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linux-kernel-module-cheat/oops.html
Ciro Santilli 0a2a05fe28 8e6cdcb14b
2025-05-10 07:50:53 +01:00

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<meta name="description" content="The perfect emulation setup to study and develop the &lt;&lt;linux-kernel&gt;&gt; v5.9.2, kernel modules, &lt;&lt;qemu-buildroot-setup,QEMU&gt;&gt;, &lt;&lt;gem5-buildroot-setup,gem5&gt;&gt; and x86_64, ARMv7 and ARMv8 &lt;&lt;userland-assembly,userland&gt;&gt; and &lt;&lt;baremetal-setup,baremetal&gt;&gt; assembly, &lt;&lt;c,ANSI C&gt;&gt;, &lt;&lt;cpp,C++&gt;&gt; and &lt;&lt;posix,POSIX&gt;&gt;. EVERYTHING is built from source. &lt;&lt;gdb&gt;&gt; and &lt;&lt;kgdb&gt;&gt; just work. Powered by &lt;&lt;about-the-qemu-buildroot-setup,Buildroot&gt;&gt; and &lt;&lt;about-the-baremetal-setup,crosstool-NG&gt;&gt;. Highly automated. Thoroughly documented. Automated &lt;&lt;test-this-repo,tests&gt;&gt;. "Tested" in an Ubuntu 20.04 host.">
<title>Linux Kernel Module Cheat</title>
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</head>
<body id="oops" class="article">
<div id="header">
<h1>Linux Kernel Module Cheat</h1>
</div>
<div id="content">
<div class="sect3">
<h4 id="oops"><a class="anchor" href="#oops"></a><a class="link" href="#oops">17.6.2. Kernel oops</a></h4>
<div class="paragraph">
<p>On oops, the shell still lives after.</p>
</div>
<div class="paragraph">
<p>However we:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>leave the normal control flow, and <code>oops after</code> never gets printed: an interrupt is serviced</p>
</li>
<li>
<p>cannot <code>rmmod oops</code> afterwards</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>It is possible to make <code>oops</code> lead to panics always with:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>echo 1 &gt; /proc/sys/kernel/panic_on_oops
insmod oops.ko</pre>
</div>
</div>
<div class="paragraph">
<p>An oops stack trace looks like:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
IP: myinit+0x18/0x30 [oops]
PGD dccf067 P4D dccf067 PUD dcc1067 PMD 0
Oops: 0002 [#1] SMP NOPTI
Modules linked in: oops(O+)
CPU: 0 PID: 53 Comm: insmod Tainted: G O 4.16.0 #6
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.11.0-0-g63451fca13-prebuilt.qemu-project.org 04/01/2014
RIP: 0010:myinit+0x18/0x30 [oops]
RSP: 0018:ffffc900000d3cb0 EFLAGS: 00000282
RAX: 000000000000000b RBX: ffffffffc0000000 RCX: ffffffff81e3e3a8
RDX: 0000000000000001 RSI: 0000000000000086 RDI: ffffffffc0001033
RBP: ffffc900000d3e30 R08: 69796d2073706f6f R09: 000000000000013b
R10: ffffea0000373280 R11: ffffffff822d8b2d R12: 0000000000000000
R13: ffffffffc0002050 R14: ffffffffc0002000 R15: ffff88000dc934c8
FS: 00007ffff7ff66a0(0000) GS:ffff88000fc00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000000dcd2000 CR4: 00000000000006f0
Call Trace:
do_one_initcall+0x3e/0x170
do_init_module+0x5b/0x210
load_module+0x2035/0x29d0
? SyS_finit_module+0xa8/0xb0
SyS_finit_module+0xa8/0xb0
do_syscall_64+0x6f/0x310
? trace_hardirqs_off_thunk+0x1a/0x32
entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x7ffff7b36206
RSP: 002b:00007fffffffeb78 EFLAGS: 00000206 ORIG_RAX: 0000000000000139
RAX: ffffffffffffffda RBX: 000000000000005c RCX: 00007ffff7b36206
RDX: 0000000000000000 RSI: 000000000069e010 RDI: 0000000000000003
RBP: 000000000069e010 R08: 00007ffff7ddd320 R09: 0000000000000000
R10: 00007ffff7ddd320 R11: 0000000000000206 R12: 0000000000000003
R13: 00007fffffffef4b R14: 0000000000000000 R15: 0000000000000000
Code: &lt;c7&gt; 04 25 00 00 00 00 00 00 00 00 e8 b2 33 09 c1 31 c0 c3 0f 1f 44
RIP: myinit+0x18/0x30 [oops] RSP: ffffc900000d3cb0
CR2: 0000000000000000
---[ end trace 3cdb4e9d9842b503 ]---</pre>
</div>
</div>
<div class="paragraph">
<p>To find the line that oopsed, look at the <code>RIP</code> register:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>RIP: 0010:myinit+0x18/0x30 [oops]</pre>
</div>
</div>
<div class="paragraph">
<p>and then on GDB:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>./run-gdb</pre>
</div>
</div>
<div class="paragraph">
<p>run</p>
</div>
<div class="literalblock">
<div class="content">
<pre>info line *(myinit+0x18)</pre>
</div>
</div>
<div class="paragraph">
<p>which gives us the correct line:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>Line 7 of "/root/linux-kernel-module-cheat/out/kernel_modules/x86_64/kernel_modules/panic.c" starts at address 0xbf00001c &lt;myinit+28&gt; and ends at 0xbf00002c &lt;myexit&gt;.</pre>
</div>
</div>
<div class="paragraph">
<p>This-did not work on <code>arm</code> due to <a href="gdb-step-debug-kernel-module-arm.html">GDB step debug kernel module insmodded by init on ARM</a> so we need to either:</p>
</div>
<div class="ulist">
<ul>
<li>
<p><a href="gdb-module-init.html">GDB module_init</a></p>
</li>
<li>
<p><a href="kernel-module-stack-trace-to-source-line.html">Kernel module stack trace to source line</a> post-mortem method</p>
</li>
</ul>
</div>
<div class="ulist">
<ul>
</ul>
</div>
</div>
<div class="paragraph nav-footer">
<p> Previous: <a href="panic-trace-show-addresses-instead-of-symbols.html">Panic trace show addresses instead of symbols</a> | ↑ Up: <a href="kernel-panic-and-oops.html">Kernel panic and oops</a> | ⌂ Home: <a href="README.html">Linux Kernel Module Cheat</a> | Next: <a href="dump-stack.html">dump_stack</a> →</p>
</div>
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