GDB(4)

📦 version 15.1, in freebsd 14.5, freebsd 15.1, nixpkgs unstable

💖 GDB(4) — external kernel debugger 💖

okay bestie, this is GDB(4) from the Kernel Interfaces Manual, and it’s giving “external kernel debugger” main character energy. Let’s get into it, because this one is long and kinda iconic ngl. 🫶

✨ NAME ✨

  • gdb — external kernel debugger

🌸 SYNOPSIS 🌸

makeoptions DEBUG=-g
options DDB

🦋 DESCRIPTION 🦋

The gdb kernel debugger is a variation of gdb(1) (ports/devel/gdb) 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) (ports/devel/gdb) 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.

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.

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"

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
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 ?? ()

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 💻

okay so the whole user interface to gdb is via gdb(1) (ports/devel/gdb), which means all your regular gdb(1) (ports/devel/gdb) commands still work in here too. This section is specifically about the extra kernel-debugging extensions that get installed in the kernel build directory. 💖

🛠️ Debugging environment 🛠️

These macros are the little handles that manipulate the debugging environment:

  • ddb — switch back to ddb(4). This one is only meaningful when you're doing remote debugging, bestie.
  • getsyms — display kldstat information for the target machine and invite the user to paste it back in. This is required because gdb does not allow data to be passed to shell scripts. It's 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, so 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 🧚

These macros are convenience functions intended to make things easier than the standard gdb(1) (ports/devel/gdb) 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 the 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 the current stack frame in various formats.
  • xp1 — show the second parameter of the current stack frame in various formats.
  • xp2 — show the third parameter of the current stack frame in various formats.
  • xp3 — show the fourth parameter of the current stack frame in various formats.
  • xp4 — show the fifth parameter of the current stack frame in various formats.
  • xs — show the last 12 words on the stack in hexadecimal.
  • xxp — show the register contents and the first ten parameters.
  • z — single step 1 instruction (over calls) and show the next instruction.
  • zs — single step 1 instruction (through calls) and show the next instruction.

👀 Examining other processes 👀

These macros access other processes. The gdb debugger does not understand the concept of multiple processes, so they effectively bypass the entire gdb environment. iconic workaround behavior, honestly 💅

  • 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) (ports/devel/gdb) 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) (ports/devel/gdb), 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) (ports/devel/gdb) debugger was never designed to debug kernels, and it is not a very good match. Many problems exist. not me finding out gdb was kinda fighting for its life this whole time 😭

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 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.

Many of these commands only work on the ia32 architecture.

source/version: Debian · date: May 17, 2016 · page: GDB(4) — okay bye bestie, go debug slayfully 💅💖🎀

📦 where this page comes from