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     The FreeBSD Documentation Project

     $FreeBSD: doc/en_US.ISO_8859-1/books/handbook/kerneldebug/chapter.sgml,v 1.26 2000/11/07 09:08:11 jkoshy Exp $

<chapter id="kerneldebug">
  <title>Kernel Debugging</title>
  <para><emphasis>Contributed by &a.paul; and &a.joerg;</emphasis></para>
    <title>Debugging a Kernel Crash Dump with <command>gdb</command></title>
    <para>Here are some instructions for getting kernel debugging working on a
      crash dump.  They assume 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 <literal>config kernel</literal> line), or
      you can specify an alternate using the
      &man.dumpon.8; command.  The best way to use &man.dumpon.8; is to set
      the <literal>dumpdev</literal> variable in
      <filename>/etc/rc.conf</filename>.  Typically you want to specify one of
      the swap devices specified in <filename>/etc/fstab</filename>. Dumps to
      non-swap devices, tapes for example, are currently not supported. Config
      your kernel using <command>config <option>-g</option></command>.  See <link
	linkend="kernelconfig">Kernel Configuration</link> for details on
      configuring the FreeBSD kernel.</para>

    <para>Use the &man.dumpon.8; 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 &man.swapon.8;).  This is normally arranged
      via <filename>/etc/rc.conf</filename> and <filename>/etc/rc</filename>.
      Alternatively, you can hard-code the dump device via the
      <literal>dump</literal> clause in the <literal>config</literal> line of
      your kernel config file.  This is deprecated and should be used only if
      you want a crash dump from a kernel that crashes during booting.</para>
      <para>In the following, the term <command>gdb</command> refers to
        the debugger <command>gdb</command> run in <quote>kernel debug 
        mode</quote>.  This can be accomplished by starting the 
        <command>gdb</command> with the option <option>-k</option>.  In 
        kernel debug mode, <command>gdb</command> changes its prompt to 
      <para>If you are using FreeBSD 3 or earlier, you should make a stripped
        copy of the debug kernel, rather than installing the large debug
        kernel itself:</para>

      <screen>&prompt.root; <userinput>cp kernel kernel.debug</userinput>
&prompt.root; <userinput>strip -g kernel</userinput></screen>

      <para>This stage isn't necessary, but it is recommended.  (In
        FreeBSD 4 and later releases this step is performed automatically
        at the end of the kernel <command>make</command> process.)
        When the kernel has been stripped, either automatically or by
        using the commands above, you may install it as usual by typing
        <command>make install</command>.</para>

      <para>Note that older releases of FreeBSD (up to but not including
        3.1) used a.out kernels by default, which must have their symbol
        tables permanently resident in physical memory.  With the larger
        symbol table in an unstripped debug kernel, this is wasteful.
        Recent FreeBSD releases use ELF kernels where this is no longer a

    <para>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 <option>-s</option> flag at the boot prompt, and
      then perform the following steps:</para>

    <screen>&prompt.root; <userinput>fsck -p</userinput>
&prompt.root; <userinput>mount -a -t ufs</userinput>       # so your file system for /var/crash is writable
&prompt.root; <userinput>savecore -N /kernel.panicked /var/crash</userinput>
&prompt.root; <userinput>exit</userinput>                  # ...to multi-user</screen>
    <para>This instructs &man.savecore.8; 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.</para>
    <para>Now, after a crash dump, go to
      <filename>/sys/compile/WHATEVER</filename> and run
      <command>gdb <option>-k</option></command>.  From <command>gdb</command> do:
      <screen><userinput>symbol-file kernel.debug</userinput>
<userinput>exec-file /var/crash/kernel.0</userinput>
<userinput>core-file /var/crash/vmcore.0</userinput></screen>
      and voila, you can debug the crash dump using the kernel sources just
      like you can for any other program.</para>

    <para>Here is a script log of a <command>gdb</command> session
      illustrating the procedure.  Long lines have been folded to improve
      readability, and the lines are numbered for reference. Despite this, it
      is a real-world error trace taken during the development of the pcvt
      console driver.</para>
<screen> 1:Script started on Fri Dec 30 23:15:22 1994
 2:&prompt.root; <userinput>cd /sys/compile/URIAH</userinput>
 3:&prompt.root; <userinput>gdb -k kernel /var/crash/vmcore.1</userinput> 
 4:Reading symbol data from /usr/src/sys/compile/URIAH/kernel
 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:<prompt>(kgdb)</prompt> <userinput>where</userinput>
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:<prompt>(kgdb)</prompt> <userinput>up 10</userinput>
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:<prompt>(kgdb)</prompt> <userinput>frame frame-&gt;tf_ebp frame-&gt;tf_eip</userinput>
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-&gt;t_line].l_open)(dev, tp));
41:<prompt>(kgdb)</prompt> <userinput>list</userinput>
43:399             tp-&gt;t_state |= TS_CARR_ON;
44:400             tp-&gt;t_cflag |= CLOCAL;  /* cannot be a modem (:-) */
46:402     #if PCVT_NETBSD || (PCVT_FREEBSD >= 200)
47:403             return ((*linesw[tp-&gt;t_line].l_open)(dev, tp));
48:404     #else
49:405             return ((*linesw[tp-&gt;t_line].l_open)(dev, tp, flag));
50:406     #endif /* PCVT_NETBSD || (PCVT_FREEBSD >= 200) */
51:407     }
52:<prompt>(kgdb)</prompt> <userinput>print tp</userinput>
53:Reading in symbols for ../../i386/i386/cons.c...done.
54:$1 = (struct tty *) 0x1bae
55:<prompt>(kgdb)</prompt> <userinput>print tp-&gt;t_line</userinput>
56:$2 = 1767990816
57:<prompt>(kgdb)</prompt> <userinput>up</userinput>
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:<prompt>(kgdb)</prompt> <userinput>up</userinput>
62:#2  0xf0132c34 in spec_open ()
63:<prompt>(kgdb)</prompt> <userinput>up</userinput>
64:#3  0xf012d014 in vn_open ()
65:<prompt>(kgdb)</prompt> <userinput>up</userinput>
66:#4  0xf012a183 in open ()
67:<prompt>(kgdb)</prompt> <userinput>up</userinput>
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-&gt;sy_call)(p, args, rval);
74:<prompt>(kgdb)</prompt> <userinput>up</userinput>
75:Initial frame selected; you cannot go up.
76:<prompt>(kgdb)</prompt> <userinput>quit</userinput>
77:&prompt.root; <userinput>exit</userinput>
80:Script done on Fri Dec 30 23:18:04 1994</screen>
    <para>Comments to the above script:</para>
	<term>line 6:</term>

	  <para>This is a dump taken from within DDB (see below), hence the
	    panic comment <quote>because you said to!</quote>, and a rather
	    long stack trace; the initial reason for going into DDB has been a
	    page fault trap though.</para>
	<term>line 20:</term>

	  <para>This is the location of function <function>trap()</function>
	    in the stack trace.</para>
	<term>line 36:</term>

	  <para>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 do not have a new core
	    dump handy &lt;g&gt;, my kernel has not panicked for a rather long
	    time.) From looking at the code in source line 403, there is a
	    high probability that either the pointer access for
	    <quote>tp</quote> was messed up, or the array access was out of
	<term>line 52:</term>

	  <para>The pointer looks suspicious, but happens to be a valid
	<term>line 56:</term>

	  <para>However, it obviously points to garbage, so we have found our
	    error! (For those unfamiliar with that particular piece of code:
	    <literal>tp-&gt;t_line</literal> refers to the line discipline  of
	    the console device here, which must be a rather small integer
    <title>Debugging a Crash Dump with DDD</title>
    <para>Examining a kernel crash dump with a graphical debugger like
      <command>ddd</command> is also possible.  Add the <option>-k</option>
      option to the <command>ddd</command> command line you would use
      normally.  For example;</para>

    <screen>&prompt.root; <userinput>ddd -k /var/crash/kernel.0 /var/crash/vmcore.0</userinput></screen>
    <para>You should then be able to go about looking at the crash dump using
      <command>ddd</command>'s graphical interface.</para>
    <title>Post-Mortem Analysis of a Dump</title>
    <para>What do you do if a kernel dumped core but you did not expect it,
      and it is therefore not compiled using <command>config -g</command>? Not
      everything is lost here.  Do not panic!</para>

    <para>Of course, you still need to enable crash dumps.  See above on the
      options you have to specify in order to do this.</para>
    <para>Go to your kernel config directory
      and edit your configuration file.  Uncomment (or add, if it does not
      exist) the following line</para>

makeoptions    DEBUG=-g                #Build kernel with gdb(1) debug symbols</programlisting>

    <para>Rebuild the kernel.  Due to the time stamp change on the Makefile,
      there will be some other object files rebuild, for example
      <filename>trap.o</filename>.  With a bit of luck, the added
      <option>-g</option> option will not change anything for the generated
      code, so you will 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 &man.size.1; command.  If there is a
      mismatch, you probably need to give up here.</para>

    <para>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 <command>gdb <option>-k</option></command>
      session until you know enough.</para>

    <para>All this is not guaranteed to work, but it will do it fine in most
    <title>On-Line Kernel Debugging Using DDB</title>
    <para>While <command>gdb <option>-k</option></command> as an off-line 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

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

    <para>To configure your kernel to include DDB, add the option line
options DDB</programlisting>

      to your config file, and rebuild.  (See <link
	linkend="kernelconfig">Kernel Configuration</link> for details on
      configuring the FreeBSD kernel.</para>

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

    <para>The second scenario is a hot-key on the keyboard, usually
      Ctrl-Alt-ESC.  For syscons, this can be remapped; some of the
      distributed maps do this, so watch out.  There is an option available
      for serial consoles that allows the use of a serial line BREAK on the
      console line to enter DDB (<literal>options BREAK_TO_DEBUGGER</literal>
      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.</para>

    <para>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.</para>

    <para>The DDB commands roughly resemble some <command>gdb</command>
      commands.  The first thing you probably need to do is to set a
    <screen><userinput>b function-name</userinput>
<userinput>b address</userinput></screen>
    <para>Numbers are taken hexadecimal by default, but to make them distinct
      from symbol names; hexadecimal numbers starting with the letters
      <literal>a-f</literal> need to be preceded with <literal>0x</literal>
      (this is optional for other numbers).  Simple expressions are allowed,
      for example: <literal>function-name + 0x103</literal>.</para>

    <para>To continue the operation of an interrupted kernel, simply
    <para>To get a stack trace, use:</para>
      <para>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.</para>
    <para>If you want to remove a breakpoint, use</para>
<userinput>del address-expression</userinput></screen>
    <para>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; this can be
      obtained from:</para>
    <screen><userinput>show b</userinput></screen>
    <para>To single-step the kernel, try:</para>
    <para>This will step into functions, but you can make DDB trace them until
      the matching return statement is reached by:</para>

      <para>This is different from <command>gdb</command>'s
	<command>next</command> statement; it is like <command>gdb</command>'s
    <para>To examine data from memory, use (for example):
      <screen><userinput>x/wx 0xf0133fe0,40</userinput>
<userinput>x/hd db_symtab_space</userinput>
<userinput>x/bc termbuf,10</userinput>
<userinput>x/s stringbuf</userinput></screen>
      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:</para>
    <screen><userinput>x ,10</userinput></screen>
    <para>Similarly, use

      <screen><userinput>x/ia foofunc,10</userinput></screen>
      to disassemble the first 0x10 instructions of
      <function>foofunc</function>, and display them along with their offset
      from the beginning of <function>foofunc</function>.</para>

    <para>To modify memory, use the write command:</para>
    <screen><userinput>w/b termbuf 0xa 0xb 0</userinput>
<userinput>w/w 0xf0010030 0 0</userinput></screen>
    <para>The command modifier
      specifies the size of the data to be written, the first following
      expression is the address to write to and the remainder is interpreted
      as data to write to successive memory locations.</para>

    <para>If you need to know the current registers, use:</para>

    <screen><userinput>show reg</userinput></screen>

    <para>Alternatively, you can display a single register value by e.g.

      <screen><userinput>p $eax</userinput></screen>

      and modify it by:</para>

    <screen><userinput>set $eax new-value</userinput></screen>
    <para>Should you need to call some kernel functions from DDB, simply

    <screen><userinput>call func(arg1, arg2, ...)</userinput></screen>
    <para>The return value will be printed.</para>
    <para>For a &man.ps.1; style summary of all running processes, use:</para>
    <para>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:</para>

    <para>This will cause your kernel to dump core and reboot, so you can
      later analyze the core on a higher level with <command>gdb</command>.  This command
      usually must be followed by another <command>continue</command>
    <screen><userinput>call boot(0)</userinput></screen>
    <para>Which might be a good way to cleanly shut down the running system,
      <function>sync()</function> 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.</para>
    <screen><userinput>call cpu_reset()</userinput></screen>
    <para>is the final way out of disaster and almost the same as hitting the
      Big Red Button.</para>

    <para>If you need a short command summary, simply type:</para>
    <para>However, it is highly recommended to have a printed copy of the
	&man.ddb.4; manual page ready for a debugging
      session.  Remember that it is hard to read the on-line manual while
      single-stepping the kernel.</para>
    <title>On-Line Kernel Debugging Using Remote GDB</title>
    <para>This feature has been supported since FreeBSD 2.2, and it is
      actually a very neat one.</para>
    <para>GDB has already supported <emphasis>remote debugging</emphasis> for
      a long time.  This is done using a very simple protocol along a serial
      line.  Unlike the other methods described above, you will need two
      machines for doing this.  One is the host providing the debugging
      environment, including all the sources, and a copy of the kernel binary
      with all the symbols in it, and the other one is the target machine that
      simply runs a similar copy of the very same kernel (but stripped of the
      debugging information).</para>

    <para>You should configure the kernel in question with <command>config
	-g</command>, include <option>DDB</option> into the configuration, and
      compile it as usual.  This gives a large blurb of a binary, due to the
      debugging information.  Copy this kernel to the target machine, strip
      the debugging symbols off with <command>strip -x</command>, and boot it
      using the <option>-d</option> boot option.  Connect the serial line
      of the target machine that has "flags 080" set on its sio device
      to any serial line of the debugging host.
      Now, on the debugging machine, go to the compile directory of the target
      kernel, and start <command>gdb</command>:</para>

    <screen>&prompt.user; <userinput>gdb -k kernel</userinput>
GDB is free software and you are welcome to distribute copies of it
 under certain conditions; type "show copying" to see the conditions.
There is absolutely no warranty for GDB; type "show warranty" for details.
GDB 4.16 (i386-unknown-freebsd),
Copyright 1996 Free Software Foundation, Inc...
<prompt>(kgdb)</prompt> </screen>
    <para>Initialize the remote debugging session (assuming the first serial
      port is being used) by:</para>

    <screen><prompt>(kgdb)</prompt> <userinput>target remote /dev/cuaa0</userinput></screen>
    <para>Now, on the target host (the one that entered DDB right before even
      starting the device probe), type:</para>
    <screen>Debugger("Boot flags requested debugger")
Stopped at Debugger+0x35: movb	$0, edata+0x51bc
<prompt>db&gt;</prompt> <userinput>gdb</userinput></screen>
    <para>DDB will respond with:</para>
    <screen>Next trap will enter GDB remote protocol mode</screen>
    <para>Every time you type <command>gdb</command>, the mode will be toggled
      between remote GDB and local DDB.  In order to force a next trap
      immediately, simply type <command>s</command> (step).  Your hosting GDB
      will now gain control over the target kernel:</para>
    <screen>Remote debugging using /dev/cuaa0
Debugger (msg=0xf01b0383 "Boot flags requested debugger")
    at ../../i386/i386/db_interface.c:257
    <para>You can use this session almost as any other GDB session, including
      full access to the source, running it in gud-mode inside an Emacs window
      (which gives you an automatic source code display in another Emacs
      window) etc.</para>
    <title>Debugging Loadable Modules Using GDB</title>
    <para>When debugging a panic that occurred within a module, or
      using remote GDB against a machine that uses dynamic modules,
      you need to tell GDB how to obtain symbol information for those

    <para>First, you need to build the module(s) with debugging

    <screen>&prompt.root; <userinput>cd /sys/modules/linux</userinput>
&prompt.root; <userinput>make clean; make COPTS=-g</userinput></screen>
    <para>If you are using remote GDB, you can run
      <command>kldstat</command> on the target machine to find out
      where the module was loaded:</para>

    <screen>&prompt.root; <userinput>kldstat</userinput>
Id Refs Address    Size     Name
 1    4 0xc0100000 1c1678   kernel
 2    1 0xc0a9e000 6000     linprocfs.ko
 3    1 0xc0ad7000 2000     warp_saver.ko
 4    1 0xc0adc000 11000    linux.ko

    <para>If you are debugging a crash dump, you'll need to walk the
      <literal>linker_files</literal> list, starting at
      <literal>linker_files->tqh_first</literal> and following the
      <literal>link.tqe_next</literal> pointers until you find the
      entry with the <literal>filename</literal> you are looking for.
      The <literal>address</literal> member of that entry is the load
      address of the module.</para>
    <para>Next, you need to find out the offset of the text section
      within the module:</para>

    <screen>&prompt.root; <userinput>objdump --section-headers /sys/modules/linux/linux.ko | grep text</userinput>
  3 .rel.text     000016e0  000038e0  000038e0  000038e0  2**2
 10 .text         00007f34  000062d0  000062d0  000062d0  2**2</screen>

    <para>The one you want is the <literal>.text</literal> section,
      section 10 in the above example.  The fourth numerical field
      (sixth field overall) is the offset in hex of the text section
      within the file (0x62d0 in our example).  Add this to the load
      address reported by <command>kldstat</command> to obtain the
      address of the module text in memory.</para>
    <para>Take the load address of the module (as reported by
      <command>kldstat</command>) and add the offset of the text
      section within the module (0x62d0 + 0xc0adc000 = c0ae22d0 in our
      example).  This is the address that the module code was
      relocated to.  Use the <command>add-symbol-file</command>
      command in GDB to tell the debugger about the module:</para>

    <screen><prompt>(kgdb)</prompt> <userinput>add-symbol-file /sys/modules/linux/linux.ko 0xc0ae22d0</userinput>
add symbol table from file "/sys/modules/linux/linux.ko" at text_addr = 0xc0ae22d0?
(y or n) <userinput>y</userinput>
Reading symbols from /sys/modules/linux/linux.ko...done.
    <para>You now have access to all the symbols in the module.</para>
    <title>Debugging a Console Driver</title>

    <para>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 <option>-h</option> at the <prompt>Boot:</prompt> 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

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