NAME
ddb
—
kernel debugger
DESCRIPTION
The ddb
debugger provides a means for
debugging the kernel, and analysing the kernel after a system crash
("panic"), with a
gdb(1)-like syntax.
ddb
is invoked upon a kernel panic when
the sysctl(8) ddb.panic is set to 1. It may be
invoked from the console when the sysctl ddb.console
is set to 1, using any of the following methods:
- Using the key sequence
Ctrl-Alt-Esc
. - Sending a
BREAK
when using a serial console. - Writing to the sysctl ddb.trigger.
- For i386 and amd64 architectures, using the key sequence
Ctrl-Alt-Delete
when the sysctl machdep.kbdreset is set to 2.
ddb
prompts for commands on the console
with:
ddb>
The general syntax of a ddb
command
is:
/
modifiers]
[address][,
count]To save typing, ddb
makes use of a context
inferred from previous commands. In this context, the current location is
called dot. The examine
,
search
, show struct
, and
write
commands update dot to
be that of the last address examined or the last location modified, and have
intuitive effects on next and
prev. All the other commands do not change
dot, and set next to be the
same. (See VARIABLES.)
An expression can be used in place of address (see EXPRESSIONS). Omitting address in a command uses the last value of dot. A missing count is taken to be 1 for printing commands or infinity for stack traces. Entering a blank line causes the last command to be repeated using next in place of address, a count of 1, and no modifiers.
ddb
has a feature like
more(1) for the output. If the number of lines output in response to
one command exceeds the number set in the $lines
variable, it displays the message
‘--db_more--
’ and waits for a
response.
The valid responses are:
- ⟨
space
⟩ - One more page.
- ⟨
return
⟩ - One more line.
q
- Abort the current command, and return to the command input mode.
The following command line editing keys are provided:
^b
- back one character
^f
- forward one character
^a
- beginning of line
^e
- end of line
^w
- erase word back
^h
| ⟨del
⟩- erase previous character
^d
- erase next character
^k
- delete to end of line
^u
- delete line
^p
- previous in command history
^n
- next in command history
^r
- redraw line
^t
- exchange the two characters to the left of the cursor
COMMANDS
The following commands may be typed at the
‘ddb>
’ prompt. Some commands
consist of more than one word, and if only the first word or words are
entered, the possible alternatives to complete the command are displayed and
no other action is performed.
help
- List the available commands.
- [
e
]x
[amine
] [/bhlqaAxzodurcsmiI
] [addr][,
count] - Display the contents at address addr according to
the formats in the modifier. If no format is specified, the last formats
specified for this command are used.
The format characters are:
/b
- look at by bytes (8 bits)
/h
- look at by half words (16 bits)
/l
- look at by long words (32 bits) (default)
/q
- look at by long longs (64 bits) (only available on 64-bit platforms)
/a
- print the location being displayed
/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 are displayed as an octal escape code (e.g., '\000').
/s
- display the null-terminated string at the location. Non-printing characters are displayed as octal escapes.
/m
- display in unsigned hex with character dump at the end of each line. The location is also displayed in hex at the beginning of each line.
/i
- display as an instruction
/I
- display as an alternate format instruction depending on the machine:
- alpha
- Print affected register contents for every instruction.
- amd64,
- i386
- Do not skip padding to the next long word boundary for unconditional jumps.
- m88k
- Decode instructions for the opposite CPU model (e.g. m88110 when running on an m88100 processor).
The value of next is set to the addr plus the size of the data examined.
p
[rint
] [/axzodurc
] [addr]- Print addr according to the modifier character. The
valid modifiers are a subset of those from the
examine
command, and act as described there. If no modifier is specified, the last one specified in a previous use ofprint
is used.For example,
print/x $eax
will print something like this:
xxxxxx
pp
[rint
] [addr]- Pretty-print addr using CTF debug symbols included in the kernel binary image. The CTF section is normally added by running ctfstrip(1) as part of building a new kernel.
w
[rite
] [/bhl
] [addr] expr [expr ...]- Write the value of each expr expression at
succeeding locations start at addr. The write unit
size can be specified using one of the modifiers:
The value of next is set to addr plus the size of values written.
Warning: since there is no delimiter between expressions, the command may not parse as you expect. It is best to enclose each expression in parentheses.
set
$
name [=
] expr- Set the named variable or register with the value of expr. For valid variable names, see VARIABLES.
boot
how- Reboot the machine depending on how:
boot sync
- Sync disks and reboot.
boot crash
- Dump core and reboot.
boot dump
- Sync disks, dump core and reboot.
boot halt
- Just halt.
boot reboot
- Just reboot.
boot poweroff
- Power down the machine whenever possible; if it fails, just halt.
break
[addr][,
count]- Set a break point at addr. If
count is supplied,
ddb
allows the breakpoint to be silently hit (count - 1) times before stopping at the break point. d
[elete
] [addr]- Delete the break point set with the
break
command. s
[tep
] [/p
] [,
count]- Single step count times. If the
/p
modifier is specified, print each instruction at each step. Otherwise, only print the last instruction.Warning: depending on machine type, it may not be possible to single-step through some low-level code paths. On machines with software-emulated single-stepping (e.g., alpha), stepping through code executed by interrupt handlers will probably do the wrong thing.
call
name(
expr [,
expr ...])
- Call the function named by name 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.
c
[ontinue
] [/c
]- Continue execution until a breakpoint or watchpoint. If the
/c
modifier is given, instructions are counted while executing.Warning: when counting with
/c
,ddb
is really silently single-stepping. This means that single-stepping on low-level code may cause strange behavior. watch
addr [,
size]- Set a watchpoint for the region starting at addr.
Execution stops and control returns to
ddb
when an attempt is made to modify a watched region. The size argument 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 on some systems (e.g., i386).
dwatch
addr- Delete the watchpoint at address addr that was
previously set with a
watch
command. hangman
[/s
[0-9
]]- This is a tiny and handy tool for random kernel hangs analysis, of which its depth is controlled by the optional argument of the default value of five. It uses some sophisticated heuristics to spot the global symbol that caused the hang. Since the discovering algorithm is a probabilistic one, you may spend substantial time to figure the exact symbol name. This smart thing requires a little of your attention, the input it accepts is mostly of the same format as that of the famous hangman(6) game, to which it, apparently, is obliged by the name. Hint: the nm(1) utility might help.
until
[/p
]- Stop at the next "call" or "return" instruction. If
the
/p
modifier is specified,ddb
prints the call nesting depth and the cumulative instruction count at each call or return. Otherwise, it stays silent until the matching return is hit. match
[/p
]- Stop at the next matching return instruction. If the
/p
modifier is specified,ddb
prints the call nesting depth and the cumulative instruction count at each call or return. Otherwise, it remains mostly quiet. next
[/p
]- The
next
command is a synonym formatch
. kill
pid- Send an uncatchable
SIGABRT
signal to the process specified by the pid argument. trace
[/tu
] [frameaddr][,
count]- Show the stack trace. The
/t
modifier interprets the frameaddr argument as the TID of a process and shows the stack trace of that process. frameaddr is subject to the radix; use the 0t prefix to enter a decimal TID. The/t
modifier is not supported on all platforms. The/u
modifier shows the stack trace of user space; if omitted, the kernel stack is traced instead. The count argument is the limit on the number of frames to be followed. If count is omitted, all frames are printed.Warning: user space stack trace is valid only if the machine dependent code supports it.
search
[/bhl
] [addr] value [mask] [,
count]- Search memory for a value beginning at addr. This
command might fail in interesting ways if it doesn't find the searched-for
value. This is because
ddb
doesn't always recover from touching bad memory. The optional count argument limits the search. The modifiers are the same as those of thewrite
command.The next address is set to the address where value is found, or just after where the search area finishes.
show
what- Displays various things, depending on what:
show bcstats
- Prints the buffer cache statistics.
show breaks
- Prints a list of all breakpoints that have been set with the
break
command. show buf
[/f
] addr- Prints the
struct buf
at addr. If the/f
modifier is specified output will also includesoftdep
printout, if those are available. show extents
- Prints a detailed list of all extents.
show locks
[addr]- Prints the list of locks held by a thread. If an optional address is
not specified,
curproc
is assumed. Theoption WITNESS
is required for this command to be available. show malloc
[addr]- Prints malloc debugging information if available. If an optional address is specified, only information about that address is printed.
show map
[/f
] addr- Prints the
vm_map
at addr. If the/f
modifier is specified the complete map is printed. show mbuf
addr- Prints the
struct mbuf
header at addr. Depending on the mbuf flagsstruct pkthdr
andstruct m_ext
are printed as well. show mount
[/f
] addr- Prints the
struct mount
at addr. If the/f
modifier is specified prints out allvnodes
(see alsoshow vnode
) and also allbufs
(see alsoshow buf
) on all those vnodes. show nfsnode
[/f
] addr- Prints the
struct nfsnode
at addr. If the/f
modifier is specified prints out additional information as well. show nfsreq
[/f
] addr- Prints the
struct nfsreq
at addr. If the/f
modifier is specified prints out additional information as well. show object
[/f
] addr- Prints the
vm_object
at addr. If the/f
modifier is specified the complete object is printed. show page
[/f
] addr- Prints the
vm_page
at addr. If the/f
modifier is specified the complete page is printed. show panic
- Prints the panic string.
show pool
[/p
] addr- Prints the
pool
at addr. Valid modifiers:/p
- Print the pagelist for this pool.
show proc
[addr]- Prints the
struct proc
at addr. If an optional address is not specifiedcurproc
is assumed. show registers
[/u
]- Display the register set. If the
/u
modifier is specified, it displays user registers (or the currently saved registers) instead of the kernel's. Note: The/u
modifier is not supported on every machine, in which case incorrect information may be displayed. show socket
addr- Prints the
struct socket
at addr. If the socket is spliced, thestruct sosplice
associated with the socket is printed as well. show struct
name [addr]- Prints the content of the memory at addr as a struct name. Nested structures and bit fields are not printed. Character arrays are printed as bytes.
show uvmexp
- Displays a selection of uvm counters and statistics.
show vnode
[/f
] addr- Prints the
struct vnode
at addr. If the/f
modifier is specified prints out allbufs
(see alsoshow buf
) currently attached to thisvnode
. show watches
- Displays all watchpoints set with the
watch
command. show witness
[/b
]- Prints the current order list. If the
/b
modifier is specified, the list of found lock order violations is printed instead. Theoption WITNESS
is required for this command to be available. show all procs
[/anow
]- Display information on all processes.
/n
- (Default) Show process information in a ps(1)-like format. Information printed includes process ID, thread ID, parent process ID, UID, process status, process flags, process wait channel message and process command name.
/a
- Shows the 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 theshow map
command. /o
- Shows non-idle threads that were on CPU when ddb was entered. Information printed includes thread ID, process ID, UID, process flags, thread flags, current CPU, and command name.
/w
- Shows each thread's ID, command, process group, wait channel address, and wait channel message.
show all bufs
[/f
]- Display information about all buffers in the system.
/f
- For each buffer, print a more detailed output. See the
show buf
command for more information.
show all callout
- Display the contents of the callout table.
show all pools
[/a
]- Display information about all system pools in a format similar to
vmstat(8).
/a
- Displays “interesting” address information.
show all locks
- Prints the list of locks held by all threads in the system. The
option WITNESS
is required for this command to be available. show all mounts
[/f
]- Display information on all mounted filesystems.
/f
- For each filesystem, list all its struct vnode addresses. These
addresses can be used in the
show vnode
command.
show all nfsnodes
[/f
]- Display information about all nfsnodes in the system.
/f
- For each nfsnode, print a more detailed output. See the
show nfsnode
command for more information.
show all nfsreqs
[/f
]- Display information for all outstanding NFS requests.
/f
- For each NFS requests, print a more detailed output. See the
show nfsreq
command for more information.
show all vnodes
[/f
]- Display information about all vnodes in the system.
/f
- For each vnode, print a more detailed output. See the
show vnode
command for more information.
callout
- A synonym for the
show all callout
command. ps
[/anow
]- A synonym for
show all procs
. mac
[hine
] subcommand [args ...]- Perform a platform-specific command.
The following commands are supported by multiprocessor kernels on these platforms: amd64, i386, macppc, mips64, and sparc64.
cpuinfo
- Display the state of each CPU.
ddbcpu
N- Stop the current CPU and start handling
ddb
on the selected CPU. startcpu
[N]- Resume normal processing on the selected CPU, or all CPUs if none is specified.
stopcpu
[N]- Stop normal processing on the selected CPU, or all CPUs (except the
one handling
ddb
) if none is specified.
Other platform-specific commands:
arm:
frame
addr- Display the trapframe at addr.
i386:
sysregs
- Display the contents of the privileged registers: IDTR, GDTR, LDTR, TR, CR0, CR2, CR3, and CR4.
m88k:
ddbcpu
N- Stop the current CPU and start handling
ddb
on the selected CPU. frame
addr- Display the trapframe at addr.
regs
- Display the registers from when
ddb
was entered. searchframe
[addr]- Search for and display stack exception frames, starting from addr if given, else the address in register r31, and stopping at the next 8k boundary.
where
- Display where the current CPU was stopped.
mips64:
tlb
[/p
asid] [/c
] [tlb]trap
??
sh:
cache
[addr]- Display the cache, starting from addr, defaulting to 0.
frame
- Display the switch and trap frames.
tlb
- Display the TLB.
sparc64:
ctx
- Display the context addresses for all threads.
VARIABLES
ddb
denotes registers and variables by
$
name. Register names can be
found with the show registers
command.
Some variable names are suffixed with numbers, and some may have a
modifier following a colon immediately after the variable name. For example,
register variables can have the ‘:u
’
modifier to indicate a user register (e.g.,
‘$eax:u
’).
Built-in debugger variables currently supported are:
- $radix
- Input and output radix.
- $maxoff
- Addresses are printed as
symbol
+
offset unless offset is greater than $maxoff. - $maxwidth
- The width of the displayed lines.
- $lines
- The number of lines to page. This is used by the “more” feature.
- $tabstops
- Tab stop width.
- $log
- Controls whether the output of
ddb
will also appear in the system message buffer.
These variables can also be controlled outside
ddb
via the ‘ddb’
sysctl(8) hierarchy.
EXPRESSIONS
Almost all expression operators in C are supported except for
‘~
’,
‘^
’, and unary
‘&
’. Special rules for expressions
in ddb
are:
- identifier
- The name of a symbol. It is translated to the address (or value) of the
symbol. ‘
.
’ and ‘:
’ can be used in the identifier. The following can be accepted as an identifier, if supported by an object format dependent routine:- [filename
:
]func[:linenumber] - [filename:]variable
- filename[:linenumber]
emulator::mach_msg_trap
’) to specify other than kernel symbols. - [filename
- number
- The radix is determined by the first two letters:
‘
0x
’: hex, ‘0o
’: octal, ‘0t
’: decimal, otherwise, the value of $radix is used. .
- dot: the current address.
+
- next: the next address.
..
- The address of the start of the last line examined. Unlike
dot or next, this is only
changed by the
examine
orwrite
command. '
- The last address explicitly specified.
$
variable- The value of a register or variable. The name may be followed by a
‘
:
’ and modifiers as described above with identifier. - expr
#
expr - A binary operator which rounds up the left hand side to the next multiple of right hand side.
*
expr- Indirection. It may be followed by a ':' and modifiers as described above.
SEE ALSO
ctfstrip(1), gdb(1), nm(1), witness(4), sysctl.conf(5), hangman(6), crash(8), sysctl(8), extent(9), pool(9), uvm_init(9)
HISTORY
This kernel facility first appeared in the MACH 2 operating system developed by CMU. Hangman (which stands for "hangs maniacal analyzer") first appeared in OpenBSD 1.2.