stacksize - 1)) + $stacksize

                    while ($stackp < $stack_top)
                            if (*($stackp) > _stext && *($stackp) < _sinittext)
                                    info symbol *($stackp)
                            end
                            set $stackp += 4
                    end
                    set $next_th=(((char *)$next_t->thread_group.next) - $pid_off)
                    while ($next_th != $next_t)
                            set $next_th=(struct task_struct *)$next_th
                            printf "\npid %d; comm %s:\n", $next_t.pid, $next_t.comm
                            printf "===================\n"
                            set var $stackp = $next_t.thread.sp
                            set var $stack_top = ($stackp & ~($stacksize - 1)) + stacksize

                            while ($stackp < $stack_top)
                                    if (*($stackp) > _stext && *($stackp) < _sinittext)
                                            info symbol *($stackp)
                                    end
                                    set $stackp += 4
                            end
                            set $next_th=(((char *)$next_th->thread_group.next) - $pid_off)
                    end
                    set $next_t=(char *)($next_t->tasks.next) - $tasks_off
            end
    end
    document bttnobp
            dump all thread stack traces on a kernel compiled with !CONFIG_FRAME_POINTER
    end

    define btthreadstack
            set var $pid_task = $arg0

            printf "\npid %d; comm %s:\n", $pid_task.pid, $pid_task.comm
            printf "task struct: "
            print $pid_task
            printf "===================\n"
            set var $stackp = $pid_task.thread.sp
            set var $stacksize = sizeof(union thread_union)
            set var $stack_top = ($stackp & ~($stacksize - 1)) + $stacksize
            set var $stack_bot = ($stackp & ~($stacksize - 1))

            set $stackp = *((unsigned long *) $stackp)
            while (($stackp < $stack_top) && ($stackp > $stack_bot))
                    set var $addr = *(((unsigned long *) $stackp) + 1)
                    info symbol $addr
                    set $stackp = *((unsigned long *) $stackp)
            end
    end
    document btthreadstack
             dump a thread stack using the given task structure pointer
    end

    define btt
            set $tasks_off=((size_t)&((struct task_struct *)0)->tasks)
            set $pid_off=((size_t)&((struct task_struct *)0)->thread_group.next)
            set $init_t=&init_task
            set $next_t=(((char *)($init_t->tasks).next) - $tasks_off)
            while ($next_t != $init_t)
                    set $next_t=(struct task_struct *)$next_t
                    btthreadstack $next_t

                    set $next_th=(((char *)$next_t->thread_group.next) - $pid_off)
                    while ($next_th != $next_t)
                            set $next_th=(struct task_struct *)$next_th
                            btthreadstack $next_th
                            set $next_th=(((char *)$next_th->thread_group.next) - $pid_off)
                    end
                    set $next_t=(char *)($next_t->tasks.next) - $tasks_off
            end
    end
    document btt
            dump all thread stack traces on a kernel compiled with CONFIG_FRAME_POINTER
    end

    define btpid
            set var $pid = $arg0
            set $tasks_off=((size_t)&((struct task_struct *)0)->tasks)
            set $pid_off=((size_t)&((struct task_struct *)0)->thread_group.next)
            set $init_t=&init_task
            set $next_t=(((char *)($init_t->tasks).next) - $tasks_off)
            set var $pid_task = 0

            while ($next_t != $init_t)
                    set $next_t=(struct task_struct *)$next_t

                    if ($next_t.pid == $pid)
                            set $pid_task = $next_t
                    end

                    set $next_th=(((char *)$next_t->thread_group.next) - $pid_off)
                    while ($next_th != $next_t)
                            set $next_th=(struct task_struct *)$next_th
                            if ($next_th.pid == $pid)
                                    set $pid_task = $next_th
                            end
                            set $next_th=(((char *)$next_th->thread_group.next) - $pid_off)
                    end
                    set $next_t=(char *)($next_t->tasks.next) - $tasks_off
            end

            btthreadstack $pid_task
    end
    document btpid
            backtrace of pid
    end

    define trapinfo
            set var $pid = $arg0
            set $tasks_off=((size_t)&((struct task_struct *)0)->tasks)
            set $pid_off=((size_t)&((struct task_struct *)0)->thread_group.next)
            set $init_t=&init_task
            set $next_t=(((char *)($init_t->tasks).next) - $tasks_off)
            set var $pid_task = 0

            while ($next_t != $init_t)
                    set $next_t=(struct task_struct *)$next_t

                    if ($next_t.pid == $pid)
                            set $pid_task = $next_t
                    end

                    set $next_th=(((char *)$next_t->thread_group.next) - $pid_off)
                    while ($next_th != $next_t)
                            set $next_th=(struct task_struct *)$next_th
                            if ($next_th.pid == $pid)
                                    set $pid_task = $next_th
                            end
                            set $next_th=(((char *)$next_th->thread_group.next) - $pid_off)
                    end
                    set $next_t=(char *)($next_t->tasks.next) - $tasks_off
            end

            printf "Trapno %ld, cr2 0x%lx, error_code %ld\n", $pid_task.thread.trap_no, \
                                    $pid_task.thread.cr2, $pid_task.thread.error_code

    end
    document trapinfo
            Run info threads and lookup pid of thread #1
            'trapinfo <pid>' will tell you by which trap & possibly
            address the kernel panicked.
    end

    define dump_record
            set var $desc = $arg0
            set var $info = $arg1
            if ($argc > 2)
                    set var $prev_flags = $arg2
            else
                    set var $prev_flags = 0
            end

            set var $prefix = 1
            set var $newline = 1

            set var $begin = $desc->text_blk_lpos.begin % (1U << prb->text_data_ring.size_bits)
            set var $next = $desc->text_blk_lpos.next % (1U << prb->text_data_ring.size_bits)

            # handle data-less record
            if ($begin & 1)
                    set var $text_len = 0
                    set var $log = ""
            else
                    # handle wrapping data block
                    if ($begin > $next)
                            set var $begin = 0
                    end

                    # skip over descriptor id
                    set var $begin = $begin + sizeof(long)

                    # handle truncated message
                    if ($next - $begin < $info->text_len)
                            set var $text_len = $next - $begin
                    else
                            set var $text_len = $info->text_len
                    end

                    set var $log = &prb->text_data_ring.data[$begin]
            end

            # prev & LOG_CONT && !(info->flags & LOG_PREIX)
            if (($prev_flags & 8) && !($info->flags & 4))
                    set var $prefix = 0
            end

            # info->flags & LOG_CONT
            if ($info->flags & 8)
                    # (prev & LOG_CONT && !(prev & LOG_NEWLINE))
                    if (($prev_flags & 8) && !($prev_flags & 2))
                            set var $prefix = 0
                    end
                    # (!(info->flags & LOG_NEWLINE))
                    if (!($info->flags & 2))
                            set var $newline = 0
                    end
            end

            if ($prefix)
                    printf "[%5lu.%06lu] ", $info->ts_nsec / 1000000000, $info->ts_nsec % 1000000000
            end
            if ($text_len)
                    eval "printf \"%%%d.%ds\", $log", $text_len, $text_len
            end
            if ($newline)
                    printf "\n"
            end

            # handle dictionary data

            set var $dict = &$info->dev_info.subsystem[0]
            set var $dict_len = sizeof($info->dev_info.subsystem)
            if ($dict[0] != '\0')
                    printf " SUBSYSTEM="
                    set var $idx = 0
                    while ($idx < $dict_len)
                            set var $c = $dict[$idx]
                            if ($c == '\0')
                                    loop_break
                            else
                                    if ($c < ' ' || $c >= 127 || $c == '\\')
                                            printf "\\x%02x", $c
                                    else
                                            printf "%c", $c
                                    end
                            end
                            set var $idx = $idx + 1
                    end
                    printf "\n"
            end

            set var $dict = &$info->dev_info.device[0]
            set var $dict_len = sizeof($info->dev_info.device)
            if ($dict[0] != '\0')
                    printf " DEVICE="
                    set var $idx = 0
                    while ($idx < $dict_len)
                            set var $c = $dict[$idx]
                            if ($c == '\0')
                                    loop_break
                            else
                                    if ($c < ' ' || $c >= 127 || $c == '\\')
                                            printf "\\x%02x", $c
                                    else
                                            printf "%c", $c
                                    end
                            end
                            set var $idx = $idx + 1
                    end
                    printf "\n"
            end
    end
    document dump_record
            Dump a single record. The first parameter is the descriptor,
            the second parameter is the info, the third parameter is
            optional and specifies the previous record's flags, used for
            properly formatting continued lines.
    end

    define dmesg
            # definitions from kernel/printk/printk_ringbuffer.h
            set var $desc_committed = 1
            set var $desc_finalized = 2
            set var $desc_sv_bits = sizeof(long) * 8
            set var $desc_flags_shift = $desc_sv_bits - 2
            set var $desc_flags_mask = 3 << $desc_flags_shift
            set var $id_mask = ~$desc_flags_mask

            set var $desc_count = 1U << prb->desc_ring.count_bits
            set var $prev_flags = 0

            set var $id = prb->desc_ring.tail_id.counter
            set var $end_id = prb->desc_ring.head_id.counter

            while (1)
                    set var $desc = &prb->desc_ring.descs[$id % $desc_count]
                    set var $info = &prb->desc_ring.infos[$id % $desc_count]

                    # skip non-committed record
                    set var $state = 3 & ($desc->state_var.counter >> $desc_flags_shift)
                    if ($state == $desc_committed || $state == $desc_finalized)
                            dump_record $desc $info $prev_flags
                            set var $prev_flags = $info->flags
                    end

                    set var $id = ($id + 1) & $id_mask
                    if ($id == $end_id)
                            loop_break
                    end
            end
    end
    document dmesg
            print the kernel ring buffer
    end