DDB(4)

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:

command[/modifier] address [,count]

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:

,count

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:

  ACORN26
bsw Writes one or two bytes to the IObus. Takes an address and a value. Use the “b” modifier to write a single byte and the “h” modifier to write two bytes.
frame
    Given a trap frame address, print out the trap frame.
irqstat
    Display the IRQ statistics

  ALPHA
cpu Switch to another cpu.

  AMD64
cpu Switch to another cpu.

  ARM32
frame
    Given a trap frame address, print out the trap frame.

  HPPA
frame
    Without an address the default trap frame is printed. Otherwise, the trap frame address can be given, or, when the “l” modifier is used, an LWP address.

  I386
cpu Switch to another cpu.

  IA64
vector
    Without a vector, information about all 256 vectors is shown. Otherwise, the given vector is shown.

  MIPS
cp0 Dump CP0 (coprocessor 0) register values.
kvtop
    Print the physical address for a given kernel virtual address.
tlb Print out the Translation Lookaside Buffer (TLB). Only works in NetBSD kernels compiled with DEBUG option.

  POWERPC 4xx
ctx Print process MMU context information.
pv  Print PA-VA mapping information.
reset
    Reset the system.
tf  Display the contents of the trapframe.
tlb Display instruction translation storage buffer information.
dcr Set the DCR register. Must be between 0x00 and 0x3ff.
user
    Display user memory. Use the “i” modifier to get instruction decoding.

  POWERPC OEA
bat Print BAT registers and translations.
mmu Print MMU registers.

  SH3
tlb Print TLB entries.
cache
    Print cache entries.
frame
    Print switch frame and trap frames.
stack
    Print kernel stack usage. Only works in NetBSD kernels compiled with the KSTACK_DEBUG option.

  SPARC
cpu Switch to another cpu.
prom
    Enter the Sun PROM monitor.
proc
    Display some information about the LWP pointed to, or curlwp.
pcb Display information about the “struct pcb” listed.
page
    Display the pointer to the “struct vm_page” for this physical address.

  SPARC64
ctx Print process context information.
cpu Switch to another cpu.
dtlb
    Print data translation look-aside buffer context information.
dtsb
    Display data translation storage buffer information.
kmap
    Display information about the listed mapping in the kernel pmap. Use the “f” modifier to get a full listing.
extract
    Extract the physical address for a given virtual address from the kernel pmap.
fpstate
    Dump the FPU state.
itlb
    Print instruction translation look-aside buffer context information.
itsb
    Display instruction translation storage buffer information.
lwp Display a struct lwp
pcb Display information about the “struct pcb” listed.
pctx
    Attempt to change process context.
page
    Display the pointer to the “struct vm_page” for this physical address.
phys
    Display physical memory.
pmap
    Display the pmap. Use the “f” modifier to get a fuller listing.
proc
    Display some information about the process pointed to, or curproc.
prom
    Enter the OFW PROM.
pv  Display the “struct pv_entry” pointed to.
sir Reset the machine and enter prom (do a Software Initiated Reset).
stack
    Dump the window stack. Use the “u” modifier to get userland information.
tf  Display full trap frame state. This is most useful for inclusion with bug reports.
ts  Display trap state.
traptrace
    Display or set trap trace information. Use the “r” and “f” modifiers to get reversed and full information, respectively.
watch
    Set or clear a physical or virtual hardware watchpoint. Pass the address to be watched, or “0” (or omit the address) to clear the watchpoint. Optional modifiers are “p” for physical address, “r” for trap on read access (default: trap on write access only), “b” for 8 bit width, “h” for 16 bit, “l” for 32 bit or “L” for 64 bit.
window
    Print register window information. Argument is a stack frame number (0 is top of stack, which is used when no index is given).

  SUN2, SUN3 and SUN3X
abort
    Drop into monitor via abort (allows continue).
halt
    Exit to Sun PROM monitor as in halt(8).
reboot
    Reboot the machine as in reboot(8).
pgmap
    Given an address, print the address, segment map, page map, and Page Table Entry (PTE).

  VAX
cpu Switch to another cpu.

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.