2024-01-09 04:00:05 +01:00
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<title>SecBSD: Crash Reports</title>
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<meta name="description" content="How to report an SecBSD kernel crash">
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<h1><a href="index.html">
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<img src="/img/logo.png" alt="[SecBSD]"></a>
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<li><a href="index.html">About</a></li>
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2024-06-22 05:14:38 +02:00
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2024-01-09 04:00:05 +01:00
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</header>
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<div class="box">
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<b>█</b>
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<b>SecBSD: Crash Reports</b><br><br>
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<b class="purple">Minimum information for kernel problems</b>
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<p>Familiarize yourself with <a href="report.html">the general bug
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reporting procedures</a> first.
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All of that will apply.
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When reporting a kernel panic or crash, please remember:
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</p>
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<ul>
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<li>We need the console output on the screen.
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Capture it and save it.
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Serial consoles are best, but if you are on a VGA console you can
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<a href="https://www.openbsd.org/faq/faq7.html">scroll the console back</a>
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and take readable pictures with a phone or camera.
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<li>If the kernel panicked we need the traceback.
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It may be displayed on the screen.
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If you are at a
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<a href="https://man.openbsd.org/ddb.4">ddb</a>>
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prompt, type <kbd>trace</kbd>.
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If you are running SMP, use the <kbd>mach ddbcpu N</kbd> command for each
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of the <var>N</var> processors you have and repeat the <kbd>trace</kbd>
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command for each processor.
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<li>We need the process list.
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Use the command <kbd>ps</kbd> to get that.
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</ul>
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<p>
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Reports without the above information are useless.
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This is the minimum we need to be able to track down the issue.
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</p>
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<b class="purple">Additional information you can send</b>
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<p>
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In some situations more information is desirable.
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Below are outlined some additional steps you can take in certain situations:
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<ul>
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<li>
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If your crash appears to involve filesystems.
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The following additional things would be helpful
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<ul>
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<li>The output of the
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<a href="https://man.openbsd.org/ddb.4">ddb</a>> command
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<kbd>show uvm</kbd>
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<li>The output of the
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<a href="https://man.openbsd.org/ddb.4">ddb</a>>
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command <kbd>show bcstats</kbd>
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<li>The output of the <kbd>mount</kbd> command from your running machine, so
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we know what filesystems are mounted and how.
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</ul>
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<li> ... XXX boot crash? XXX
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<li> ... XXX show regs? XXX
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</ul>
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<b>Lost the panic message?</b>
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<p>
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Under some circumstances, you may lose the very first message of a panic,
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stating the reason for the panic.
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</p>
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<pre class="cmdbox">
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ddb> <b>show panic</b>
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0: kernel: page fault trap, code=0
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ddb>
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</pre>
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<b>Note for SMP systems</b>
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<p>
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You should get a trace from each processor as part of your report:
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</p>
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<pre class="cmdbox">
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ddb{0}> <b>trace</b>
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pool_get(d05e7c20,0,dab19ef8,d0169414,80) at pool_get+0x226
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fxp_add_rfabuf(d0a62000,d3c12b00,dab19f10,dab19f10) at fxp_add_rfabuf+0xa5
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fxp_intr(d0a62000) at fxp_intr+0x1e7
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Xintr_ioapic0() at Xintr_ioapic0+0x6d
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--- interrupt ---
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idle_loop+0x21:
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ddb{0}> <b>machine ddbcpu 1</b>
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Stopped at Debugger+0x4: leave
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ddb{1}> <b>trace</b>
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Debugger(d0319e28,d05ff5a0,dab1bee8,d031cc6e,d0a61800) at Debugger+0x4
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i386_ipi_db(d0a61800,d05ff5a0,dab1bef8,d01eb997) at i386_ipi_db+0xb
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i386_ipi_handler(b0,d05f0058,dab10010,d01d0010,dab10010) at i386_ipi_handler+0x
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4a
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Xintripi() at Xintripi+0x47
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--- interrupt ---
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i386_softintlock(0,58,dab10010,dab10010,d01e0010) at i386_softintlock+0x37
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Xintrltimer() at Xintrltimer+0x47
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--- interrupt ---
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idle_loop+0x21:
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ddb{1}>
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</pre>
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<p>
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Repeat the <code>machine ddbcpu x</code> followed by <code>trace</code> for each
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processor in your machine.
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</p>
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<b>How do I gather further information from a kernel crash?</b>
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<p>
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A typical kernel crash on SecBSD might look like this:
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<pre class="cmdbox">
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kernel: page fault trap, code=0
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Stopped at <b>pf_route+0x263</b>: mov 0x40(%edi),%edx
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ddb>
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</pre>
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<p>
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This crash happened at offset <code>0x263</code> in the function <code>pf_route</code>.
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</p>
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<p>
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The first command to run from the
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<a href="https://man.openbsd.org/ddb">ddb(4)</a> prompt is <code>trace</code>:
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<pre class="cmdbox">
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ddb> <b>trace</b>
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<b>pf_route</b>(e28cb7e4,e28bc978,2,1fad,d0b8b120) at <b>pf_route+0x263</b>
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pf_test(2,1f4ad,e28cb7e4,b4c1) at pf_test+0x706
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pf_route(e28cbb00,e28bc978,2,d0a65440,d0b8b120) at pf_route+0x207
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pf_test(2,d0a65440,e28cbb00,d023c282) at pf_test+0x706
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ip_output(d0b6a200,0,0,0,0) at ip_output+0xb67
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icmp_send(d0b6a200,0,1,a012) at icmp_send+0x57
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icmp_reflect(d0b6a200,0,1,0,3) at icmp_reflect+0x26b
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icmp_input(d0b6a200,14,0,0,d0b6a200) at icmp_input+0x42c
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ipv4_input(d0b6a200,e289f140,d0a489e0,e289f140) at ipv4_input+0x6eb
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ipintr(10,10,e289f140,e289f140,e28cbd38) at ipintr+0x8d
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Bad frame pointer: 0xe28cbcac
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ddb>
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</pre>
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<p>
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This tells us what function calls lead to the crash.
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</p>
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<p>
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To find out the particular line of C code that caused the crash, you can
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do the following:
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</p>
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<p>
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Find the source file where the crashing function is defined.
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In this example, that would be <code>pf_route()</code> in <code>/sys/net/pf.c</code>.
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Use <a href="https://man.openbsd.org/objdump">objdump(1)</a> to get the
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disassembly:
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<pre class="cmdbox">
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$ <b>cd /sys/arch/$(uname -m)/compile/GENERIC</b>
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$ <b>objdump -dlr obj/pf.o >/tmp/pf.dis</b>
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</pre>
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<p>
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In the output, grep for the function name:
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<pre class="cmdbox">
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$ <b>grep "<pf_route>:" /tmp/pf.dis</b>
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0000<b>7d88</b> <pf_route>:
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</pre>
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<p>
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Take this first hex number <code>7d88</code> and add the offset <code>0x263</code> from
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the <code>Stopped at</code> line:
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<pre class="cmdbox">
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$ <b>printf '%x\n' $((0x7d88 + 0x263))</b>
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7feb
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</pre>
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<p>
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Scroll down to the line <code>7feb</code>.
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The assembler instruction should match the one quoted in the <code>Stopped at</code>
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line.
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Then scroll up to the nearest C line number:
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<pre class="cmdbox">
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$ <b>more /tmp/pf.dis</b>
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/sys/net/pf.c:<b>3872</b>
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7fe7: 0f b7 43 02 movzwl 0x2(%ebx),%eax
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<b>7feb</b>: 8b 57 40 <b>mov 0x40(%edi),%edx</b>
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7fee: 39 d0 cmp %edx,%eax
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7ff0: 0f 87 92 00 00 00 ja 8088 <pf_route+0x300>
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</pre>
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<p>
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So, it's precisely line <code>3872</code> of <code>pf.c</code> that crashes:
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<pre class="cmdbox">
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$ <b>nl -ba /sys/net/pf.c | sed -n 3872p</b>
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3872 if ((u_int16_t)ip->ip_len <= ifp->if_mtu) {
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</pre>
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<p>
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The kernel that produced the crash output and the object file for objdump must
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be compiled from the exact same source file, otherwise the offsets won't match.
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</p>
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<p>
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If you provide both the ddb trace output and the relevant objdump section,
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that's very helpful.
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</p>
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</div>
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</body>
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</html>
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