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https://github.com/cirosantilli/linux-kernel-module-cheat.git
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568 lines
36 KiB
HTML
568 lines
36 KiB
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<meta name="description" content="The perfect emulation setup to study and develop the <<linux-kernel>> v5.9.2, kernel modules, <<qemu-buildroot-setup,QEMU>>, <<gem5-buildroot-setup,gem5>> and x86_64, ARMv7 and ARMv8 <<userland-assembly,userland>> and <<baremetal-setup,baremetal>> assembly, <<c,ANSI C>>, <<cpp,C++>> and <<posix,POSIX>>. EVERYTHING is built from source. <<gdb>> and <<kgdb>> just work. Powered by <<about-the-qemu-buildroot-setup,Buildroot>> and <<about-the-baremetal-setup,crosstool-NG>>. Highly automated. Thoroughly documented. Automated <<test-this-repo,tests>>. "Tested" in an Ubuntu 20.04 host.">
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<title>Linux Kernel Module Cheat</title>
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</head>
|
||
<body id="gdb-step-debug-early-boot" class="article">
|
||
<div id="header">
|
||
<h1>Linux Kernel Module Cheat</h1>
|
||
</div>
|
||
<div id="content">
|
||
<div class="sect2">
|
||
<h3 id="gdb-step-debug-early-boot"><a class="anchor" href="#gdb-step-debug-early-boot"></a><a class="link" href="#gdb-step-debug-early-boot">3.5. GDB step debug early boot</a></h3>
|
||
<div class="paragraph">
|
||
<p>TODO successfully debug the very first instruction that the Linux kernel runs, before <code>start_kernel</code>!</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Break at the very first instruction executed by QEMU:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./run-gdb --no-continue</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Note however that early boot parts appear to be relocated in memory somehow, and therefore:</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p>you won’t see the source location in GDB, only assembly</p>
|
||
</li>
|
||
<li>
|
||
<p>you won’t be able to break by symbol in those early locations</p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Further discussion at: <a href="linux-kernel-entry-point.html">Linux kernel entry point</a>.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>In the specific case of gem5 aarch64 at least:</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p>gem5 relocates the kernel in memory to a fixed location, see e.g. <a href="https://gem5.atlassian.net/browse/GEM5-787" class="bare">https://gem5.atlassian.net/browse/GEM5-787</a></p>
|
||
</li>
|
||
<li>
|
||
<p><code>--param 'system.workload.early_kernel_symbols=True</code> should in theory duplicate the symbols to the correct physical location, but it was broken at one point: <a href="https://gem5.atlassian.net/browse/GEM5-785" class="bare">https://gem5.atlassian.net/browse/GEM5-785</a></p>
|
||
</li>
|
||
<li>
|
||
<p>gem5 executes directly from vmlinux, so there is no decompression code involved, so you actually immediately start running the "true" first instruction from <code>head.S</code> as described at: <a href="https://stackoverflow.com/questions/18266063/does-linux-kernel-have-main-function/33422401#33422401" class="bare">https://stackoverflow.com/questions/18266063/does-linux-kernel-have-main-function/33422401#33422401</a></p>
|
||
</li>
|
||
<li>
|
||
<p>once the MMU gets turned on at kernel symbol <code>__primary_switched</code>, the virtual address matches the ELF symbols, and you start seeing correct symbols without the need for <code>early_kernel_symbols</code>. This can be observed clearly with <code>function_trace = True</code>: <a href="https://stackoverflow.com/questions/64049487/how-to-trace-executed-guest-function-symbol-names-with-their-timestamp-in-gem5/64049488#64049488" class="bare">https://stackoverflow.com/questions/64049487/how-to-trace-executed-guest-function-symbol-names-with-their-timestamp-in-gem5/64049488#64049488</a> which produces:</p>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>0: _kernel_flags_le_lo32 (12500)
|
||
12500: __crc_tcp_add_backlog (1000)
|
||
13500: __crc_crypto_alg_tested (6500)
|
||
20000: __crc_tcp_add_backlog (10000)
|
||
30000: __crc_crypto_alg_tested (500)
|
||
30500: __crc_scsi_is_host_device (5000)
|
||
35500: __crc_crypto_alg_tested (1500)
|
||
37000: __crc_scsi_is_host_device (4000)
|
||
41000: __crc_crypto_alg_tested (3000)
|
||
44000: __crc_tcp_add_backlog (263500)
|
||
307500: __crc_crypto_alg_tested (975500)
|
||
1283000: __crc_tcp_add_backlog (77191500)
|
||
78474500: __crc_crypto_alg_tested (1000)
|
||
78475500: __crc_scsi_is_host_device (19500)
|
||
78495000: __crc_crypto_alg_tested (500)
|
||
78495500: __crc_scsi_is_host_device (13500)
|
||
78509000: __primary_switched (14000)
|
||
78523000: memset (21118000)
|
||
99641000: __primary_switched (2500)
|
||
99643500: start_kernel (11000)</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>so we see that <code><em>primary_switched</code> is the first non-trash symbol (non-<code></em>crc_*</code> and non-<code><em>kernel_flags</em>*</code>, which are just informative symbols, not actual executable code)</p>
|
||
</div>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p><a href="linux-kernel-entry-point.html">Linux kernel entry point</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="linux-kernel-arch-agnostic-entry-point.html">Linux kernel arch-agnostic entry point</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="linux-kernel-early-boot-messages.html">Linux kernel early boot messages</a></p>
|
||
</li>
|
||
</ul>
|
||
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|
||
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|
||
<div class="paragraph nav-footer">
|
||
<p>← Previous: <a href="bypass-lx-symbols.html">Bypass lx-symbols</a> | ↑ Up: <a href="gdb.html">GDB step debug</a> | ⌂ Home: <a href="README.html">Linux Kernel Module Cheat</a> | Next: <a href="linux-kernel-entry-point.html">Linux kernel entry point</a> →</p>
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