NAME

ddb — in-kernel debugger

SYNOPSIS

options DDB

To enable history editing:
options DDB_HISTORY_SIZE=integer

To disable entering ddb upon kernel panic:
options DDB_ONPANIC=0

To enable teeing all ddb output to the kernel msgbuf:
options DDB_TEE_MSGBUF=1

To specify commands which will be executed on each entry to ddb:
options DDB_COMMANDONENTER="trace;show registers" In this case, "trace" and then "show registers" will be executed automatically.

To enable extended online help:
options DDB_VERBOSE_HELP.

DESCRIPTION

ddb is the in-kernel debugger. It may be entered at any time via a special key sequence, and optionally may be invoked when the kernel panics.

ENTERING THE DEBUGGER

Unless DDB_ONPANIC is set to 0, ddb will be activated whenever the kernel would otherwise panic.

ddb may also be activated from the console. In general, sending a break on a serial console will activate ddb. There are also key sequences for each port that will activate ddb from the keyboard:

alpha

<Ctrl>-<Alt>-<Esc> on PC style keyboards.

amd64

<Ctrl>-<Alt>-<Esc>

<Break> on serial console.

amiga

<LAlt>-<LAmiga>-<F10>

atari

<Alt>-<LeftShift>-<F9>

hp300

<Shift>-<Reset>

hpcarm

<Ctrl>-<Alt>-<Esc>

hpcmips

<Ctrl>-<Alt>-<Esc>

hpcsh

<Ctrl>-<Alt>-<Esc>

hppa

<Ctrl>-<Alt>-<Esc> on PC style keyboards.

+++++ (five plus signs) on PDC console

<Break> on serial console.

i386

<Ctrl>-<Alt>-<Esc>

<Break> on serial console.

mac68k

<Command>-<Power>, or the Interrupt switch.

macppc

Some models: <Command>-<Option>-<Power>

mvme68k

Abort switch on CPU card.

pmax

<Do> on LK-201 rcons console.

<Break> on serial console.

sparc

<L1>-A, or <Stop>-A on a Sun keyboard.

<Break> on serial console.

sparc64

<L1>-A, or <Stop>-A on a Sun keyboard.

<Break> on serial console.

sun3

<L1>-A, or <Stop>-A on a Sun keyboard.

<Break> on serial console.

vax

<Esc>-<Shift>-D on serial console.

x68k

Interrupt switch on the body.

xen dom0

<Ctrl>-<Alt>-<Esc> on PC style keyboards.

+++++ (five plus signs) on serial console.

xen domU

+++++ (five plus signs) on serial console.

zaurus

<Ctrl>-<Alt>-<Esc>

The key sequence to activate ddb can be changed by modifying “hw.cnmagic” with sysctl(8). If the console is not dedicated to ddb the sequence should not be easily typed by accident. In addition, ddb may be explicitly activated by the debugging code in the kernel if DDB is configured.

Commands can be automatically run when ddb is entered by using options DDB_COMMANDONENTER or by setting ddb.commandonenter with sysctl(8). Multiple commands can be separated by a semi-colon.

COMMAND SYNTAX

The general command syntax is:

The current memory location being edited is referred to as dot, and the next location is next. They are displayed as hexadecimal numbers.

Commands that examine and/or modify memory update dot to the address of the last line examined or the last location modified, and set next to the next location to be examined or modified. Other commands don't change dot, and set next to be the same as dot.

A blank line repeats the previous command from the address next with the previous count and no modifiers. Specifying address sets dot to the address. If address is omitted, dot is used. A missing count is taken to be 1 for printing commands, and infinity for stack traces.

The syntax:

repeats the previous command, just as a blank line does, but with the specified count.

ddb has a more(1)-like functionality; if a number of lines in a command's output exceeds the number defined in the lines variable, then ddb displays “--db more--” and waits for a response, which may be one of:

<return>

one more line.

<space>

one more page.

q

abort the current command, and return to the command input mode.

You can set lines variable to zero to disable this feature.

If ddb history editing is enabled (by defining the

options DDB_HISTORY_SIZE=num

kernel option), then a history of the last num commands is kept. The history can be manipulated with the following key sequences:

<Ctrl>-P

retrieve previous command in history (if any).

<Ctrl>-N

retrieve next command in history (if any).

COMMANDS

ddb supports the following commands:

!address[(expression[,...])]

A synonym for call.

break[/u] address[,count]

Set a breakpoint at address. If count is supplied, continues (count-1) times before stopping at the breakpoint. If the breakpoint is set, a breakpoint number is printed with ‘#’. This number can be used to delete the breakpoint, or to add conditions to it.

If /u is specified, set a breakpoint at a user-space address. Without /u, address is considered to be in the kernel-space, and an address in the wrong space will be rejected, and an error message will be emitted. This modifier may only be used if it is supported by machine dependent routines.

Warning: if a user text is shadowed by a normal user-space debugger, user-space breakpoints may not work correctly. Setting a breakpoint at the low-level code paths may also cause strange behavior.

bt[/ul] [frame-address][,count]

A synonym for trace.

bt/t[/ul] [pid][,count]

A synonym for trace/t.

bt/a[/ul] [lwpaddr][,count]

A synonym for trace/a.

call address[(expression[,...])]

Call the function specified by address with the argument(s) listed in parentheses. Parentheses may be omitted if the function takes no arguments. The number of arguments is currently limited to 10.

continue[/c]

Continue execution until a breakpoint or watchpoint. If /c is specified, count instructions while executing. Some machines (e.g., pmax) also count loads and stores.

Warning: when counting, the debugger is really silently single-stepping. This means that single-stepping on low-level may cause strange behavior.

delete address | #number

Delete a breakpoint. The target breakpoint may be specified by address, as per break, or by the breakpoint number returned by break if it's prefixed with ‘#’.

dmesg [count]

Prints the contents of the kernel message buffer. The optional count argument will limit printing to at most the last count bytes of the message buffer.

dwatch address

Delete the watchpoint at address that was previously set with watch command.

examine[/modifier] address[,count]

Display the address locations according to the format in modifier. Multiple modifier formats display multiple locations. If modifier isn't specified, the modifier from the last use of examine is used.

The valid format characters for modifier are:

b

examine bytes (8 bits).

h

examine half-words (16 bits).

l

examine words (legacy “long”, 32 bits).

L

examine long words (implementation dependent)

a

print the location being examined.

A

print the location with a line number if possible.

x

display in unsigned hex.

z

display in signed hex.

o

display in unsigned octal.

d

display in signed decimal.

u

display in unsigned decimal.

r

display in current radix, signed.

c

display low 8 bits as a character. Non-printing characters as displayed as an octal escape code (e.g., ‘\000’).

s

display the NUL terminated string at the location. Non-printing characters are displayed as octal escapes.

m

display in unsigned hex with a character dump at the end of each line. The location is displayed as hex at the beginning of each line.

i

display as a machine instruction.

I

display as a machine instruction, with possible alternative formats depending upon the machine:

alpha

print register operands

m68k

use Motorola syntax

vax

don't assume that each external label is a procedure entry mask

kill pid[,signal_number]

Send a signal to the process specified by the pid. Note that pid is interpreted using the current radix (see trace/t command for details). If signal_number isn't specified, the SIGTERM signal is sent.

match[/p]

A synonym for next.

next[/p]

Stop at the matching return instruction. If /p is specified, print the call nesting depth and the cumulative instruction count at each call or return. Otherwise, only print when the matching return is hit.

print[/axzodurc] address [address ...]

Print addresses address according to the modifier character, as per examine. Valid modifiers are: /a, /x, /z, /o, /d, /u, /r, and /c (as per examine). If no modifier is specified, the most recent one specified is used. address may be a string, and is printed “as-is”. For example:

print/x "eax = " $eax "\necx = " $ecx "\n"

will produce:

eax = xxxxxx 
ecx = yyyyyy

ps[/a][/n][/w][/l]

A synonym for show all procs.

reboot [flags]

Reboot, using the optionally supplied boot flags, which is a bitmask supporting the same values as for reboot(2). Some of the more useful flags:

Value	Name	Description
0x1	RB_ASKNAME	Ask for file name to reboot from
0x2	RB_SINGLE	Reboot to single user mode
0x4	RB_NOSYNC	Don't sync before reboot
0x8	RB_HALT	Halt instead of reboot
0x40	RB_KDB	Boot into kernel debugger
0x100	RB_DUMP	Dump unconditionally before reboot
0x808	RB_POWERDOWN	Power off (or at least halt)

Note: Limitations of the command line interface preclude specification of a boot string.

search[/bhl] address value [mask] [,count]

Search memory from address for value. The unit size is specified with a modifier character, as per examine. Valid modifiers are: /b, /h, and /l. If no modifier is specified, /l is used.

This command might fail in interesting ways if it doesn't find value. This is because ddb doesn't always recover from touching bad memory. The optional count limits the search.

set $variable [=] expression

Set the named variable or register to the value of expression. Valid variable names are described in VARIABLES.

show all callout

Display information about callouts in the system. See callout(9) for more information on callouts.

show all pages

Display basic information about all physical pages managed by the VM system. For more detailed information about a single page, use show page.

show all pools[/clp]

Display all pool information. Modifiers are the same as show pool.

show all procs[/a][/n][/w][/l]

Display all process information. Valid modifiers:

/n

show process information in a ps(1) style format. Information printed includes: process ID, parent process ID, process group, UID, process status, process flags, number of LWPs, command name, and process wait channel message.

/a

show each process ID, command name, kernel virtual addresses of each process' proc structure, u-area, and vmspace structure. The vmspace address is also the address of the process' vm_map structure, and can be used in the show map command.

/w

show each LWP ID, process ID, command name, system call emulation, priority, wait channel message and wait channel address. LWPs currently running on a CPU are marked with the '>' sign.

/l

show each LWP ID, process ID, process status, CPU ID the LWP runs on, process flags, kernel virtual address of LWP structure, LWP name and wait channel message. LWPs currently running on a CPU are marked with the '>' sign. This is the default.

show arptab

Dump the entire AF_INET routing table. This command is available only on systems which support inet and ARP.

show breaks

Display all breakpoints.

show buf[/f] address

Print the struct buf at address. The /f does nothing at this time.

show event[/f][/i][/m][/t]

Print all the non-zero evcnt(9) event counters. Valid modifiers:

/f

event counters with a count of zero are printed as well.

/i

interrupted counters will be displayed.

/m

misc counters will be displayed.

/t

trap counters will be displayed.

If none of /i, /m or /t are specified, all are shown. You can combine any of these. For example, the modifier /itf will select both interrupt and trap events, including those that are non-zero.

show files address

Display information about the vnodes of the files that are currently open by the process associated with the proc structure at address. This address can be found using the show all procs /a command. If the kernel is compiled with options LOCKDEBUG then details about the locking of the underlying uvm object will also be displayed.

show lock address

Display information about a lock at address. This command is useful only if a kernel is compiled with options LOCKDEBUG.

show map[/f] address

Print the vm_map at address. If /f is specified, the complete map is printed.

show mount[/f] address

Print the mount structure at address. If /f is specified, the complete vnode list is printed.

show mbuf[/c] address

Print the mbuf structure at address. If /c is specified, the mbufs in the chain are followed.

show ncache address

Dump the namecache list associated with vnode at address.

show object[/f] address

Print the vm_object at address. If /f is specified, the complete object is printed.

show page[/f] address

Print the vm_page at address. If /f is specified, the complete page is printed.

show panic

Print the current "panic" string.

show pool[/clp] address

Print the pool at address. Valid modifiers:

/c

Print the cachelist and its statistics for this pool.

/l

Print the log entries for this pool.

/p

Print the pagelist for this pool.

show proc[/ap] address | pid

Show information about a process and its LWPs. LWPs currently running on a CPU are marked with the '>' sign.

/a

The argument passed is the kernel virtual address of LWP structure.

/p

The argument passed is a PID. Note that pid is interpreted using the current radix (see trace/t command for details). This is the default.

show registers[/u]

Display the register set. If /u is specified, display user registers instead of kernel registers or the currently save one.

Warning: support for /u is machine dependent. If not supported, incorrect information will be displayed.

show sched_qs

Print the state of the scheduler's run queues. For each run queue that has an LWP, the run queue index and the list of LWPs will be shown. If the run queue has LWPs, but the sched_whichqs bit is not set for that queue, the queue index will be prefixed with a ‘!’.

show uvmexp

Print a selection of UVM counters and statistics.

show kernhist

Dumps the kernel histories. This command is available only if a kernel is compiled with options KERNHIST or options UVMHIST.

show vnode[/f] address

Print the vnode at address. If /f is specified, the complete vnode is printed.

show watches

Display all watchpoints.

sifting[/F] string

Search the symbol tables for all symbols of which string is a substring, and display them. If /F is specified, a character is displayed immediately after each symbol name indicating the type of symbol.

For a.out(5)-format symbol tables, absolute symbols display @, text segment symbols display *, data segment symbols display +, BSS segment symbols display -, and filename symbols display /. For ELF-format symbol tables, object symbols display +, function symbols display *, section symbols display &, and file symbols display /.

To sift for a string beginning with a number, escape the first character with a backslash as:

sifting \386

step[/p] [,count]

Single-step count times. If /p is specified, print each instruction at each step. Otherwise, only print the last instruction.

Warning: depending on the machine type, it may not be possible to single-step through some low-level code paths or user-space code. On machines with software-emulated single-stepping (e.g., pmax), stepping through code executed by interrupt handlers will probably do the wrong thing.

sync

Sync the disks, force a crash dump, and then reboot.

trace[/u[l]] [frame-address][,count]

Stack trace from frame-address. If /u is specified, trace user-space, otherwise trace kernel-space. count is the number of frames to be traced. If count is omitted, all frames are printed. If /l is specified, the trace is printed and also stored in the kernel message buffer.

Warning: user-space stack trace is valid only if the machine dependent code supports it.

trace/t[l] [pid][,count]

Stack trace by “thread” (process, on NetBSD) rather than by stack frame address. Note that pid is interpreted using the current radix, whilst ps displays pids in decimal; prefix pid with ‘0t’ to force it to be interpreted as decimal (see VARIABLES section for radix). If /l is specified, the trace is printed and also stored in the kernel message buffer.

Warning: trace by pid is valid only if the machine dependent code supports it.

trace/a[l] [lwpaddr][,count]

Stack trace by light weight process (LWP) address rather than by stack frame address. If /l is specified, the trace is printed and also stored in the kernel message buffer.

Warning: trace by LWP address is valid only if the machine dependent code supports it.

until[/p]

Stop at the next call or return instruction. If /p is specified, print the call nesting depth and the cumulative instruction count at each call or return. Otherwise, only print when the matching return is hit.

watch address[,size]

Set a watchpoint for a region. Execution stops when an attempt to modify the region occurs. size defaults to 4.

If you specify a wrong space address, the request is rejected with an error message.

Warning: attempts to watch wired kernel memory may cause an unrecoverable error in some systems such as i386. Watchpoints on user addresses work the best.

whatis address

Describe what an address is.

write[/bhlBHL] address expression [expression ...]

Write the expressions at succeeding locations. The unit size is specified with a modifier character, as per examine. Valid modifiers are: /b, /h, and /l. If no modifier is specified, /l is used.

Specifying the modifiers in upper case, /B, /H, /L, will prevent ddb from reading the memory location first, which is useful for avoiding side effects when writing to I/O memory regions.

Warning: since there is no delimiter between expressions, strange things may occur. It's best to enclose each expression in parentheses.

x[/modifier] address[,count]

A synonym for examine.

MACHINE-SPECIFIC COMMANDS

The "glue" code that hooks ddb into the NetBSD kernel for any given port can also add machine specific commands to the ddb command parser. All of these commands are preceded by the command word machine to indicate that they are part of the machine-specific command set (e.g. machine reboot). Some of these commands are:

VARIABLES

ddb accesses registers and variables as $name. Register names are as per the show registers command. Some variables are suffixed with numbers, and may have a modifier following a colon immediately after the variable name. For example, register variables may have a ‘u’ modifier to indicate user register (e.g., $eax:u).

Built-in variables currently supported are:

lines

The number of lines. This is used by the more feature. When this variable is set to zero the more feature is disabled.

maxoff

Addresses are printed as 'symbol'+offset unless offset is greater than maxoff.

maxwidth

The width of the displayed line. ddb wraps the current line by printing new line when maxwidth column is reached. When this variable is set to zero ddb doesn't perform any wrapping.

onpanic

If greater than zero (the default is 1), ddb will be invoked when the kernel panics. If the kernel configuration option

options DDB_ONPANIC=0

is used, onpanic will be initialized to off, causing a stack trace to be printed and the system to be rebooted instead of ddb being entered. Other useful settings are -1, which suppresses the stack trace before reboot, and 2, which causes a stack trace to be printed and ddb to be entered.

fromconsole

If non-zero (the default), the kernel allows to enter ddb from the console (by break signal or special key sequence). If the kernel configuration option

options DDB_FROMCONSOLE=0

is used, fromconsole will be initialized to off.

radix

Input and output radix.

tabstops

Tab stop width.

tee_msgbuf

If explicitly set to non zero (zero is the default) all ddb output will not only be displayed on screen but also be fed to the msgbuf. The default of the variable can be set using the kernel configuration option

options DDB_TEE_MSGBUF=1

which will initialize tee_msgbuf to be 1. This option is especially handy for poor souls who don't have a serial console but want to recall ddb output from a crash investigation. This option is more generic than the /l command modifier possible for selected commands as discussed above to log the output. Mixing both /l and this setting can give double loggings.

All built-in variables are accessible via sysctl(3).

EXPRESSIONS

Almost all expression operators in C are supported, except ‘~’, ‘^’, and unary ‘&’. Special rules in ddb are:

identifier

name of a symbol. It is translated to the address (or value) of it. ‘.’ and ‘:’ can be used in the identifier. If supported by an object format dependent routine, [filename:]function[:line number], [filename:]variable, and filename[:line number], can be accepted as a symbol. The symbol may be prefixed with symbol_table_name:: (e.g., emulator::mach_msg_trap) to specify other than kernel symbols.

number

number. Radix is determined by the first two characters: ‘0x’ - hex, ‘0o’ - octal, ‘0t’ - decimal, otherwise follow current radix.

.

dot

+

next

..

address of the start of the last line examined. Unlike dot or next, this is only changed by the examine or write commands.

"

last address explicitly specified.

$name

register name or variable. It is translated to the value of it. It may be followed by a ‘:’ and modifiers as described above.

#

a binary operator which rounds up the left hand side to the next multiple of right hand side.

*expr

expression indirection. It may be followed by a ‘:’ and modifiers as described above.

SEE ALSO

reboot(2), options(4), crash(8), reboot(8), sysctl(8), cnmagic(9)

HISTORY

The ddb kernel debugger was written as part of the MACH project at Carnegie-Mellon University.