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https://github.com/cirosantilli/linux-kernel-module-cheat.git
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577 lines
36 KiB
HTML
577 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="arm-big-endian-mode" class="article">
|
||
<div id="header">
|
||
<h1>Linux Kernel Module Cheat</h1>
|
||
</div>
|
||
<div id="content">
|
||
<div class="sect4">
|
||
<h5 id="arm-big-endian-mode"><a class="anchor" href="#arm-big-endian-mode"></a><a class="link" href="#arm-big-endian-mode">30.1.3.2. ARM big endian mode</a></h5>
|
||
<div class="paragraph">
|
||
<p>ARM can switch between big and little endian mode on the fly!</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>However, everyone only uses little endian, so the big endian ecosystem is not as supported.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>TODO is there any advantage of using big endian?</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Here Peter mentions that QEMU does "support" big endian in theory, but that there are no machines for it not sure what that implies: <a href="https://stackoverflow.com/questions/41571643/emulatin-big-endian-arm-system-with-qemu" class="bare">https://stackoverflow.com/questions/41571643/emulatin-big-endian-arm-system-with-qemu</a></p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>We can try it out quickly in user mode with:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>touch userland/arch/aarch64/freestanding/linux/hello.S
|
||
./build-userland --arch aarch64 --ccflags=-mbig-endian userland/arch/aarch64/freestanding/linux/hello.S
|
||
./run --arch aarch64 --userland userland/arch/aarch64/freestanding/linux/hello.S</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>and it fails with:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>Invalid ELF image for this architecture</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>From this we can guess that the big endian metadata is actually stored in the <a href="elf.html">ELF</a> file, and confirm that with:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./run-toolchain \
|
||
--arch aarch64 \
|
||
readelf \
|
||
-- \
|
||
--file-header "$(./getvar --arch aarch64 userland_build_dir)/arch/aarch64/freestanding/linux/hello.out" \
|
||
;</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>which contains:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>Data: 2's complement, big endian</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>instead of the default:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>Data: 2's complement, little endian</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>TODO does the Linux kernel support running big endian executables? I tried after building the big endian executable:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./build-buildroot --arch aarch64
|
||
./run --arch aarch64 --eval-after ./arch/aarch64/freestanding/linux/hello.out</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>but that failed with:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>/lkmc/arch/aarch64/freestanding/linux/hello.out: line 1: ELF@x@0@8@: not found
|
||
/lkmc/arch/aarch64/freestanding/linux/hello.out: line 2: @@: not found
|
||
/lkmc/arch/aarch64/freestanding/linux/hello.out: line 3: syntax error: unexpected ")"</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>TODO:</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p>can you compile the Linux kernel itself as big endian? Looks like yes since there is a <a href="https://github.com/torvalds/linux/blob/v5.1/arch/arm64/Kconfig#L791"><code>config CPU_BIG_ENDIAN</code></a> See also: <a href="https://unix.stackexchange.com/questions/378829/getting-big-endian-linux-build-to-boot-on-arm-with-u-boot" class="bare">https://unix.stackexchange.com/questions/378829/getting-big-endian-linux-build-to-boot-on-arm-with-u-boot</a></p>
|
||
</li>
|
||
<li>
|
||
<p>how can be is the endianess be checked and modified in the CPU?</p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
</ul>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph nav-footer">
|
||
<p>← Previous: <a href="arm-thumb-encoding.html">ARM Thumb encoding</a> | ↑ Up: <a href="arm-instruction-encodings.html">ARM instruction encodings</a> | ⌂ Home: <a href="README.html">Linux Kernel Module Cheat</a> | Next: <a href="arm-branch-instructions.html">ARM branch instructions</a> →</p>
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