KMDB(1) User Commands KMDB(1)


kmdb - in situ kernel debugger


Boot-time Loading

ok boot [device-specifier] -k [-d] [boot-flags]

ok boot [device-specifier] kmdb [-d] [boot-flags]


kernel$ /platform/i86pc/kernel/$ISADIR/unix -k [-d] [boot-flags]

Runtime Loading

mdb -K


kmdb is an interactive kernel debugger which implements the user
interface and functionality of mdb(1) in a live kernel context. kmdb
provides features that allow for the control of kernel execution and for
the inspection and modification of live kernel state. kmdb can be loaded
at the beginning of a boot session or after the system is booted.

This man page describes the features and functionality that are unique to
kmdb or different in kmdb as compared to mdb(1). For more information on
mdb(1) or further details on the features and functionality implemented
by kmdb, see the mdb(1) man page and the Modular Debugger Guide.

Loading and Unloading

Boot-time Loading
When requested, the kernel runtime linker (krtld)
loads kmdb prior to the transfer of control to the
kernel. If the -d flag is used, the debugger gains
control of the system prior to the execution of the
initial function in the 'unix' object. If -d is not
used, kmdb is loaded but does not gain control until
such time as it is explicitly entered. See the
Debugger Entry section below. For a list of the boot
commands which cause kmdb to be loaded at boot, see
the SYNOPSIS section above.

Boot-loaded kmdb can be unloaded only by means of a
system reboot.

Some features of kmdb rely on the presence of kernel
services and are not immediately available to boot-
loaded kmdb. In particular, the loading and
unloading of dmods is not available until the module
subsystem is initialized. Requests are queued until
they can be processed. Similarly, translation of
virtual addresses to physical addresses is not be
available until the VM system has been initialized.
Attempted translations fail until translation
facilities are available.

Run-time Loading
kmdb can also be loaded after the system has booted,
using the -K flag to mdb(1). When loaded in this
fashion, it will immediately gain control of the
system. Run-time-loaded kmdb can be unloaded using
the -U flag to mdb(1) or from within the debugger
with the -u flag to the ::quit dcmd.

Terminal types
When loaded, kmdb attempts to determine the proper
terminal type in use on the system console. If the
system being debugged has an attached keyboard and
local display that are both used for the system
console, kmdb uses the terminal type appropriate for
the machine: 'sun' for SPARC; 'sun-color' for x86.
When a serial console is in use, boot-loaded kmdb
defaults to a terminal type 'vt100'. Run-time-loaded
kmdb defaults to the terminal type requested by
mdb(1). mdb(1) requests the terminal type specified
by the value of the TERM environment variable unless
overridden by the -T flag. ::term can be used to
view the current terminal type.

Debugger Entry

Debugger entry can be requested explicitly or implicitly. Implicit entry,
encountered when breakpoints or other execution control features are
used, is discussed in the Execution Control section.

The primary means for explicit debugger entry is with the keyboard abort
sequence for systems with local consoles and the BREAK character for
those with serial consoles. The abort sequence is STOP-A or Shift-Pause
for SPARC systems with local consoles, and F1-A or Shift-Pause for x86
systems with local consoles. See kbd(1) for a discussion of the abort
sequence and for instructions on disabling it.

A second way to request entry into the debugger is with the mdb(1)
command. Invocations of mdb(1) with the -K flag after the debugger is
loaded trigger debugger entry.

If the kernel panics and kmdb is loaded, by default, the panic routine
enters kmdb for live debugging. If a dump device is specified, and you
enter ::cont, the debugger exits and a crash dump is performed. To
prevent the kernel from entering kmdb when panicking, you can set the
nopanicdebug variable to 1. Set the nopanicdebug variable to 1 using kmdb
or including the following a line in /etc/system:

set nopanicdebug = 1

This can be useful if you want to keep kmdb loaded, but always want a
panic to trigger a crash dump without entering the debugger.

Execution Control

For the most part, the execution control facilities provided by kmdb for
the kernel mirror those provided by the mdb(1) process target.
Breakpoints (::bp), watchpoints (::wp), ::continue, and the various
flavors of ::step can be used.

In contrast to the unlimited user process watchpoints supplied by the
kernel, kmdb is restricted to a set of CPU watchpoints that limit the
number, size, and type of watchpoints allowed. The ::wp command does not
allow a watchpoint to be created if it is incompatible with the
watchpoints supported by the hardware.

Debugger modules (dmods)
As with mdb(1), kmdb is installed with a number of subsystem-specific
debugger modules, or dmods. The dmods are loaded and unloaded
automatically with the loading and unloading of the subsystems that they
support. The dmods can also be explicitly loaded and unloaded using
::load and ::unload.

kmdb uses kernel facilities to load and unload dmods and must resume
system execution to perform each requested action. When a dmod load or
unload is complete, the system is stopped and the debugger is
automatically re-entered. For a dmod load, processing is completed when
the load of a requested dmod succeeds or fails. Status messages are
provided in either case.

Processor-specific functionality
Some functionality is specific to an individual processor type. An
example of such functionality is the branch tracing provided by various
x86 processors. Access to these processor-specific features is provided
with processor-specific dcmds that are present only on systems that
support them. The availability of processor-specific support is indicated
in the output of the ::status dcmd. The debugger relies on the kernel to
determine the processor type. Even though the debugger might provide
support for a given processor type, the support is not exposed until the
kernel has progressed to the point at which processor identification has

Kernel Macros

The debugger provides access to a set of macros that are precompiled into
the debugger. Only the precompiled macros are available. Unlike with
mdb(1), the $< dcmd may not be used to load macros from arbitrary
locations. Use the $M command to list the available macros.

Built-in dcmds
This section lists dcmds that are unique to kmdb or those with behavior
that differs in kmdb as compared to mdb(1).

[address]::bp [+/-dDestT] [-c cmd] [-n count] sym ...
address :b [cmd ...]

Set a breakpoint at the specified locations. The ::bp dcmd sets a
breakpoint at each address or symbol specified, including an optional
address specified by an explicit expression preceding the dcmd, and
each string or immediate value following the dcmd. The arguments can
be symbol names or immediate values denoting a particular virtual
address of interest.

If a symbol name is specified, the name may refer to a symbol that
cannot yet be evaluated. It might consist of an object name and
function name in a load object that has not yet been opened. In such
a case, the breakpoint is deferred and is not active in the target
until an object matching the given name is loaded. The breakpoint is
automatically enabled when the load object is opened.

The -d, -D, -e, -s, -t, -T, -c, and -n options have the same meaning
as they do for the ::evset dcmd. See mdb(1) for a description of
::evset. If the :b form of the dcmd is used, a breakpoint is set only
at the virtual address specified by the expression preceding the
dcmd. The arguments following the :b dcmd are concatenated together
to form the callback string. If this string contains meta-characters,
it must be quoted.

::branches [-v]
(x86 only)

Display the last branches taken by the CPU. This dcmd is supported
only on x86 systems, and is available only when processor-specific
support is detected and enabled. The number and type of branches
displayed is dependent on the capabilities of the branch tracing
facilities provided by the CPU. When the -v option is used, the
instructions prior to a given branch are displayed.

[function] ::call [arg [arg ...]]

Call the specified function using the specified arguments. The called
function must be listed as a function in the symbol table for a
loaded module. String arguments are passed by reference. When the
call completes, the return value of the function is displayed.

This dcmd must be used with extreme caution. The kernel will not be
resumed when the call is made. The function being called may not make
any assumptions regarding the availability of any kernel services,
and must not perform operations or calls that may block. The user
must also beware of any side-effects introduced by the called
function, as kernel stability might be affected.

[addr] ::cpuregs [-c cpuid]

Display the current general purpose register set for the specified
CPU, in the format used by ::regs.

[addr] ::cpustack [-c cpuid]

Print a C stack backtrace for the specified CPU. The backtrace
displayed is for the point at which the specified CPU entered or was
stopped by the debugger.

addr[,len] ::in [-L len]
(x86 only)

Read len bytes from the I/O port specified by addr. The value of the
-L option, if provided, takes precedence over the value of the repeat
count. The read length must be 1, 2, or 4 bytes, and the port address
must have the same alignment as the length.

addr[,len] ::out [-L len] value
(x86 only)

Write value to the len-byte I/O port specified by addr. The value of
the -L option, if provided, takes precedence over the value of the
repeat count. The write length must be 1, 2, or 4 bytes and the port
address must have the same alignment as the length.

::quit [-u]

Causes the debugger to exit. When the -u option is used, the system
is resumed and the debugger is unloaded. The -u option may not be
used if the debugger was loaded at boot. When the -u option is not
used, SPARC systems will exit to the boot PROM ok prompt. The go
command can be used to re-enter the debugger. On x86 systems, a
prompt is displayed that requests permission to reboot the machine.

::step [over|out|branch]

Step the target one instruction. The optional over argument is used
to step over subroutine calls. When the optional out argument is
specified, the target program continues until control returns from
the current function.

The optional branch argument is available only on x86 systems when
processor-specific support is detected and enabled. When ::step
branch is specified, the target program continues until the next
branching instruction is encountered.

On SPARC systems, the ::step dcmd may not be used to step 'ta'
instructions. Similarly, it may not be used on x86 systems to step
'int' instructions. If the step results in a trap that cannot be
resolved by the debugger, a message to that effect is printed and the
step will fail.


Use the specified CPU as the representative. Stack traces, general
purpose register dumps, and similar functionality use the new
representative CPU as the data source. Full execution control
functionality is available on the new representative CPU.


Display the current terminal type.

addr[,len]::wp [+/-dDestT] [-rwx] [-pi] [-n count] [-c cmd]
addr[,len]:a [cmd ...]
addr[,len]:p [cmd ...]
addr[,len]:w [cmd ...]

Set a watchpoint at the specified address, interpreted by default as
a virtual address. If the -p option is used, the address is
interpreted as a physical address. On x86 platforms, watchpoints can
be set on I/O ports using the -i option. When the -i option is used,
the address is interpreted as that of an I/O port.

The length in bytes of the watched region can be set by specifying an
optional repeat count preceding the dcmd. If no length is explicitly
set, the default is one byte. The ::wp dcmd allows the watchpoint to
be configured to trigger on any combination of read (-r option),
write (-w option), or execute (-x option) access.

The -d, -D, -e, -s, -t, -T, -c, and -n options have the same meaning
as they do for the ::evset dcmd. See mdb(1) for a description of
::evset. The :a dcmd sets a read access watchpoint at the specified
address. The :p dcmd sets an execute access watchpoint at the
specified address. The :w dcmd sets a write access watchpoint at the
specified address. The arguments following the :a, :p, and :w dcmds
are concatenated together to form the callback string. If the string
contains meta-characters, it must be quoted.


See attributes(7) for descriptions of the following attributes:

|Interface Stability | Evolving |


kbd(1), mdb(1), system(5), attributes(7), boot(8), dumpadm(8), kernel(8)

Modular Debugger Guide:


Limitations on Memory Available to the Debugger

The memory region available to the debugger is allocated when the
debugger is loaded, and is fixed at that point. If dcmds attempt to
allocate more memory than is available, they will, if possible, be
terminated. The debugger will attempt to recover gracefully from an out-
of-memory situation, but may be unable to, and may be forced to terminate
the system. This constraint is especially acute on 32-bit x86 systems.

Performance Impact

System performance will be negatively impacted by the loading of kmdb, as
the debugger will consume kernel memory and other limited system

illumos February 17, 2023 KMDB(1)