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GDB(4)			 BSD Kernel Interfaces Manual			GDB(4)

NAME
     gdb — external kernel debugger

SYNOPSIS
     makeoptions DEBUG=-g
     options DDB

DESCRIPTION
     The gdb kernel debugger is a variation of gdb(1) which understands some
     aspects of the FreeBSD kernel environment.	 It can be used in a number of
     ways:

     ·	 It can be used to examine the memory of the processor on which it
	 runs.

     ·	 It can be used to analyse a processor dump after a panic.

     ·	 It can be used to debug another system interactively via a serial or
	 firewire link.	 In this mode, the processor can be stopped and single
	 stepped.

     ·	 With a firewire link, it can be used to examine the memory of a
	 remote system without the participation of that system.  In this
	 mode, the processor cannot be stopped and single stepped, but it can
	 be of use when the remote system has crashed and is no longer
	 responding.

     When used for remote debugging, gdb requires the presence of the ddb(4)
     kernel debugger.  Commands exist to switch between gdb and ddb(4).

PREPARING FOR DEBUGGING
     When debugging kernels, it is practically essential to have built a ker‐
     nel with debugging symbols (makeoptions DEBUG=-g).	 It is easiest to per‐
     form operations from the kernel build directory, by default
     /usr/obj/usr/src/sys/GENERIC.

     First, ensure you have a copy of the debug macros in the directory:

	   make gdbinit

     This command performs some transformations on the macros installed in
     /usr/src/tools/debugscripts to adapt them to the local environment.

   Inspecting the environment of the local machine
     To look at and change the contents of the memory of the system you are
     running on,

	   gdb -k -wcore kernel.debug /dev/mem

     In this mode, you need the -k flag to indicate to gdb(1) that the “dump
     file” /dev/mem is a kernel data file.  You can look at live data, and if
     you include the -wcore option, you can change it at your peril.  The sys‐
     tem does not stop (obviously), so a number of things will not work.  You
     can set breakpoints, but you cannot “continue” execution, so they will
     not work.

   Debugging a crash dump
     By default, crash dumps are stored in the directory /var/crash.  Investi‐
     gate them from the kernel build directory with:

	   gdb -k kernel.debug /var/crash/vmcore.29

     In this mode, the system is obviously stopped, so you can only look at
     it.

   Debugging a live system with a remote link
     In the following discussion, the term “local system” refers to the system
     running the debugger, and “remote system” refers to the live system being
     debugged.

     To debug a live system with a remote link, the kernel must be compiled
     with the option options DDB.  The option options BREAK_TO_DEBUGGER
     enables the debugging machine stop the debugged machine once a connection
     has been established by pressing ‘^C’.

   Debugging a live system with a remote serial link
     When using a serial port for the remote link on the i386 platform, the
     serial port must be identified by setting the flag bit 0x80 for the spec‐
     ified interface.  Generally, this port will also be used as a serial con‐
     sole (flag bit 0x10), so the entry in /boot/device.hints should be:

	   hint.sio.0.flags="0x90"

   Debugging a live system with a remote firewire link
     As with serial debugging, to debug a live system with a firewire link,
     the kernel must be compiled with the option options DDB.

     A number of steps must be performed to set up a firewire link:

     ·	 Ensure that both systems have firewire(4) support, and that the ker‐
	 nel of the remote system includes the dcons(4) and dcons_crom(4)
	 drivers.  If they are not compiled into the kernel, load the KLDs:

	       kldload firewire

	 On the remote system only:

	       kldload dcons
	       kldload dcons_crom

	 You should see something like this in the dmesg(8) output of the
	 remote system:

	       fwohci0: BUS reset
	       fwohci0: node_id=0x8800ffc0, gen=2, non CYCLEMASTER mode
	       firewire0: 2 nodes, maxhop <= 1, cable IRM = 1
	       firewire0: bus manager 1
	       firewire0: New S400 device ID:00c04f3226e88061
	       dcons_crom0: <dcons configuration ROM> on firewire0
	       dcons_crom0: bus_addr 0x22a000

	 It is a good idea to load these modules at boot time with the follow‐
	 ing entry in /boot/loader.conf:

	       dcons_crom_enable="YES"

	 This ensures that all three modules are loaded.  There is no harm in
	 loading dcons(4) and dcons_crom(4) on the local system, but if you
	 only want to load the firewire(4) module, include the following in
	 /boot/loader.conf:

	       firewire_enable="YES"

     ·	 Next, use fwcontrol(8) to find the firewire node corresponding to the
	 remote machine.  On the local machine you might see:

	       # fwcontrol
	       2 devices (info_len=2)
	       node	   EUI64	status
		  1  0x00c04f3226e88061	     0
		  0  0x000199000003622b	     1

	 The first node is always the local system, so in this case, node 0 is
	 the remote system.  If there are more than two systems, check from
	 the other end to find which node corresponds to the remote system.
	 On the remote machine, it looks like this:

	       # fwcontrol
	       2 devices (info_len=2)
	       node	   EUI64	status
		  0  0x000199000003622b	     0
		  1  0x00c04f3226e88061	     1

     ·	 Next, establish a firewire connection with dconschat(8):

	       dconschat -br -G 5556 -t 0x000199000003622b

	 0x000199000003622b is the EUI64 address of the remote node, as deter‐
	 mined from the output of fwcontrol(8) above.  When started in this
	 manner, dconschat(8) establishes a local tunnel connection from port
	 localhost:5556 to the remote debugger.	 You can also establish a con‐
	 sole port connection with the -C option to the same invocation
	 dconschat(8).	See the dconschat(8) manpage for further details.

	 The dconschat(8) utility does not return control to the user.	It
	 displays error messages and console output for the remote system, so
	 it is a good idea to start it in its own window.

     ·	 Finally, establish connection:

	       # gdb kernel.debug
	       GNU gdb 5.2.1 (FreeBSD)
	       (political statements omitted)
	       Ready to go.  Enter 'tr' to connect to the remote target
	       with /dev/cuad0, 'tr /dev/cuad1' to connect to a different port
	       or 'trf portno' to connect to the remote target with the firewire
	       interface.  portno defaults to 5556.

	       Type 'getsyms' after connection to load kld symbols.

	       If you are debugging a local system, you can use 'kldsyms' instead
	       to load the kld symbols.	 That is a less obnoxious interface.
	       (gdb) trf
	       0xc21bd378 in ?? ()

	 The trf macro assumes a connection on port 5556.  If you want to use
	 a different port (by changing the invocation of dconschat(8) above),
	 use the tr macro instead.  For example, if you want to use port 4711,
	 run dconschat(8) like this:

	       dconschat -br -G 4711 -t 0x000199000003622b

	 Then establish connection with:

	       (gdb) tr localhost:4711
	       0xc21bd378 in ?? ()

   Non-cooperative debugging a live system with a remote firewire link
     In addition to the conventional debugging via firewire described in the
     previous section, it is possible to debug a remote system without its
     cooperation, once an initial connection has been established.  This cor‐
     responds to debugging a local machine using /dev/mem.  It can be very
     useful if a system crashes and the debugger no longer responds.  To use
     this method, set the sysctl(8) variables hw.firewire.fwmem.eui64_hi and
     hw.firewire.fwmem.eui64_lo to the upper and lower halves of the EUI64 ID
     of the remote system, respectively.  From the previous example, the
     remote machine shows:

	   # fwcontrol
	   2 devices (info_len=2)
	   node	       EUI64	    status
	      0	 0x000199000003622b	 0
	      1	 0x00c04f3226e88061	 1

     Enter:

	   # sysctl -w hw.firewire.fwmem.eui64_hi=0x00019900
	   hw.firewire.fwmem.eui64_hi: 0 -> 104704
	   # sysctl -w hw.firewire.fwmem.eui64_lo=0x0003622b
	   hw.firewire.fwmem.eui64_lo: 0 -> 221739

     Note that the variables must be explicitly stated in hexadecimal.	After
     this, you can examine the remote machine's state with the following
     input:

	   # gdb -k kernel.debug /dev/fwmem0.0
	   GNU gdb 5.2.1 (FreeBSD)
	   (messages omitted)
	   Reading symbols from /boot/kernel/dcons.ko...done.
	   Loaded symbols for /boot/kernel/dcons.ko
	   Reading symbols from /boot/kernel/dcons_crom.ko...done.
	   Loaded symbols for /boot/kernel/dcons_crom.ko
	   #0  sched_switch (td=0xc0922fe0) at /usr/src/sys/kern/sched_4bsd.c:621
	   0xc21bd378 in ?? ()

     In this case, it is not necessary to load the symbols explicitly.	The
     remote system continues to run.

COMMANDS
     The user interface to gdb is via gdb(1), so gdb(1) commands also work.
     This section discusses only the extensions for kernel debugging that get
     installed in the kernel build directory.

   Debugging environment
     The following macros manipulate the debugging environment:

     ddb     Switch back to ddb(4).  This command is only meaningful when per‐
	     forming remote debugging.

     getsyms
	     Display kldstat information for the target machine and invite
	     user to paste it back in.	This is required because gdb does not
	     allow data to be passed to shell scripts.	It is necessary for
	     remote debugging and crash dumps; for local memory debugging use
	     kldsyms instead.

     kldsyms
	     Read in the symbol tables for the debugging machine.  This does
	     not work for remote debugging and crash dumps; use getsyms
	     instead.

     tr interface
	     Debug a remote system via the specified serial or firewire inter‐
	     face.

     tr0     Debug a remote system via serial interface /dev/cuad0.

     tr1     Debug a remote system via serial interface /dev/cuad1.

     trf     Debug a remote system via firewire interface at default port
	     5556.

     The commands tr0, tr1 and trf are convenience commands which invoke tr.

   The current process environment
     The following macros are convenience functions intended to make things
     easier than the standard gdb(1) commands.

     f0	     Select stack frame 0 and show assembler-level details.

     f1	     Select stack frame 1 and show assembler-level details.

     f2	     Select stack frame 2 and show assembler-level details.

     f3	     Select stack frame 3 and show assembler-level details.

     f4	     Select stack frame 4 and show assembler-level details.

     f5	     Select stack frame 5 and show assembler-level details.

     xb	     Show 12 words in hex, starting at current ebp value.

     xi	     List the next 10 instructions from the current eip value.

     xp	     Show the register contents and the first four parameters of the
	     current stack frame.

     xp0     Show the first parameter of current stack frame in various for‐
	     mats.

     xp1     Show the second parameter of current stack frame in various for‐
	     mats.

     xp2     Show the third parameter of current stack frame in various for‐
	     mats.

     xp3     Show the fourth parameter of current stack frame in various for‐
	     mats.

     xp4     Show the fifth parameter of current stack frame in various for‐
	     mats.

     xs	     Show the last 12 words on stack in hexadecimal.

     xxp     Show the register contents and the first ten parameters.

     z	     Single step 1 instruction (over calls) and show next instruction.

     zs	     Single step 1 instruction (through calls) and show next instruc‐
	     tion.

   Examining other processes
     The following macros access other processes.  The gdb debugger does not
     understand the concept of multiple processes, so they effectively bypass
     the entire gdb environment.

     btp pid
	     Show a backtrace for the process pid.

     btpa    Show backtraces for all processes in the system.

     btpp    Show a backtrace for the process previously selected with
	     defproc.

     btr ebp
	     Show a backtrace from the ebp address specified.

     defproc pid
	     Specify the PID of the process for some other commands in this
	     section.

     fr frame
	     Show frame frame of the stack of the process previously selected
	     with defproc.

     pcb proc
	     Show some PCB contents of the process proc.

   Examining data structures
     You can use standard gdb(1) commands to look at most data structures.
     The macros in this section are convenience functions which typically dis‐
     play the data in a more readable format, or which omit less interesting
     parts of the structure.

     bp	     Show information about the buffer header pointed to by the vari‐
	     able bp in the current frame.

     bpd     Show the contents (char *) of bp->data in the current frame.

     bpl     Show detailed information about the buffer header (struct bp)
	     pointed at by the local variable bp.

     bpp bp  Show summary information about the buffer header (struct bp)
	     pointed at by the parameter bp.

     bx	     Print a number of fields from the buffer header pointed at in by
	     the pointer bp in the current environment.

     vdev    Show some information of the vnode pointed to by the local vari‐
	     able vp.

   Miscellaneous macros
     checkmem
	     Check unallocated memory for modifications.  This assumes that
	     the kernel has been compiled with options DIAGNOSTIC This causes
	     the contents of free memory to be set to 0xdeadc0de.

     dmesg   Print the system message buffer.  This corresponds to the
	     dmesg(8) utility.	This macro used to be called msgbuf.  It can
	     take a very long time over a serial line, and it is even slower
	     via firewire or local memory due to inefficiencies in gdb.	 When
	     debugging a crash dump or over firewire, it is not necessary to
	     start gdb to access the message buffer: instead, use an appropri‐
	     ate variation of

		   dmesg -M /var/crash/vmcore.0 -N kernel.debug
		   dmesg -M /dev/fwmem0.0 -N kernel.debug

     kldstat
	     Equivalent of the kldstat(8) utility without options.

     pname   Print the command name of the current process.

     ps	     Show process status.  This corresponds in concept, but not in
	     appearance, to the ps(1) utility.	When debugging a crash dump or
	     over firewire, it is not necessary to start gdb to display the
	     ps(1) output: instead, use an appropriate variation of

		   ps -M /var/crash/vmcore.0 -N kernel.debug
		   ps -M /dev/fwmem0.0 -N kernel.debug

     y	     Kludge for writing macros.	 When writing macros, it is convenient
	     to paste them back into the gdb window.  Unfortunately, if the
	     macro is already defined, gdb insists on asking

		   Redefine foo?

	     It will not give up until you answer ‘y’.	This command is that
	     answer.  It does nothing else except to print a warning message
	     to remind you to remove it again.

SEE ALSO
     gdb(1), ps(1), ddb(4), firewire(4), dconschat(8), dmesg(8), fwcontrol(8),
     kldload(8)

AUTHORS
     This man page was written by Greg Lehey ⟨grog@FreeBSD.org⟩.

BUGS
     The gdb(1) debugger was never designed to debug kernels, and it is not a
     very good match.  Many problems exist.

     The gdb implementation is very inefficient, and many operations are slow.

     Serial debugging is even slower, and race conditions can make it diffi‐
     cult to run the link at more than 9600 bps.  Firewire connections do not
     have this problem.

     The debugging macros “just growed”.  In general, the person who wrote
     them did so while looking for a specific problem, so they may not be gen‐
     eral enough, and they may behave badly when used in ways for which they
     were not intended, even if those ways make sense.

     Many of these commands only work on the ia32 architecture.

BSD			       February 8, 2005				   BSD
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