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https://github.com/cirosantilli/linux-kernel-module-cheat.git
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571 lines
36 KiB
HTML
571 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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|
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</head>
|
||
<body id="your-first-glibc-hack" class="article">
|
||
<div id="header">
|
||
<h1>Linux Kernel Module Cheat</h1>
|
||
</div>
|
||
<div id="content">
|
||
<div class="sect4">
|
||
<h5 id="your-first-glibc-hack"><a class="anchor" href="#your-first-glibc-hack"></a><a class="link" href="#your-first-glibc-hack">2.2.2.3. Your first glibc hack</a></h5>
|
||
<div class="paragraph">
|
||
<p>We use <a href="libc-choice.html">glibc as our default libc now</a>, and it is tracked as an unmodified submodule at <a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/submodules/glibc">submodules/glibc</a>, at the exact same version that Buildroot has it, which can be found at: <a href="https://github.com/buildroot/buildroot/blob/2018.05/package/glibc/glibc.mk#L13">package/glibc/glibc.mk</a>. Buildroot 2018.05 applies no patches.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Let’s hack up the <code>puts</code> function:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./build-buildroot -- glibc-reconfigure</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>with the patch:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>diff --git a/libio/ioputs.c b/libio/ioputs.c
|
||
index 706b20b492..23185948f3 100644
|
||
--- a/libio/ioputs.c
|
||
+++ b/libio/ioputs.c
|
||
@@ -38,8 +38,9 @@ _IO_puts (const char *str)
|
||
if ((_IO_vtable_offset (_IO_stdout) != 0
|
||
|| _IO_fwide (_IO_stdout, -1) == -1)
|
||
&& _IO_sputn (_IO_stdout, str, len) == len
|
||
+ && _IO_sputn (_IO_stdout, " hacked", 7) == 7
|
||
&& _IO_putc_unlocked ('\n', _IO_stdout) != EOF)
|
||
- result = MIN (INT_MAX, len + 1);
|
||
+ result = MIN (INT_MAX, len + 1 + 7);
|
||
|
||
_IO_release_lock (_IO_stdout);
|
||
return result;</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>And then:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./run --eval-after './c/hello.out'</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>outputs:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>hello hacked</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Lol!</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>We can also test our hacked glibc on <a href="user-mode-simulation.html">User mode simulation</a> with:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./run --userland userland/c/hello.c</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>I just noticed that this is actually a good way to develop glibc for other archs.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>In this example, we got away without recompiling the userland program because we made a change that did not affect the glibc ABI, see this answer for an introduction to ABI stability: <a href="https://stackoverflow.com/questions/2171177/what-is-an-application-binary-interface-abi/54967743#54967743" class="bare">https://stackoverflow.com/questions/2171177/what-is-an-application-binary-interface-abi/54967743#54967743</a></p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Note that for arch agnostic features that don’t rely on bleeding kernel changes that you host doesn’t yet have, you can develop glibc natively as explained at:</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p><a href="https://stackoverflow.com/questions/10412684/how-to-compile-my-own-glibc-c-standard-library-from-source-and-use-it/52454710#52454710" class="bare">https://stackoverflow.com/questions/10412684/how-to-compile-my-own-glibc-c-standard-library-from-source-and-use-it/52454710#52454710</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://stackoverflow.com/questions/847179/multiple-glibc-libraries-on-a-single-host/52454603#52454603" class="bare">https://stackoverflow.com/questions/847179/multiple-glibc-libraries-on-a-single-host/52454603#52454603</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://stackoverflow.com/questions/2856438/how-can-i-link-to-a-specific-glibc-version/52550158#52550158" class="bare">https://stackoverflow.com/questions/2856438/how-can-i-link-to-a-specific-glibc-version/52550158#52550158</a> more focus on symbol versioning, but no one knows how to do it, so I answered</p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Tested on a30ed0f047523ff2368d421ee2cce0800682c44e + 1.</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
</ul>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph nav-footer">
|
||
<p>← Previous: <a href="your-first-kernel-module-hack.html">Your first kernel module hack</a> | ↑ Up: <a href="how-to-hack-stuff.html">How to hack stuff</a> | ⌂ Home: <a href="README.html">Linux Kernel Module Cheat</a> | Next: <a href="your-first-binutils-hack.html">Your first Binutils hack</a> →</p>
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