NAMEgdb - external kernel debugger
SYNOPSISmakeoptions DEBUG=-g options DDB
DESCRIPTIONThe 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: +
PREPARING FOR DEBUGGINGWhen 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: +
COMMANDSThe 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/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 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 ALSOgdb(1), ps(1), ddb(4), firewire(4), dconschat(8), dmesg(8), fwcontrol(8), kldload(8)
AUTHORSThis man page was written by Greg Lehey <grog@FreeBSD.org>.
BUGSThe 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 growed''. 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. GDB(4)