DDB(4) OpenBSD Programmer's Manual DDB(4)NAMEddb - 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:
command [/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. Multiple modifier formats display multiple
locations. 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.
m68k Use Motorola assembly syntax.
m88k Decode instructions for the opposite CPU
model (e.g. m88110 when running on an m88100
processor).
vax Don't assume that each external label is a
procedure entry mask.
The value of next is set to the addr plus the size of the
data examined.
print [/axzodurc] [addr [addr ...]]
Print each 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 of print
is used. The addr argument can be a string, and it is
printed as a literal.
For example,
print/x "eax = " $eax "\necx = " $ecx "\n"
will print something like this:
eax = xxxxxx
ecx = yyyyyy
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:
/b byte (8 bits)
/h half word (16 bits)
/l long word (32 bits) (default)
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.
Valid variable names are described below.
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.
If the break point is successfully set, a break point number
is displayed, in the form #number. This can later be used in
deleting the break point or for adding conditions to it.
d[elete] [addr | #number]
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 for match.
trace [/pu] [frameaddr] [,count]
Show the stack trace. The /p modifier interprets the
frameaddr argument as the PID of a process and shows the
stack trace of that process. The /p 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 the write 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 include softdep printout, if
those are available.
show extents
Prints a detailed list of all extents.
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 flags struct pkthdr and struct m_ext are
printed as well.
show mount [/f] addr
Prints the struct mount at addr. If the /f modifier is
specified prints out all vnodes (see also show vnode)
and also all bufs (see also show 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 offset offset
Prints a list of the known kernel structure fields
which occur at the given offset from the beginning of
the struct, as well as their size. The option
DDB_STRUCT is required for this command to be
available.
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 [/clp] addr
Prints the pool at addr. 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 [addr]
Prints the struct proc at addr. If an optional address
is not specified curproc 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 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. The
option DDB_STRUCT is required for this command to be
available.
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 all bufs (see also show buf)
currently attached to this vnode.
show watches
Displays all watchpoints set with the watch command.
show all procs [/anw]
Display information on all processes.
/n (Default) Show process information in a ps(1)-like
format. Information printed includes process ID,
parent process ID, process group, UID, process
status, process flags, process command name, and
process wait channel message.
/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 the show map command.
/w Shows each process' PID, command, system call
emulation, 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 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 [/anw] A synonym for show all procs.
VARIABLESddb 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]
The symbol may be prefixed with
`symboltablename::' (e.g.,
`emulator::mach_msg_trap') to specify other than
kernel symbols.
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 or write 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 ALSOgdb(1), nm(1), sysctl.conf(5), hangman(6), kgdb(7), crash(8), sysctl(8),
extent(9), pool(9), uvm(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.
OpenBSD 4.9 November 27, 2010 OpenBSD 4.9
