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https://github.com/cirosantilli/linux-kernel-module-cheat.git
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551 lines
36 KiB
HTML
551 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>>. <<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="gem5-m5out-system-dmesg-file" class="article">
|
||
<div id="header">
|
||
<h1>Linux Kernel Module Cheat</h1>
|
||
</div>
|
||
<div id="content">
|
||
<div class="sect3">
|
||
<h4 id="gem5-m5out-system-dmesg-file"><a class="anchor" href="#gem5-m5out-system-dmesg-file"></a><a class="link" href="#gem5-m5out-system-dmesg-file">24.10.2. gem5 <code>m5out/system.workload.dmesg</code> file</a></h4>
|
||
<div class="paragraph">
|
||
<p>This file used to be called just <code>m5out/system.dmesg</code>, but the name was changed after the workload refactorings of March 2020.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>This file is capable of showing terminal messages that are <code>printk</code> before the serial is enabled as described at: <a href="linux-kernel-early-boot-messages.html">Linux kernel early boot messages</a>.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>The file is dumped only on kernel panics which gem5 can detect by the PC address: <a href="exit-gem5-on-panic.html">Exit gem5 on panic</a>.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>This mechanism can be very useful to debug the Linux kernel boot if problems happen before the serial is enabled.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>This magic mechanism works by activating an event when the PC reaches the <code>printk</code> address, much like gem5 <a href="exit-gem5-on-panic.html">can detect <code>panic</code> by PC</a> and then parsing printk function arguments and buffers!</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>The relevant source is at <a href="https://github.com/gem5/gem5/blob/cd69bb50414450c3bb5ef41dce676b75fd42c0ee/src/kern/linux/printk.cc"><code>src/kern/linux/printk.c</code></a>.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>We can test this mechanism in a controlled way by hacking a <code>panic()</code> into the kernel next to a <code>printk</code> that shows up before the serial is enabled, e.g. on Linux v5.4.3 we could do:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
|
||
index f296d89be757..3e79916322c2 100644
|
||
--- a/kernel/trace/ftrace.c
|
||
+++ b/kernel/trace/ftrace.c
|
||
@@ -6207,6 +6207,7 @@ void __init ftrace_init(void)
|
||
|
||
pr_info("ftrace: allocating %ld entries in %ld pages\n",
|
||
count, count / ENTRIES_PER_PAGE + 1);
|
||
+ panic("foobar");
|
||
|
||
last_ftrace_enabled = ftrace_enabled = 1;</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>With this, after the panic, <code>system.workload.dmesg</code> contains on LKMC d09a0d97b81582cc88381c4112db631da61a048d aarch64:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>[0.000000] Booting Linux on physical CPU 0x0000000000 [0x410fd070]
|
||
[0.000000] Linux version 5.4.3-dirty (lkmc@f7688b48ac46e9a669e279f1bc167722d5141eda) (gcc version 8.3.0 (Buildroot 2019.11-00002-g157ac499cf)) #1 SMP Thu Jan 1 00:00:00 UTC 1970
|
||
[0.000000] Machine model: V2P-CA15
|
||
[0.000000] Memory limited to 256MB
|
||
[0.000000] efi: Getting EFI parameters from FDT:
|
||
[0.000000] efi: UEFI not found.
|
||
[0.000000] On node 0 totalpages: 65536
|
||
[0.000000] DMA32 zone: 1024 pages used for memmap
|
||
[0.000000] DMA32 zone: 0 pages reserved
|
||
[0.000000] DMA32 zone: 65536 pages, LIFO batch:15
|
||
[0.000000] percpu: Embedded 29 pages/cpu s79960 r8192 d30632 u118784
|
||
[0.000000] pcpu-alloc: s79960 r8192 d30632 u118784 alloc=29*4096
|
||
[0.000000] pcpu-alloc: [0] 0
|
||
[0.000000] Detected PIPT I-cache on CPU0
|
||
[0.000000] CPU features: detected: ARM erratum 832075
|
||
[0.000000] CPU features: detected: EL2 vector hardening
|
||
[0.000000] ARM_SMCCC_ARCH_WORKAROUND_1 missing from firmware
|
||
[0.000000] Built 1 zonelists, mobility grouping on. Total pages: 64512
|
||
[0.000000] Kernel command line: earlyprintk=pl011,0x1c090000 lpj=19988480 rw loglevel=8 mem=256MB root=/dev/sda console_msg_format=syslog nokaslr norandmaps panic=-1 printk.devkmsg=on printk.time=y rw console=ttyAMA0 - lkmc_home=/lkmc
|
||
[0.000000] Dentry cache hash table entries: 32768 (order: 6, 262144 bytes, linear)
|
||
[0.000000] Inode-cache hash table entries: 16384 (order: 5, 131072 bytes, linear)
|
||
[0.000000] mem auto-init: stack:off, heap alloc:off, heap free:off
|
||
[0.000000] Memory: 233432K/262144K available (6652K kernel code, 792K rwdata, 2176K rodata, 896K init, 659K bss, 28712K reserved, 0K cma-reserved)
|
||
[0.000000] SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
|
||
[0.000000] ftrace: allocating 22067 entries in 87 pages</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>So we see that messages up to the <code>ftrace</code> do show up!</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
</ul>
|
||
</div>
|
||
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|
||
<div class="paragraph nav-footer">
|
||
<p>← Previous: <a href="gem5-m5out-system-terminal-file.html">gem5 m5out/system.terminal file</a> | ↑ Up: <a href="m5out-directory.html">m5out directory</a> | ⌂ Home: <a href="README.html">Linux Kernel Module Cheat</a> | Next: <a href="gem5-m5out-stats-txt-file.html">gem5 m5out/stats.txt file</a> →</p>
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