freebsd-dev/share/doc/handbook/kerneldebug.sgml

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<chapt><heading>Kernel Debugging<label id="kerneldebug"></heading>

<p><em>Contributed by &a.paul; and &a.joerg;</em>

<sect><heading>Debugging a kernel crash dump with kgdb</heading>

  <p>Here are some instructions for getting kernel debugging
  working on a crash dump, it assumes that you have enough swap
  space for a crash dump.  If you have multiple swap
  partitions and the first one is too small to hold the dump,
  you can configure your kernel to use an alternate dump device
  (in the <tt>config kernel</tt> line), or
  you can specify an alternate using the dumpon(8) command.  
  Dumps to non-swap devices,
  tapes for example, are currently not supported.  Config your
  kernel using <tt>config -g</tt>.
  See <ref id="kernelconfig" name="Kernel Configuration"> for
  details on configuring the FreeBSD kernel.

  Use the <tt>dumpon(8)</tt> command to tell the kernel where to dump
  to (note that this will have to be done after configuring the
  partition in question as swap space via <tt>swapon(8)</tt>).  This is
  normally arranged via <tt>/etc/sysconfig</tt> and <tt>/etc/rc</tt>.  
  Alternatively, you can
  hard-code the dump device via the `dump' clause in the `config' line
  of your kernel config file.  This is deprecated, use only if you
  want a crash dump from a kernel that crashes during booting.

  <em><bf>Note:</bf> In the following, the term `<tt>kgdb</tt>' refers
  to <tt>gdb</tt> run in `kernel debug mode'.  This can be accomplished by
  either starting the <tt>gdb</tt> with the option <tt>-k</tt>, or by linking
  and starting it under the name <tt>kgdb</tt>.  This is not being
  done by default, however.</em>

  When the kernel has been built make a copy of it, say
  <tt>kernel.debug</tt>, and then run <tt>strip -d</tt> on the
  original. Install the original as normal.  You may also install
  the unstripped kernel, but symbol table lookup time for some
  programs will drastically increase, and since
  the whole kernel is loaded entirely at boot time and cannot be
  swapped out later, several megabytes of
  physical memory will be wasted.

  If you are testing a new kernel, for example by typing the new
  kernel's name at the boot prompt, but need to boot a different
  one in order to get your system up and running again, boot it
  only into single user state using the <tt>-s</tt> flag at the
  boot prompt, and then perform the following steps:
<tscreen><verb>
  fsck -p
  mount -a -t ufs       # so your file system for /var/crash is writable
  savecore -N /kernel.panicked /var/crash
  exit                  # ...to multi-user
</verb></tscreen>
  This instructs <tt>savecore(8)</tt> to use another kernel for symbol name
  extraction.  It would otherwise default to the currently running kernel
  and most likely not do anything at all since the crash dump and the
  kernel symbols differ.

  Now, after a crash dump, go to <tt>/sys/compile/WHATEVER</tt> and run
  <tt>kgdb</tt>.  From <tt>kgdb</tt> do:
<tscreen><verb>
  symbol-file kernel.debug
  exec-file /var/crash/kernel.0
  core-file /var/crash/vmcore.0
</verb></tscreen>
  and voila, you can debug the crash dump using the kernel sources
  just like you can for any other program.

  Here's a script log of a <tt>kgdb</tt> session illustrating the
  procedure.  Long
  lines have been folded to improve readability, and the lines are
  numbered for reference.  Despite this, it's a real-world error
  trace taken during the development of the pcvt console driver.
<tscreen><verb>
   1:Script started on Fri Dec 30 23:15:22 1994
   2:uriah # cd /sys/compile/URIAH
   3:uriah # kgdb kernel /var/crash/vmcore.1 
   4:Reading symbol data from /usr/src/sys/compile/URIAH/kernel...done.
   5:IdlePTD 1f3000
   6:panic: because you said to!
   7:current pcb at 1e3f70
   8:Reading in symbols for ../../i386/i386/machdep.c...done.
   9:(kgdb) where
  10:#0  boot (arghowto=256) (../../i386/i386/machdep.c line 767)
  11:#1  0xf0115159 in panic ()
  12:#2  0xf01955bd in diediedie () (../../i386/i386/machdep.c line 698)
  13:#3  0xf010185e in db_fncall ()
  14:#4  0xf0101586 in db_command (-266509132, -266509516, -267381073)
  15:#5  0xf0101711 in db_command_loop ()
  16:#6  0xf01040a0 in db_trap ()
  17:#7  0xf0192976 in kdb_trap (12, 0, -272630436, -266743723)
  18:#8  0xf019d2eb in trap_fatal (...)
  19:#9  0xf019ce60 in trap_pfault (...)
  20:#10 0xf019cb2f in trap (...)
  21:#11 0xf01932a1 in exception:calltrap ()
  22:#12 0xf0191503 in cnopen (...)
  23:#13 0xf0132c34 in spec_open ()
  24:#14 0xf012d014 in vn_open ()
  25:#15 0xf012a183 in open ()
  26:#16 0xf019d4eb in syscall (...)
  27:(kgdb) up 10
  28:Reading in symbols for ../../i386/i386/trap.c...done.
  29:#10 0xf019cb2f in trap (frame={tf_es = -260440048, tf_ds = 16, tf_\
  30:edi = 3072, tf_esi = -266445372, tf_ebp = -272630356, tf_isp = -27\
  31:2630396, tf_ebx = -266427884, tf_edx = 12, tf_ecx = -266427884, tf\
  32:_eax = 64772224, tf_trapno = 12, tf_err = -272695296, tf_eip = -26\
  33:6672343, tf_cs = -266469368, tf_eflags = 66066, tf_esp = 3072, tf_\
  34:ss = -266427884}) (../../i386/i386/trap.c line 283)
  35:283                             (void) trap_pfault(&amp;frame, FALSE);
  36:(kgdb) frame frame->tf_ebp frame->tf_eip
  37:Reading in symbols for ../../i386/isa/pcvt/pcvt_drv.c...done.
  38:#0  0xf01ae729 in pcopen (dev=3072, flag=3, mode=8192, p=(struct p\
  39:roc *) 0xf07c0c00) (../../i386/isa/pcvt/pcvt_drv.c line 403)
  40:403             return ((*linesw[tp->t_line].l_open)(dev, tp));
  41:(kgdb) list
  42:398        
  43:399             tp->t_state |= TS_CARR_ON;
  44:400             tp->t_cflag |= CLOCAL;  /* cannot be a modem (:-) */
  45:401     
  46:402     #if PCVT_NETBSD || (PCVT_FREEBSD >= 200)
  47:403             return ((*linesw[tp->t_line].l_open)(dev, tp));
  48:404     #else
  49:405             return ((*linesw[tp->t_line].l_open)(dev, tp, flag));
  50:406     #endif /* PCVT_NETBSD || (PCVT_FREEBSD >= 200) */
  51:407     }
  52:(kgdb) print tp
  53:Reading in symbols for ../../i386/i386/cons.c...done.
  54:$1 = (struct tty *) 0x1bae
  55:(kgdb) print tp->t_line
  56:$2 = 1767990816
  57:(kgdb) up
  58:#1  0xf0191503 in cnopen (dev=0x00000000, flag=3, mode=8192, p=(st\
  59:ruct proc *) 0xf07c0c00) (../../i386/i386/cons.c line 126)
  60:       return ((*cdevsw[major(dev)].d_open)(dev, flag, mode, p));
  61:(kgdb) up
  62:#2  0xf0132c34 in spec_open ()
  63:(kgdb) up
  64:#3  0xf012d014 in vn_open ()
  65:(kgdb) up
  66:#4  0xf012a183 in open ()
  67:(kgdb) up
  68:#5  0xf019d4eb in syscall (frame={tf_es = 39, tf_ds = 39, tf_edi =\
  69: 2158592, tf_esi = 0, tf_ebp = -272638436, tf_isp = -272629788, tf\
  70:_ebx = 7086, tf_edx = 1, tf_ecx = 0, tf_eax = 5, tf_trapno = 582, \
  71:tf_err = 582, tf_eip = 75749, tf_cs = 31, tf_eflags = 582, tf_esp \
  72:= -272638456, tf_ss = 39}) (../../i386/i386/trap.c line 673)
  73:673             error = (*callp->sy_call)(p, args, rval);
  74:(kgdb) up
  75:Initial frame selected; you cannot go up.
  76:(kgdb) quit
  77:uriah # exit
  78:exit
  79:
  80:Script done on Fri Dec 30 23:18:04 1994
</verb></tscreen>
  Comments to the above script:
  
<descrip>
<tag/line 6:/  This is a dump taken from within DDB (see below), hence the
            panic comment ``because you said to!'', and a rather long
            stack trace; the initial reason for going into DDB has been
            a page fault trap though.
<tag/line 20:/  This is the location of function <tt>trap()</tt>
	    in the stack trace. 
<tag/line 36:/  Force usage of a new stack frame; this is no longer
            necessary now.  The stack frames are supposed to point to
            the right locations now, even in case of a trap.
            (I don't have a new core dump handy &lt;g&gt;, my kernel
            didn't panic for rather long.)
            From looking at the code in source line 403,
            there's a high probability that either the pointer
            access for ``tp'' was messed up, or the array access was
            out of bounds.
<tag/line 52:/  The pointer looks suspicious, but happens to be a valid
            address.
<tag/line 56:/ However, it obviously points to garbage, so we have found our
            error!  (For those unfamiliar with that particular piece
            of code: <tt>tp-&gt;t_line</tt> refers to the line discipline 
	    of the console device here, which must be a rather small integer
            number.)
</descrip>  


<sect><heading>Post-mortem analysis of a dump</heading>

<p>What do you do if a kernel dumped core but you did not expect
  it, and it's therefore not compiled using <tt>config -g</tt>?
  Not everything is lost here.  Don't panic!

  Of course, you still need to enable crash dumps.  See above
  on the options you've got in order to do this.

  Go to your kernel compile directory, and edit the line
  containing <tt>COPTFLAGS?=-O</tt>.  Add the <tt>-g</tt> option
  there (but <em>don't</em> change anything on the level of
  optimization).  If you do already know roughly the probable
  location of the failing piece of code (e.g., the <tt>pcvt</tt>
  driver in the example above), remove all the object files for
  this code.  Rebuild the kernel. Due to the time stamp change on
  the Makefile, there will be some other object files rebuild,
  for example <tt>trap.o</tt>.  With a bit of luck, the added
  <tt>-g</tt> option won't change anything for the generated
  code, so you'll finally get a new kernel with similar code to
  the faulting one but some debugging symbols.  You should at
  least verify the old and new sizes with the <tt>size(1)</tt> command. If
  there is a mismatch, you probably need to give up here.

  Go and examine the dump as described above.  The debugging
  symbols might be incomplete for some places, as can be seen in
  the stack trace in the example above where some functions are
  displayed without line numbers and argument lists.  If you need
  more debugging symbols, remove the appropriate object files and
  repeat the <tt>kgdb</tt> session until you know enough.

  All this is not guaranteed to work, but it will do it fine in
  most cases.

<sect><heading>On-line kernel debugging using DDB</heading>

<p>While <tt>kgdb</tt> as an offline debugger provides a very
  high level of user interface, there are some things it cannot do.
  The most important ones being breakpointing and single-stepping
  kernel code.

  If you need to do low-level debugging on your kernel, there's
  an on- line debugger available called DDB.  It allows to
  setting breakpoints, single-steping kernel functions, examining
  and changing kernel variables, etc.  However, it cannot not
  access kernel source files, and only has access to the global
  and static symbols, not to the full debug information like
  <tt>kgdb</tt>.

  To configure your kernel to include DDB, add the option line
<tscreen><verb>
        options DDB
</verb></tscreen>
  to your config file, and rebuild.  (See <ref id="kernelconfig"
  name="Kernel Configuration"> for details on configuring the
  FreeBSD kernel. Note that if you have an older version of the
  boot blocks, your debugger symbols might not be loaded at all.
  Update the boot blocks, the recent ones do load the DDB symbols
  automagically.)

  Once your DDB kernel is running, there are several ways to
  enter DDB.  The first, and earliest way is to type the boot
  flag <tt>-d</tt> right at the boot prompt.  The kernel will
  start up in debug mode and enter DDB prior to any device
  probing.  Hence you are able to even debug the device
  probe/attach functions.

  The second scenario is a hot-key on the keyboard, usually
  Ctrl-Alt-ESC.  For syscons, this can be remapped, and some of
  the distributed maps do this, so watch out.
  There's an option
  available for serial consoles
  that allows the use of a serial line BREAK on the console line to
  enter DDB (``<tt>options BREAK_TO_DEBUGGER</tt>''
  in the kernel config file).  It is not the default since there are a lot of
  crappy serial adapters around that gratuitously generate a
  BREAK condition for example when pulling the cable.

  The third way is that any panic condition will branch to DDB if
  the kernel is configured to use it.  
  For this reason, it is not wise to
  configure a kernel with DDB for a machine running unattended.

  The DDB commands roughly resemble some <tt>gdb</tt> commands.  The first you
  probably need is to set a breakpoint:
<tscreen><verb>
  b function-name
  b address
</verb></tscreen>

  Numbers are taken hexadecimal by default, but to make them
  distinct from symbol names, hexadecimal numbers starting with the
  letters <tt>a</tt>-<tt>f</tt> need to be preceded with
  <tt>0x</tt> (for other numbers, this is optional).  Simple
  expressions are allowed, for example: <tt>function-name + 0x103</tt>.

  To continue the operation of an interrupted kernel, simply type
<tscreen><verb>
  c
</verb></tscreen>
  To get a stack trace, use
<tscreen><verb>
  trace
</verb></tscreen>
  Note that when entering DDB via a hot-key, the kernel is currently
  servicing an interrupt, so the stack trace might be not of much use
  for you.

  If you want to remove a breakpoint, use
<tscreen><verb>
  del
  del address-expression
</verb></tscreen>
  The first form will be accepted immediately after a breakpoint hit,
  and deletes the current breakpoint.  The second form can remove any
  breakpoint, but you need to specify the exact address, as it can be
  obtained from
<tscreen><verb>
  show b
</verb></tscreen>
  To single-step the kernel, try
<tscreen><verb>
  s
</verb></tscreen>
  This will step into functions, but you can make DDB trace them until
  the matching return statement is reached by
<tscreen><verb>
  n
</verb></tscreen>
  <bf>Note:</bf> this is different from <tt>gdb</tt>'s `next' statement, it's like
  <tt>gdb</tt>'s `finish'.

  To examine data from memory, use (for example):
<tscreen><verb>
  x/wx 0xf0133fe0,40
  x/hd db_symtab_space
  x/bc termbuf,10
  x/s stringbuf
</verb></tscreen>
  for word/halfword/byte access, and hexadecimal/decimal/character/
  string display.  The number after the comma is the object count.
  To display the next 0x10 items, simply use
<tscreen><verb>
  x ,10
</verb></tscreen>
  Similarly, use
<tscreen><verb>
  x/ia foofunc,10
</verb></tscreen>
  to disassemble the first 0x10 instructions of <tt>foofunc</tt>, and display
  them along with their offset from the beginning of <tt>foofunc</tt>.

  To modify the memory, use the write command:
<tscreen><verb>
  w/b termbuf 0xa 0xb 0
  w/w 0xf0010030 0 0
</verb></tscreen>
  The command modifier (<tt>b</tt>/<tt>h</tt>/<tt>w</tt>)
  specifies the size of the data to be written, the first
  following expression is the address to write to, the remainder
  is interpreted as data to write to successive memory locations.

  If you need to know the current registers, use
<tscreen><verb>
  show reg
</verb></tscreen>
  Alternatively, you can display a single register value by e.g.
<tscreen><verb>
  p $eax
</verb></tscreen>
  and modify it by
<tscreen><verb>
  set $eax new-value
</verb></tscreen>

  Should you need to call some kernel functions from DDB, simply
  say
<tscreen><verb>
  call func(arg1, arg2, ...)
</verb></tscreen>
  The return value will be printed.

  For a <tt>ps(1)</tt> style summary of all running processes, use
<tscreen><verb>
  ps
</verb></tscreen>

  Now you have now examined why your kernel failed, and you wish to
  reboot.  Remember that, depending on the severity of previous
  malfunctioning, not all parts of the kernel might still be working
  as expected.  Perform one of the following actions to shut down and
  reboot your system:
<tscreen><verb>
  call diediedie()
</verb></tscreen>

  will cause your kernel to dump core and reboot, so you can
  later analyze the core on a higher level with kgdb.  This
  command usually must be followed by another
  `<tt>continue</tt>' statement.
  There is now an alias for this: `<tt>panic</tt>'.

<tscreen><verb>
  call boot(0)
</verb></tscreen>
  might be a good way to cleanly shut down the running system, <tt>sync()</tt>
  all disks, and finally reboot.  As long as the disk and file system
  interfaces of the kernel are not damaged, this might be a good way
  for an almost clean shutdown.

<tscreen><verb>
  call cpu_reset()
</verb></tscreen>
  is the final way out of disaster and almost the same as hitting
  the Big Red Button.

  If you need a short command summary, simply type
<tscreen><verb>
  help
</verb></tscreen>
  However, it's highly recommended to have a printed copy of the
  <tt>ddb(4)</tt> manual page ready for a debugging session.
  Remember that it's hard to read the on-line manual while
  single-stepping the kernel.


<sect><heading>Debugging a console driver</heading>

<p>Since you need a console driver to run DDB on, things are more
  complicated if the console driver itself is failing.  You might
  remember the use of a serial console (either with modified boot
  blocks, or by specifying <tt><bf>-h</bf></tt> at the <tt>Boot:</tt>
  prompt), and hook up a standard
  terminal onto your first serial port.  DDB works on any configured
  console driver, of course also on a serial console.