man/man4/gdb.4

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.Dd May 17, 2016
.Dt GDB 4
.Os
.Sh NAME
.Nm gdb
.Nd external kernel debugger
.Sh SYNOPSIS
.Cd "makeoptions DEBUG=-g"
.Cd "options DDB"
.Sh DESCRIPTION
The
.Nm
kernel debugger is a variation of
.Xr gdb 1
which understands some aspects of the
.Fx
kernel environment.
It can be used in a number of ways:
.Bl -bullet
.It
It can be used to examine the memory of the processor on which it runs.
.It
It can be used to analyse a processor dump after a panic.
.It
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.
.It
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.
.El
.Pp
When used for remote debugging,
.Nm
requires the presence of the
.Xr ddb 4
kernel debugger.
Commands exist to switch between
.Nm
and
.Xr ddb 4 .
.Sh PREPARING FOR DEBUGGING
When debugging kernels, it is practically essential to have built a kernel with
debugging symbols
.Pq Cd "makeoptions DEBUG=-g" .
It is easiest to perform operations from the kernel build directory, by default
.Pa /usr/obj/usr/src/sys/GENERIC .
.Pp
First, ensure you have a copy of the debug macros in the directory:
.Pp
.Dl "make gdbinit"
.Pp
This command performs some transformations on the macros installed in
.Pa /usr/src/tools/debugscripts
to adapt them to the local environment.
.Ss "Inspecting the environment of the local machine"
To look at and change the contents of the memory of the system you are running
on,
.Pp
.Dl "gdb -k -wcore kernel.debug /dev/mem"
.Pp
In this mode, you need the
.Fl k
flag to indicate to
.Xr gdb 1
that the
.Dq "dump file"
.Pa /dev/mem
is a kernel data file.
You can look at live data, and if you include the
.Fl 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
.Dq continue
execution, so they will not work.
.Ss "Debugging a crash dump"
By default, crash dumps are stored in the directory
.Pa /var/crash .
Investigate them from the kernel build directory with:
.Pp
.Dl "gdb -k kernel.debug /var/crash/vmcore.29"
.Pp
In this mode, the system is obviously stopped, so you can only look at it.
.Ss "Debugging a live system with a remote link"
In the following discussion, the term
.Dq "local system"
refers to the system running the debugger, and
.Dq "remote system"
refers to the live system being debugged.
.Pp
To debug a live system with a remote link, the kernel must be compiled with the
option
.Cd "options DDB" .
The option
.Cd "options BREAK_TO_DEBUGGER"
enables the debugging machine stop the debugged machine once a connection has
been established by pressing
.Ql ^C .
.Ss "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
.Li 0x80
for the specified interface.
Generally, this port will also be used as a serial console (flag bit
.Li 0x10 ) ,
so the entry in
.Pa /boot/device.hints
should be:
.Pp
.Dl hint.sio.0.flags="0x90"
.Ss "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
.Cd "options DDB" .
.Pp
A number of steps must be performed to set up a firewire link:
.Bl -bullet
.It
Ensure that both systems have
.Xr firewire 4
support, and that the kernel of the remote system includes the
.Xr dcons 4
and
.Xr dcons_crom 4
drivers.
If they are not compiled into the kernel, load the KLDs:
.Pp
.Dl "kldload firewire"
.Pp
On the remote system only:
.Bd -literal -offset indent
kldload dcons
kldload dcons_crom
.Ed
.Pp
You should see something like this in the
.Xr dmesg 8
output of the remote system:
.Bd -literal -offset indent
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
.Ed
.Pp
It is a good idea to load these modules at boot time with the following entry in
.Pa /boot/loader.conf :
.Pp
.Dl dcons_crom_enable="YES"
.Pp
This ensures that all three modules are loaded.
There is no harm in loading
.Xr dcons 4
and
.Xr dcons_crom 4
on the local system, but if you only want to load the
.Xr firewire 4
module, include the following in
.Pa /boot/loader.conf :
.Pp
.Dl firewire_enable="YES"
.It
Next, use
.Xr fwcontrol 8
to find the firewire node corresponding to the remote machine.
On the local machine you might see:
.Bd -literal -offset indent
# fwcontrol
2 devices (info_len=2)
node        EUI64        status
   1  0x00c04f3226e88061      0
   0  0x000199000003622b      1
.Ed
.Pp
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:
.Bd -literal -offset indent
# fwcontrol
2 devices (info_len=2)
node        EUI64        status
   0  0x000199000003622b      0
   1  0x00c04f3226e88061      1
.Ed
.It
Next, establish a firewire connection with
.Xr dconschat 8 :
.Pp
.Dl "dconschat -br -G 5556 -t 0x000199000003622b"
.Pp
.Li 0x000199000003622b
is the EUI64 address of the remote node, as determined from the output of
.Xr fwcontrol 8
above.
When started in this manner,
.Xr dconschat 8
establishes a local tunnel connection from port
.Li localhost:5556
to the remote debugger.
You can also establish a console port connection with the
.Fl C
option to the same invocation
.Xr dconschat 8 .
See the
.Xr dconschat 8
manpage for further details.
.Pp
The
.Xr 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.
.It
Finally, establish connection:
.Bd -literal -offset indent
# gdb kernel.debug
GNU gdb 5.2.1 (FreeBSD)
.Em "(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 ?? ()
.Ed
.Pp
The
.Ic trf
macro assumes a connection on port 5556.
If you want to use a different port (by changing the invocation of
.Xr dconschat 8
above), use the
.Ic tr
macro instead.
For example, if you want to use port 4711, run
.Xr dconschat 8
like this:
.Pp
.Dl "dconschat -br -G 4711 -t 0x000199000003622b"
.Pp
Then establish connection with:
.Bd -literal -offset indent
(gdb) tr localhost:4711
0xc21bd378 in ?? ()
.Ed
.El
.Ss "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
.Pa /dev/mem .
It can be very useful if a system crashes and the debugger no longer responds.
To use this method, set the
.Xr sysctl 8
variables
.Va hw.firewire.fwmem.eui64_hi
and
.Va 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:
.Bd -literal -offset indent
# fwcontrol
2 devices (info_len=2)
node        EUI64        status
   0  0x000199000003622b      0
   1  0x00c04f3226e88061      1
.Ed
.Pp
Enter:
.Bd -literal -offset indent
# 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
.Ed
.Pp
Note that the variables must be explicitly stated in hexadecimal.
After this, you can examine the remote machine's state with the following input:
.Bd -literal -offset indent
# gdb -k kernel.debug /dev/fwmem0.0
GNU gdb 5.2.1 (FreeBSD)
.Em "(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 ?? ()
.Ed
.Pp
In this case, it is not necessary to load the symbols explicitly.
The remote system continues to run.
.Sh COMMANDS
The user interface to
.Nm
is via
.Xr gdb 1 ,
so
.Xr gdb 1
commands also work.
This section discusses only the extensions for kernel debugging that get
installed in the kernel build directory.
.Ss "Debugging environment"
The following macros manipulate the debugging environment:
.Bl -tag -width indent
.It Ic ddb
Switch back to
.Xr ddb 4 .
This command is only meaningful when performing remote debugging.
.It Ic getsyms
Display
.Ic kldstat
information for the target machine and invite user to paste it back in.
This is required because
.Nm
does not allow data to be passed to shell scripts.
It is necessary for remote debugging and crash dumps; for local memory debugging
use
.Ic kldsyms
instead.
.It Ic kldsyms
Read in the symbol tables for the debugging machine.
This does not work for
remote debugging and crash dumps; use
.Ic getsyms
instead.
.It Ic tr Ar interface
Debug a remote system via the specified serial or firewire interface.
.It Ic tr0
Debug a remote system via serial interface
.Pa /dev/cuau0 .
.It Ic tr1
Debug a remote system via serial interface
.Pa /dev/cuau1 .
.It Ic trf
Debug a remote system via firewire interface at default port 5556.
.El
.Pp
The commands
.Ic tr0 , tr1
and
.Ic trf
are convenience commands which invoke
.Ic tr .
.Ss "The current process environment"
The following macros are convenience functions intended to make things easier
than the standard
.Xr gdb 1
commands.
.Bl -tag -width indent
.It Ic f0
Select stack frame 0 and show assembler-level details.
.It Ic f1
Select stack frame 1 and show assembler-level details.
.It Ic f2
Select stack frame 2 and show assembler-level details.
.It Ic f3
Select stack frame 3 and show assembler-level details.
.It Ic f4
Select stack frame 4 and show assembler-level details.
.It Ic f5
Select stack frame 5 and show assembler-level details.
.It Ic xb
Show 12 words in hex, starting at current
.Va ebp
value.
.It Ic xi
List the next 10 instructions from the current
.Va eip
value.
.It Ic xp
Show the register contents and the first four parameters of the current stack
frame.
.It Ic xp0
Show the first parameter of current stack frame in various formats.
.It Ic xp1
Show the second parameter of current stack frame in various formats.
.It Ic xp2
Show the third parameter of current stack frame in various formats.
.It Ic xp3
Show the fourth parameter of current stack frame in various formats.
.It Ic xp4
Show the fifth parameter of current stack frame in various formats.
.It Ic xs
Show the last 12 words on stack in hexadecimal.
.It Ic xxp
Show the register contents and the first ten parameters.
.It Ic z
Single step 1 instruction (over calls) and show next instruction.
.It Ic zs
Single step 1 instruction (through calls) and show next instruction.
.El
.Ss "Examining other processes"
The following macros access other processes.
The
.Nm
debugger
does not understand the concept of multiple processes, so they effectively
bypass the entire
.Nm
environment.
.Bl -tag -width indent
.It Ic btp Ar pid
Show a backtrace for the process
.Ar pid .
.It Ic btpa
Show backtraces for all processes in the system.
.It Ic btpp
Show a backtrace for the process previously selected with
.Ic defproc .
.It Ic btr Ar ebp
Show a backtrace from the
.Ar ebp
address specified.
.It Ic defproc Ar pid
Specify the PID of the process for some other commands in this section.
.It Ic fr Ar frame
Show frame
.Ar frame
of the stack of the process previously selected with
.Ic defproc .
.It Ic pcb Ar proc
Show some PCB contents of the process
.Ar proc .
.El
.Ss "Examining data structures"
You can use standard
.Xr 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.
.Bl -tag -width indent
.It Ic bp
Show information about the buffer header pointed to by the variable
.Va bp
in the current frame.
.It Ic bpd
Show the contents
.Pq Vt "char *"
of
.Va bp->data
in the current frame.
.It Ic bpl
Show detailed information about the buffer header
.Pq Vt "struct bp"
pointed at by the local variable
.Va bp .
.It Ic bpp Ar bp
Show summary information about the buffer header
.Pq Vt "struct bp"
pointed at by the parameter
.Ar bp .
.It Ic bx
Print a number of fields from the buffer header pointed at in by the pointer
.Ar bp
in the current environment.
.It Ic vdev
Show some information of the
.Vt vnode
pointed to by the local variable
.Va vp .
.El
.Ss "Miscellaneous macros"
.Bl -tag -width indent
.It Ic checkmem
Check unallocated memory for modifications.
This assumes that the kernel has been compiled with
.Cd "options DIAGNOSTIC" .
This causes the contents of free memory to be set to
.Li 0xdeadc0de .
.It Ic dmesg
Print the system message buffer.
This corresponds to the
.Xr dmesg 8
utility.
This macro used to be called
.Ic 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
.Nm .
When debugging a crash dump or over firewire, it is not necessary to start
.Nm
to access the message buffer: instead, use an appropriate variation of
.Bd -literal -offset indent
dmesg -M /var/crash/vmcore.0 -N kernel.debug
dmesg -M /dev/fwmem0.0 -N kernel.debug
.Ed
.It Ic kldstat
Equivalent of the
.Xr kldstat 8
utility without options.
.It Ic pname
Print the command name of the current process.
.It Ic ps
Show process status.
This corresponds in concept, but not in appearance, to the
.Xr ps 1
utility.
When debugging a crash dump or over firewire, it is not necessary to start
.Nm
to display the
.Xr ps 1
output: instead, use an appropriate variation of
.Bd -literal -offset indent
ps -M /var/crash/vmcore.0 -N kernel.debug
ps -M /dev/fwmem0.0 -N kernel.debug
.Ed
.It Ic y
Kludge for writing macros.
When writing macros, it is convenient to paste them
back into the
.Nm
window.
Unfortunately, if the macro is already defined,
.Nm
insists on asking
.Pp
.Dl "Redefine foo?"
.Pp
It will not give up until you answer
.Ql y .
This command is that answer.
It does nothing else except to print a warning
message to remind you to remove it again.
.El
.Sh SEE ALSO
.Xr gdb 1 ,
.Xr ps 1 ,
.Xr ddb 4 ,
.Xr firewire 4 ,
.Xr dconschat 8 ,
.Xr dmesg 8 ,
.Xr fwcontrol 8 ,
.Xr kldload 8
.Sh AUTHORS
This man page was written by
.An Greg Lehey Aq Mt grog@FreeBSD.org .
.Sh BUGS
The
.Xr gdb 1
debugger
was never designed to debug kernels, and it is not a very good match.
Many problems exist.
.Pp
The
.Nm
implementation is very inefficient, and many operations are slow.
.Pp
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.
.Pp
The debugging macros
.Dq just grew.
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.
.Pp
Many of these commands only work on the ia32 architecture.