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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 kernel with debugging
     symbols (makeoptions DEBUG=-g).  It is easiest to perform 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 system 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.  Investigate 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 specified interface.  Generally, this port
     will also be used as a serial console (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 kernel 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 following 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 determined 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 console 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/cuau0, 'tr /dev/cuau1' 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 corresponds 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 performing 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 interface.

     tr0     Debug a remote system via serial interface /dev/cuau0.

     tr1     Debug a remote system via serial interface /dev/cuau1.

     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 formats.

     xp1     Show the second parameter of current stack frame in various formats.

     xp2     Show the third parameter of current stack frame in various formats.

     xp3     Show the fourth parameter of current stack frame in various formats.

     xp4     Show the fifth parameter of current stack frame in various formats.

     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 instruction.

   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 display 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 variable 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 variable 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 appropriate 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 difficult to run the link
     at more than 9600 bps.  Firewire connections do not have this problem.

     The debugging macros “just grown”.  In general, the person who wrote them did so while
     looking for a specific problem, so they may not be general 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.