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@ -3740,7 +3740,7 @@ they will not be forgotten and will activate when the unlock happens, using the
</p>
<h4 class='subsectionHead' id='read-and-write-locks'><span class='titlemark'>0.12.3 </span> <a id='x1-470000.12.3'></a>Read and write locks</h4>
<!-- l. 1351 --><p class='noindent'>Read and write locks are specialised kinds of spinlocks so that you can exclusively
read from something or write to something. Like the earlier spinlocks example the
read from something or write to something. Like the earlier spinlocks example, the
one below shows an "irq safe" situation in which if other functions were triggered
from irqs which might also read and write to whatever you are concerned with
then they would not disrupt the logic. As before it is a good idea to keep
@ -3811,7 +3811,7 @@ which could possibly interfere with your logic then you can use the simpler
</code> or the corresponding write functions.
</p>
<h4 class='subsectionHead' id='atomic-operations'><span class='titlemark'>0.12.4 </span> <a id='x1-480000.12.4'></a>Atomic operations</h4>
<!-- l. 1360 --><p class='noindent'>If you are doing simple arithmetic: adding, subtracting or bitwise operations then
<!-- l. 1360 --><p class='noindent'>If you are doing simple arithmetic: adding, subtracting or bitwise operations, then
there is another way in the multi-CPU and multi-hyperthreaded world to stop other
parts of the system from messing with your mojo. By using atomic operations you
can be confident that your addition, subtraction or bit flip did actually happen
@ -3903,14 +3903,14 @@ below.
</p>
<h4 class='subsectionHead' id='replacement'><span class='titlemark'>0.13.1 </span> <a id='x1-500000.13.1'></a>Replacement</h4>
<!-- l. 1371 --><p class='noindent'>In Section <a href='#x1-80042'>2<!-- tex4ht:ref: sec:using_x --></a>, I said that X Window System and kernel module programming do not
mix. That is true for developing kernel modules, but in actual use, you want to be
mix. That is true for developing kernel modules. But in actual use, you want to be
able to send messages to whichever tty the command to load the module came
from.
</p><!-- l. 1374 --><p class='indent'> "tty" is an abbreviation of <span class='ecti-1000'>teletype</span>: originally a combination keyboard-printer
</p><!-- l. 1375 --><p class='indent'> "tty" is an abbreviation of <span class='ecti-1000'>teletype</span>: originally a combination keyboard-printer
used to communicate with a Unix system, and today an abstraction for the text
stream used for a Unix program, whether it is a physical terminal, an xterm on an X
display, a network connection used with ssh, etc.
</p><!-- l. 1376 --><p class='indent'> The way this is done is by using current, a pointer to the currently running task,
</p><!-- l. 1377 --><p class='indent'> The way this is done is by using current, a pointer to the currently running task,
to get the current task’s tty structure. Then, we look inside that tty structure to find
a pointer to a string write function, which we use to write a string to the
tty.
@ -3993,16 +3993,16 @@ tty.
<a id='x1-50150r75'></a><span class='ecrm-0500'>75</span><span class='ectt-0800'>module_exit(print_string_exit);</span>
<a id='x1-50152r76'></a><span class='ecrm-0500'>76</span>
<a id='x1-50154r77'></a><span class='ecrm-0500'>77</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2033'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1381 --><p class='noindent'>
<!-- l. 1382 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='flashing-keyboard-leds'><span class='titlemark'>0.13.2 </span> <a id='x1-510000.13.2'></a>Flashing keyboard LEDs</h4>
<!-- l. 1383 --><p class='noindent'>In certain conditions, you may desire a simpler and more direct way to communicate
<!-- l. 1384 --><p class='noindent'>In certain conditions, you may desire a simpler and more direct way to communicate
to the external world. Flashing keyboard LEDs can be such a solution: It is an
immediate way to attract attention or to display a status condition. Keyboard LEDs
are present on every hardware, they are always visible, they do not need any setup,
and their use is rather simple and non-intrusive, compared to writing to a tty or a
file.
</p><!-- l. 1387 --><p class='indent'> The following source code illustrates a minimal kernel module which, when
</p><!-- l. 1388 --><p class='indent'> The following source code illustrates a minimal kernel module which, when
loaded, starts blinking the keyboard LEDs until it is unloaded.
</p><!-- l. 1 --><p class='indent'>
</p>
@ -4098,33 +4098,33 @@ loaded, starts blinking the keyboard LEDs until it is unloaded.
<!-- l. 1391 --><p class='indent'> If none of the examples in this chapter fit your debugging needs
<!-- l. 1392 --><p class='indent'> If none of the examples in this chapter fit your debugging needs,
there might yet be some other tricks to try. Ever wondered what
<code> <span class='ectt-1000'>CONFIG_LL_DEBUG</span>
</code> in <code> <span class='ectt-1000'>make menuconfig</span>
</code> is good for? If you activate that you get low level access to the serial port. While this
might not sound very powerful by itself, you can patch <a href='https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/kernel/printk.c'>kernel/printk.c</a> or any other
essential syscall to print ASCII characters, thus makeing it possible to trace virtually
essential syscall to print ASCII characters, thus making it possible to trace virtually
everything what your code does over a serial line. If you find yourself porting the
kernel to some new and former unsupported architecture, this is usually amongst the
first things that should be implemented. Logging over a netconsole might also be
worth a try.
</p><!-- l. 1398 --><p class='indent'> While you have seen lots of stuff that can be used to aid debugging here, there are
</p><!-- l. 1399 --><p class='indent'> While you have seen lots of stuff that can be used to aid debugging here, there are
some things to be aware of. Debugging is almost always intrusive. Adding debug code
can change the situation enough to make the bug seem to dissappear. Thus you
should try to keep debug code to a minimum and make sure it does not show up in
production code.
</p><!-- l. 1402 --><p class='noindent'>
</p><!-- l. 1403 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='scheduling-tasks'><span class='titlemark'>0.14 </span> <a id='x1-520000.14'></a>Scheduling Tasks</h3>
<!-- l. 1404 --><p class='noindent'>There are two main ways of running tasks: tasklets and work queues. Tasklets are a
quick and easy way of scheduling a single function to be run, for example when
<!-- l. 1405 --><p class='noindent'>There are two main ways of running tasks: tasklets and work queues. Tasklets are a
quick and easy way of scheduling a single function to be run. For example, when
triggered from an interrupt, whereas work queues are more complicated but also
better suited to running multiple things in a sequence.
</p><!-- l. 1407 --><p class='noindent'>
</p><!-- l. 1409 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='tasklets'><span class='titlemark'>0.14.1 </span> <a id='x1-530000.14.1'></a>Tasklets</h4>
<!-- l. 1409 --><p class='noindent'>Here is an example tasklet module. The
<!-- l. 1411 --><p class='noindent'>Here is an example tasklet module. The
<code> <span class='ectt-1000'>tasklet_fn</span>
</code> function runs for a few seconds and in the mean time execution of the
<code> <span class='ectt-1000'>example_tasklet_init</span>
@ -4168,7 +4168,7 @@ better suited to running multiple things in a sequence.
<a id='x1-53072r35'></a><span class='ecrm-0500'>35</span>
<a id='x1-53074r36'></a><span class='ecrm-0500'>36</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2153'><span class='ectt-0800'>"Tasklet example"</span></span><span class='ectt-0800'>);</span>
<a id='x1-53076r37'></a><span class='ecrm-0500'>37</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2154'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1414 --><p class='indent'> So with this example loaded <code> <span class='ectt-1000'>dmesg</span>
<!-- l. 1416 --><p class='indent'> So with this example loaded <code> <span class='ectt-1000'>dmesg</span>
</code> should show:
@ -4180,11 +4180,11 @@ Example tasklet starts
Example tasklet init continues...
Example tasklet ends
</pre>
<!-- l. 1421 --><p class='nopar'>
</p><!-- l. 1423 --><p class='noindent'>
<!-- l. 1423 --><p class='nopar'>
</p><!-- l. 1425 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='work-queues'><span class='titlemark'>0.14.2 </span> <a id='x1-540000.14.2'></a>Work queues</h4>
<!-- l. 1425 --><p class='noindent'>To add a task to the scheduler we can use a workqueue. The kernel then uses the
<!-- l. 1427 --><p class='noindent'>To add a task to the scheduler we can use a workqueue. The kernel then uses the
Completely Fair Scheduler (CFS) to execute work within the queue.
</p><!-- l. 1 --><p class='indent'>
</p>
@ -4222,21 +4222,21 @@ Completely Fair Scheduler (CFS) to execute work within the queue.
<a id='x1-54064r32'></a><span class='ecrm-0500'>32</span>
<a id='x1-54066r33'></a><span class='ecrm-0500'>33</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2182'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span>
<a id='x1-54068r34'></a><span class='ecrm-0500'>34</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2183'><span class='ectt-0800'>"Workqueue example"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1430 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='interrupt-handlers'><span class='titlemark'>0.15 </span> <a id='x1-550000.15'></a>Interrupt Handlers</h3>
<!-- l. 1432 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='interrupt-handlers'><span class='titlemark'>0.15 </span> <a id='x1-550000.15'></a>Interrupt Handlers</h3>
<!-- l. 1434 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='interrupt-handlers1'><span class='titlemark'>0.15.1 </span> <a id='x1-560000.15.1'></a>Interrupt Handlers</h4>
<!-- l. 1434 --><p class='noindent'>Except for the last chapter, everything we did in the kernel so far we have done as a
<!-- l. 1436 --><p class='noindent'>Except for the last chapter, everything we did in the kernel so far we have done as a
response to a process asking for it, either by dealing with a special file, sending an
<code> <span class='ectt-1000'>ioctl()</span>
</code>, or issuing a system call. But the job of the kernel is not just to respond to process
requests. Another job, which is every bit as important, is to speak to the hardware
connected to the machine.
</p><!-- l. 1438 --><p class='indent'> There are two types of interaction between the CPU and the rest of the
</p><!-- l. 1440 --><p class='indent'> There are two types of interaction between the CPU and the rest of the
computer’s hardware. The first type is when the CPU gives orders to the hardware,
the other is when the hardware needs to tell the CPU something. The second, called
the order is when the hardware needs to tell the CPU something. The second, called
interrupts, is much harder to implement because it has to be dealt with when
convenient for the hardware, not the CPU. Hardware devices typically have a very
small amount of RAM, and if you do not read their information when available, it is
@ -4244,14 +4244,14 @@ lost.
</p><!-- l. 1443 --><p class='indent'> Under Linux, hardware interrupts are called IRQ’s (Interrupt ReQuests). There
</p><!-- l. 1445 --><p class='indent'> Under Linux, hardware interrupts are called IRQ’s (Interrupt ReQuests). There
are two types of IRQ’s, short and long. A short IRQ is one which is expected to take
a very short period of time, during which the rest of the machine will be blocked and
no other interrupts will be handled. A long IRQ is one which can take longer, and
during which other interrupts may occur (but not interrupts from the same
device). If at all possible, it is better to declare an interrupt handler to be
long.
</p><!-- l. 1449 --><p class='indent'> When the CPU receives an interrupt, it stops whatever it is doing (unless it is
</p><!-- l. 1451 --><p class='indent'> When the CPU receives an interrupt, it stops whatever it is doing (unless it is
processing a more important interrupt, in which case it will deal with this one
only when the more important one is done), saves certain parameters on
the stack and calls the interrupt handler. This means that certain things
@ -4263,10 +4263,10 @@ the new information at a later time (this is called the "bottom half") and
return. The kernel is then guaranteed to call the bottom half as soon as
possible – and when it does, everything allowed in kernel modules will be
allowed.
</p><!-- l. 1455 --><p class='indent'> The way to implement this is to call
</p><!-- l. 1457 --><p class='indent'> The way to implement this is to call
<code> <span class='ectt-1000'>request_irq()</span>
</code> to get your interrupt handler called when the relevant IRQ is received.
</p><!-- l. 1457 --><p class='indent'> In practice IRQ handling can be a bit more complex. Hardware is often
</p><!-- l. 1459 --><p class='indent'> In practice IRQ handling can be a bit more complex. Hardware is often
designed in a way that chains two interrupt controllers, so that all the IRQs
from interrupt controller B are cascaded to a certain IRQ from interrupt
controller A. Of course, that requires that the kernel finds out which IRQ it
@ -4280,8 +4280,8 @@ need to solve another truckload of problems. It is not enough to know if
a certain IRQs has happend, it’s also important for what CPU(s) it was
for. People still interested in more details, might want to refer to "APIC"
now.
</p><!-- l. 1466 --><p class='indent'> This function receives the IRQ number, the name of the function,
flags, a name for <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>/proc/interrupts</span></span></span> and a parameter to pass to the
</p><!-- l. 1468 --><p class='indent'> This function receives the IRQ number, the name of the function,
flags, a name for <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>/proc/interrupts</span></span></span> and a parameter to be passed to the
interrupt handler. Usually there is a certain number of IRQs available.
How many IRQs there are is hardware-dependent. The flags can include
<code> <span class='ectt-1000'>SA_SHIRQ</span>
@ -4293,16 +4293,16 @@ already a handler on this IRQ, or if you are both willing to share.
</p><!-- l. 1472 --><p class='noindent'>
</p><!-- l. 1474 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='detecting-button-presses'><span class='titlemark'>0.15.2 </span> <a id='x1-570000.15.2'></a>Detecting button presses</h4>
<!-- l. 1474 --><p class='noindent'>Many popular single board computers, such as Raspberry Pi or Beagleboards, have a
<!-- l. 1476 --><p class='noindent'>Many popular single board computers, such as Raspberry Pi or Beagleboards, have a
bunch of GPIO pins. Attaching buttons to those and then having a button press do
something is a classic case in which you might need to use interrupts, so that instead
of having the CPU waste time and battery power polling for a change in input state
of having the CPU waste time and battery power polling for a change in input state,
it is better for the input to trigger the CPU to then run a particular handling
function.
</p><!-- l. 1478 --><p class='indent'> Here is an example where buttons are connected to GPIO numbers 17 and 18 and
</p><!-- l. 1480 --><p class='indent'> Here is an example where buttons are connected to GPIO numbers 17 and 18 and
an LED is connected to GPIO 4. You can change those numbers to whatever is
appropriate for your board.
</p><!-- l. 1 --><p class='indent'>
@ -4452,14 +4452,14 @@ appropriate for your board.
<a id='x1-57286r143'></a><span class='ecrm-0500'>143</span>
<a id='x1-57288r144'></a><span class='ecrm-0500'>144</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2290'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span>
<a id='x1-57290r145'></a><span class='ecrm-0500'>145</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2291'><span class='ectt-0800'>"Handle some GPIO interrupts"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1483 --><p class='noindent'>
<!-- l. 1485 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='bottom-half'><span class='titlemark'>0.15.3 </span> <a id='x1-580000.15.3'></a>Bottom Half</h4>
<!-- l. 1485 --><p class='noindent'>Suppose you want to do a bunch of stuff inside of an interrupt routine. A common
<!-- l. 1487 --><p class='noindent'>Suppose you want to do a bunch of stuff inside of an interrupt routine. A common
way to do that without rendering the interrupt unavailable for a significant duration
is to combine it with a tasklet. This pushes the bulk of the work off into the
scheduler.
</p><!-- l. 1489 --><p class='indent'> The example below modifies the previous example to also run an additional task
</p><!-- l. 1491 --><p class='indent'> The example below modifies the previous example to also run an additional task
when an interrupt is triggered.
</p><!-- l. 1 --><p class='indent'>
</p>
@ -4624,22 +4624,22 @@ when an interrupt is triggered.
<a id='x1-58318r159'></a><span class='ecrm-0500'>159</span>
<a id='x1-58320r160'></a><span class='ecrm-0500'>160</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2412'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span>
<a id='x1-58322r161'></a><span class='ecrm-0500'>161</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2413'><span class='ectt-0800'>"Interrupt with top and bottom half"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1493 --><p class='noindent'>
<!-- l. 1495 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='crypto'><span class='titlemark'>0.16 </span> <a id='x1-590000.16'></a>Crypto</h3>
<!-- l. 1495 --><p class='noindent'>At the dawn of the internet everybody trusted everybody completely…but that did
not work out so well. When this guide was originally written it was a more innocent
<!-- l. 1497 --><p class='noindent'>At the dawn of the internet, everybody trusted everybody completely…but that did
not work out so well. When this guide was originally written, it was a more innocent
era in which almost nobody actually gave a damn about crypto - least of all kernel
developers. That is certainly no longer the case now. To handle crypto stuff the
developers. That is certainly no longer the case now. To handle crypto stuff, the
kernel has its own API enabling common methods of encryption, decryption and your
favourite hash functions.
</p><!-- l. 1500 --><p class='noindent'>
</p><!-- l. 1502 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='hash-functions'><span class='titlemark'>0.16.1 </span> <a id='x1-600000.16.1'></a>Hash functions</h4>
<!-- l. 1503 --><p class='noindent'>Calculating and checking the hashes of things is a common operation. Here is a
<!-- l. 1505 --><p class='noindent'>Calculating and checking the hashes of things is a common operation. Here is a
demonstration of how to calculate a sha256 hash within a kernel module.
</p><!-- l. 1 --><p class='indent'>
</p>
@ -4705,21 +4705,21 @@ demonstration of how to calculate a sha256 hash within a kernel module.
<a id='x1-60120r60'></a><span class='ecrm-0500'>60</span>
<a id='x1-60122r61'></a><span class='ecrm-0500'>61</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2463'><span class='ectt-0800'>"sha256 hash test"</span></span><span class='ectt-0800'>);</span>
<a id='x1-60124r62'></a><span class='ecrm-0500'>62</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2464'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1508 --><p class='indent'> Make and install the module:
<!-- l. 1510 --><p class='indent'> Make and install the module:
</p><!-- l. 1 --><p class='indent'>
</p>
<pre class='fancyvrb' id='fancyvrb68'><a id='x1-60129r1'></a><span class='ecrm-0500'>1</span><span class='ectt-1000'>make</span>
<a id='x1-60131r2'></a><span class='ecrm-0500'>2</span><span class='ectt-1000'>sudo insmod cryptosha256.ko</span>
<a id='x1-60133r3'></a><span class='ecrm-0500'>3</span><span class='ectt-1000'>dmesg</span></pre>
<!-- l. 1516 --><p class='indent'> And you should see that the hash was calculated for the test string.
</p><!-- l. 1518 --><p class='indent'> Finally, remove the test module:
<!-- l. 1518 --><p class='indent'> And you should see that the hash was calculated for the test string.
</p><!-- l. 1520 --><p class='indent'> Finally, remove the test module:
</p><!-- l. 1 --><p class='indent'>
</p>
<pre class='fancyvrb' id='fancyvrb69'><a id='x1-60136r1'></a><span class='ecrm-0500'>1</span><span class='ectt-1000'>sudo rmmod cryptosha256</span></pre>
<!-- l. 1524 --><p class='noindent'>
<!-- l. 1526 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='symmetric-key-encryption'><span class='titlemark'>0.16.2 </span> <a id='x1-610000.16.2'></a>Symmetric key encryption</h4>
<!-- l. 1526 --><p class='noindent'>Here is an example of symmetrically encrypting a string using the AES algorithm
<!-- l. 1528 --><p class='noindent'>Here is an example of symmetrically encrypting a string using the AES algorithm
and a password.
</p><!-- l. 1 --><p class='indent'>
</p>
@ -4920,10 +4920,10 @@ and a password.
<a id='x1-61390r195'></a><span class='ecrm-0500'>195</span>
<a id='x1-61392r196'></a><span class='ecrm-0500'>196</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2610'><span class='ectt-0800'>"Symmetric key encryption example"</span></span><span class='ectt-0800'>);</span>
<a id='x1-61394r197'></a><span class='ecrm-0500'>197</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2611'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1530 --><p class='noindent'>
<!-- l. 1532 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='standardizing-the-interfaces-the-device-model'><span class='titlemark'>0.17 </span> <a id='x1-620000.17'></a>Standardizing the interfaces: The Device Model</h3>
<!-- l. 1532 --><p class='noindent'>Up to this point we have seen all kinds of modules doing all kinds of things, but there
<!-- l. 1534 --><p class='noindent'>Up to this point we have seen all kinds of modules doing all kinds of things, but there
was no consistency in their interfaces with the rest of the kernel. To impose some
consistency such that there is at minimum a standardized way to start, suspend and
resume a device a device model was added. An example is show below, and you can
@ -5033,13 +5033,13 @@ functions.
<!-- l. 1538 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='optimizations'><span class='titlemark'>0.18 </span> <a id='x1-630000.18'></a>Optimizations</h3>
<!-- l. 1540 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='optimizations'><span class='titlemark'>0.18 </span> <a id='x1-630000.18'></a>Optimizations</h3>
<!-- l. 1542 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='likely-and-unlikely-conditions'><span class='titlemark'>0.18.1 </span> <a id='x1-640000.18.1'></a>Likely and Unlikely conditions</h4>
<!-- l. 1542 --><p class='noindent'>Sometimes you might want your code to run as quickly as possible,
<!-- l. 1544 --><p class='noindent'>Sometimes you might want your code to run as quickly as possible,
especially if it is handling an interrupt or doing something which might
cause noticible latency. If your code contains boolean conditions and if you
know that the conditions are almost always likely to evaluate as either
@ -5049,7 +5049,7 @@ know that the conditions are almost always likely to evaluate as either
<code> <span class='ectt-1000'>likely</span>
</code> and <code> <span class='ectt-1000'>unlikely</span>
</code> macros.
</p><!-- l. 1546 --><p class='indent'> For example, when allocating memory you are almost always expecting this to
</p><!-- l. 1548 --><p class='indent'> For example, when allocating memory you are almost always expecting this to
succeed.
</p><!-- l. 1 --><p class='indent'>
</p>
@ -5059,50 +5059,50 @@ succeed.
<a id='x1-64018r4'></a><span class='ecrm-0500'>4</span><span class='ectt-0800'>    bio = NULL;</span>
<a id='x1-64020r5'></a><span class='ecrm-0500'>5</span><span class='ectt-0800'>    </span><span id='textcolor2689'><span class='ectt-0800'>goto</span></span><span class='ectt-0800'> out;</span>
<a id='x1-64022r6'></a><span class='ecrm-0500'>6</span><span class='ectt-0800'>}</span></pre>
<!-- l. 1557 --><p class='indent'> When the <code> <span class='ectt-1000'>unlikely</span>
<!-- l. 1559 --><p class='indent'> When the <code> <span class='ectt-1000'>unlikely</span>
</code> macro is used, the compiler alters its machine instruction output, so that it
continues along the false branch and only jumps if the condition is true. That
avoids flushing the processor pipeline. The opposite happens if you use the
<code> <span class='ectt-1000'>likely</span>
</code> macro.
</p><!-- l. 1561 --><p class='noindent'>
</p><!-- l. 1563 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='common-pitfalls'><span class='titlemark'>0.19 </span> <a id='x1-650000.19'></a>Common Pitfalls</h3>
<!-- l. 1564 --><p class='noindent'>
<!-- l. 1566 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='using-standard-libraries'><span class='titlemark'>0.19.1 </span> <a id='x1-660000.19.1'></a>Using standard libraries</h4>
<!-- l. 1566 --><p class='noindent'>You can not do that. In a kernel module you can only use kernel functions, which are
<!-- l. 1568 --><p class='noindent'>You can not do that. In a kernel module, you can only use kernel functions which are
the functions you can see in <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>/proc/kallsyms</span></span></span>.
</p><!-- l. 1569 --><p class='noindent'>
</p><!-- l. 1571 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='disabling-interrupts'><span class='titlemark'>0.19.2 </span> <a id='x1-670000.19.2'></a>Disabling interrupts</h4>
<!-- l. 1571 --><p class='noindent'>You might need to do this for a short time and that is OK, but if you do not enable
<!-- l. 1573 --><p class='noindent'>You might need to do this for a short time and that is OK, but if you do not enable
them afterwards, your system will be stuck and you will have to power it
off.
</p><!-- l. 1573 --><p class='noindent'>
</p><!-- l. 1575 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='where-to-go-from-here'><span class='titlemark'>0.20 </span> <a id='x1-680000.20'></a>Where To Go From Here?</h3>
<!-- l. 1575 --><p class='noindent'>For people seriously interested in kernel programming, I recommend <a href='https://kernelnewbies.org'>kernelnewbies.org</a>
<!-- l. 1577 --><p class='noindent'>For people seriously interested in kernel programming, I recommend <a href='https://kernelnewbies.org'>kernelnewbies.org</a>
and the <a href='https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/Documentation'>Documentation</a> subdirectory within the kernel source code which is not
always easy to understand but can be a starting point for further investigation. Also,
as Linus Torvalds said, the best way to learn the kernel is to read the source code
yourself.
</p><!-- l. 1578 --><p class='indent'> If you are interested in more examples of short kernel modules then searching on
</p><!-- l. 1580 --><p class='indent'> If you are interested in more examples of short kernel modules then searching on
sites such as Github and Gitlab is a good way to start, although there is a lot of
duplication of older LKMPG examples which may not compile with newer kernel
versions. You will also be able to find examples of the use of kernel modules to attack
or compromise systems or exfiltrate data and those can be useful for thinking about
how to defend systems and learning about existing security mechanisms within the
kernel.
</p><!-- l. 1581 --><p class='indent'> I hope I have helped you in your quest to become a better programmer, or at
</p><!-- l. 1583 --><p class='indent'> I hope I have helped you in your quest to become a better programmer, or at
least to have fun through technology. And, if you do write useful kernel modules, I
hope you publish them under the GPL, so I can use them too.
</p><!-- l. 1584 --><p class='indent'> If you would like to contribute to this guide or notice anything glaringly wrong,
</p><!-- l. 1586 --><p class='indent'> If you would like to contribute to this guide or notice anything glaringly wrong,
please create an issue at <a class='url' href='https://github.com/sysprog21/lkmpg'><span class='ectt-1000'>https://github.com/sysprog21/lkmpg</span></a>.
</p><!-- l. 1586 --><p class='indent'> Happy hacking!
</p><!-- l. 1588 --><p class='indent'> Happy hacking!
</p>
</body>

138
lkmpg-for-ht.html
View File

@ -3740,7 +3740,7 @@ they will not be forgotten and will activate when the unlock happens, using the
</p>
<h4 class='subsectionHead' id='read-and-write-locks'><span class='titlemark'>0.12.3 </span> <a id='x1-470000.12.3'></a>Read and write locks</h4>
<!-- l. 1351 --><p class='noindent'>Read and write locks are specialised kinds of spinlocks so that you can exclusively
read from something or write to something. Like the earlier spinlocks example the
read from something or write to something. Like the earlier spinlocks example, the
one below shows an "irq safe" situation in which if other functions were triggered
from irqs which might also read and write to whatever you are concerned with
then they would not disrupt the logic. As before it is a good idea to keep
@ -3811,7 +3811,7 @@ which could possibly interfere with your logic then you can use the simpler
</code> or the corresponding write functions.
</p>
<h4 class='subsectionHead' id='atomic-operations'><span class='titlemark'>0.12.4 </span> <a id='x1-480000.12.4'></a>Atomic operations</h4>
<!-- l. 1360 --><p class='noindent'>If you are doing simple arithmetic: adding, subtracting or bitwise operations then
<!-- l. 1360 --><p class='noindent'>If you are doing simple arithmetic: adding, subtracting or bitwise operations, then
there is another way in the multi-CPU and multi-hyperthreaded world to stop other
parts of the system from messing with your mojo. By using atomic operations you
can be confident that your addition, subtraction or bit flip did actually happen
@ -3903,14 +3903,14 @@ below.
</p>
<h4 class='subsectionHead' id='replacement'><span class='titlemark'>0.13.1 </span> <a id='x1-500000.13.1'></a>Replacement</h4>
<!-- l. 1371 --><p class='noindent'>In Section <a href='#x1-80042'>2<!-- tex4ht:ref: sec:using_x --></a>, I said that X Window System and kernel module programming do not
mix. That is true for developing kernel modules, but in actual use, you want to be
mix. That is true for developing kernel modules. But in actual use, you want to be
able to send messages to whichever tty the command to load the module came
from.
</p><!-- l. 1374 --><p class='indent'> "tty" is an abbreviation of <span class='ecti-1000'>teletype</span>: originally a combination keyboard-printer
</p><!-- l. 1375 --><p class='indent'> "tty" is an abbreviation of <span class='ecti-1000'>teletype</span>: originally a combination keyboard-printer
used to communicate with a Unix system, and today an abstraction for the text
stream used for a Unix program, whether it is a physical terminal, an xterm on an X
display, a network connection used with ssh, etc.
</p><!-- l. 1376 --><p class='indent'> The way this is done is by using current, a pointer to the currently running task,
</p><!-- l. 1377 --><p class='indent'> The way this is done is by using current, a pointer to the currently running task,
to get the current task’s tty structure. Then, we look inside that tty structure to find
a pointer to a string write function, which we use to write a string to the
tty.
@ -3993,16 +3993,16 @@ tty.
<a id='x1-50150r75'></a><span class='ecrm-0500'>75</span><span class='ectt-0800'>module_exit(print_string_exit);</span>
<a id='x1-50152r76'></a><span class='ecrm-0500'>76</span>
<a id='x1-50154r77'></a><span class='ecrm-0500'>77</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2033'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1381 --><p class='noindent'>
<!-- l. 1382 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='flashing-keyboard-leds'><span class='titlemark'>0.13.2 </span> <a id='x1-510000.13.2'></a>Flashing keyboard LEDs</h4>
<!-- l. 1383 --><p class='noindent'>In certain conditions, you may desire a simpler and more direct way to communicate
<!-- l. 1384 --><p class='noindent'>In certain conditions, you may desire a simpler and more direct way to communicate
to the external world. Flashing keyboard LEDs can be such a solution: It is an
immediate way to attract attention or to display a status condition. Keyboard LEDs
are present on every hardware, they are always visible, they do not need any setup,
and their use is rather simple and non-intrusive, compared to writing to a tty or a
file.
</p><!-- l. 1387 --><p class='indent'> The following source code illustrates a minimal kernel module which, when
</p><!-- l. 1388 --><p class='indent'> The following source code illustrates a minimal kernel module which, when
loaded, starts blinking the keyboard LEDs until it is unloaded.
</p><!-- l. 1 --><p class='indent'>
</p>
@ -4098,33 +4098,33 @@ loaded, starts blinking the keyboard LEDs until it is unloaded.
<!-- l. 1391 --><p class='indent'> If none of the examples in this chapter fit your debugging needs
<!-- l. 1392 --><p class='indent'> If none of the examples in this chapter fit your debugging needs,
there might yet be some other tricks to try. Ever wondered what
<code> <span class='ectt-1000'>CONFIG_LL_DEBUG</span>
</code> in <code> <span class='ectt-1000'>make menuconfig</span>
</code> is good for? If you activate that you get low level access to the serial port. While this
might not sound very powerful by itself, you can patch <a href='https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/kernel/printk.c'>kernel/printk.c</a> or any other
essential syscall to print ASCII characters, thus makeing it possible to trace virtually
essential syscall to print ASCII characters, thus making it possible to trace virtually
everything what your code does over a serial line. If you find yourself porting the
kernel to some new and former unsupported architecture, this is usually amongst the
first things that should be implemented. Logging over a netconsole might also be
worth a try.
</p><!-- l. 1398 --><p class='indent'> While you have seen lots of stuff that can be used to aid debugging here, there are
</p><!-- l. 1399 --><p class='indent'> While you have seen lots of stuff that can be used to aid debugging here, there are
some things to be aware of. Debugging is almost always intrusive. Adding debug code
can change the situation enough to make the bug seem to dissappear. Thus you
should try to keep debug code to a minimum and make sure it does not show up in
production code.
</p><!-- l. 1402 --><p class='noindent'>
</p><!-- l. 1403 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='scheduling-tasks'><span class='titlemark'>0.14 </span> <a id='x1-520000.14'></a>Scheduling Tasks</h3>
<!-- l. 1404 --><p class='noindent'>There are two main ways of running tasks: tasklets and work queues. Tasklets are a
quick and easy way of scheduling a single function to be run, for example when
<!-- l. 1405 --><p class='noindent'>There are two main ways of running tasks: tasklets and work queues. Tasklets are a
quick and easy way of scheduling a single function to be run. For example, when
triggered from an interrupt, whereas work queues are more complicated but also
better suited to running multiple things in a sequence.
</p><!-- l. 1407 --><p class='noindent'>
</p><!-- l. 1409 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='tasklets'><span class='titlemark'>0.14.1 </span> <a id='x1-530000.14.1'></a>Tasklets</h4>
<!-- l. 1409 --><p class='noindent'>Here is an example tasklet module. The
<!-- l. 1411 --><p class='noindent'>Here is an example tasklet module. The
<code> <span class='ectt-1000'>tasklet_fn</span>
</code> function runs for a few seconds and in the mean time execution of the
<code> <span class='ectt-1000'>example_tasklet_init</span>
@ -4168,7 +4168,7 @@ better suited to running multiple things in a sequence.
<a id='x1-53072r35'></a><span class='ecrm-0500'>35</span>
<a id='x1-53074r36'></a><span class='ecrm-0500'>36</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2153'><span class='ectt-0800'>"Tasklet example"</span></span><span class='ectt-0800'>);</span>
<a id='x1-53076r37'></a><span class='ecrm-0500'>37</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2154'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1414 --><p class='indent'> So with this example loaded <code> <span class='ectt-1000'>dmesg</span>
<!-- l. 1416 --><p class='indent'> So with this example loaded <code> <span class='ectt-1000'>dmesg</span>
</code> should show:
@ -4180,11 +4180,11 @@ Example tasklet starts
Example tasklet init continues...
Example tasklet ends
</pre>
<!-- l. 1421 --><p class='nopar'>
</p><!-- l. 1423 --><p class='noindent'>
<!-- l. 1423 --><p class='nopar'>
</p><!-- l. 1425 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='work-queues'><span class='titlemark'>0.14.2 </span> <a id='x1-540000.14.2'></a>Work queues</h4>
<!-- l. 1425 --><p class='noindent'>To add a task to the scheduler we can use a workqueue. The kernel then uses the
<!-- l. 1427 --><p class='noindent'>To add a task to the scheduler we can use a workqueue. The kernel then uses the
Completely Fair Scheduler (CFS) to execute work within the queue.
</p><!-- l. 1 --><p class='indent'>
</p>
@ -4222,21 +4222,21 @@ Completely Fair Scheduler (CFS) to execute work within the queue.
<a id='x1-54064r32'></a><span class='ecrm-0500'>32</span>
<a id='x1-54066r33'></a><span class='ecrm-0500'>33</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2182'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span>
<a id='x1-54068r34'></a><span class='ecrm-0500'>34</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2183'><span class='ectt-0800'>"Workqueue example"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1430 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='interrupt-handlers'><span class='titlemark'>0.15 </span> <a id='x1-550000.15'></a>Interrupt Handlers</h3>
<!-- l. 1432 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='interrupt-handlers'><span class='titlemark'>0.15 </span> <a id='x1-550000.15'></a>Interrupt Handlers</h3>
<!-- l. 1434 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='interrupt-handlers1'><span class='titlemark'>0.15.1 </span> <a id='x1-560000.15.1'></a>Interrupt Handlers</h4>
<!-- l. 1434 --><p class='noindent'>Except for the last chapter, everything we did in the kernel so far we have done as a
<!-- l. 1436 --><p class='noindent'>Except for the last chapter, everything we did in the kernel so far we have done as a
response to a process asking for it, either by dealing with a special file, sending an
<code> <span class='ectt-1000'>ioctl()</span>
</code>, or issuing a system call. But the job of the kernel is not just to respond to process
requests. Another job, which is every bit as important, is to speak to the hardware
connected to the machine.
</p><!-- l. 1438 --><p class='indent'> There are two types of interaction between the CPU and the rest of the
</p><!-- l. 1440 --><p class='indent'> There are two types of interaction between the CPU and the rest of the
computer’s hardware. The first type is when the CPU gives orders to the hardware,
the other is when the hardware needs to tell the CPU something. The second, called
the order is when the hardware needs to tell the CPU something. The second, called
interrupts, is much harder to implement because it has to be dealt with when
convenient for the hardware, not the CPU. Hardware devices typically have a very
small amount of RAM, and if you do not read their information when available, it is
@ -4244,14 +4244,14 @@ lost.
</p><!-- l. 1443 --><p class='indent'> Under Linux, hardware interrupts are called IRQ’s (Interrupt ReQuests). There
</p><!-- l. 1445 --><p class='indent'> Under Linux, hardware interrupts are called IRQ’s (Interrupt ReQuests). There
are two types of IRQ’s, short and long. A short IRQ is one which is expected to take
a very short period of time, during which the rest of the machine will be blocked and
no other interrupts will be handled. A long IRQ is one which can take longer, and
during which other interrupts may occur (but not interrupts from the same
device). If at all possible, it is better to declare an interrupt handler to be
long.
</p><!-- l. 1449 --><p class='indent'> When the CPU receives an interrupt, it stops whatever it is doing (unless it is
</p><!-- l. 1451 --><p class='indent'> When the CPU receives an interrupt, it stops whatever it is doing (unless it is
processing a more important interrupt, in which case it will deal with this one
only when the more important one is done), saves certain parameters on
the stack and calls the interrupt handler. This means that certain things
@ -4263,10 +4263,10 @@ the new information at a later time (this is called the "bottom half") and
return. The kernel is then guaranteed to call the bottom half as soon as
possible – and when it does, everything allowed in kernel modules will be
allowed.
</p><!-- l. 1455 --><p class='indent'> The way to implement this is to call
</p><!-- l. 1457 --><p class='indent'> The way to implement this is to call
<code> <span class='ectt-1000'>request_irq()</span>
</code> to get your interrupt handler called when the relevant IRQ is received.
</p><!-- l. 1457 --><p class='indent'> In practice IRQ handling can be a bit more complex. Hardware is often
</p><!-- l. 1459 --><p class='indent'> In practice IRQ handling can be a bit more complex. Hardware is often
designed in a way that chains two interrupt controllers, so that all the IRQs
from interrupt controller B are cascaded to a certain IRQ from interrupt
controller A. Of course, that requires that the kernel finds out which IRQ it
@ -4280,8 +4280,8 @@ need to solve another truckload of problems. It is not enough to know if
a certain IRQs has happend, it’s also important for what CPU(s) it was
for. People still interested in more details, might want to refer to "APIC"
now.
</p><!-- l. 1466 --><p class='indent'> This function receives the IRQ number, the name of the function,
flags, a name for <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>/proc/interrupts</span></span></span> and a parameter to pass to the
</p><!-- l. 1468 --><p class='indent'> This function receives the IRQ number, the name of the function,
flags, a name for <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>/proc/interrupts</span></span></span> and a parameter to be passed to the
interrupt handler. Usually there is a certain number of IRQs available.
How many IRQs there are is hardware-dependent. The flags can include
<code> <span class='ectt-1000'>SA_SHIRQ</span>
@ -4293,16 +4293,16 @@ already a handler on this IRQ, or if you are both willing to share.
</p><!-- l. 1472 --><p class='noindent'>
</p><!-- l. 1474 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='detecting-button-presses'><span class='titlemark'>0.15.2 </span> <a id='x1-570000.15.2'></a>Detecting button presses</h4>
<!-- l. 1474 --><p class='noindent'>Many popular single board computers, such as Raspberry Pi or Beagleboards, have a
<!-- l. 1476 --><p class='noindent'>Many popular single board computers, such as Raspberry Pi or Beagleboards, have a
bunch of GPIO pins. Attaching buttons to those and then having a button press do
something is a classic case in which you might need to use interrupts, so that instead
of having the CPU waste time and battery power polling for a change in input state
of having the CPU waste time and battery power polling for a change in input state,
it is better for the input to trigger the CPU to then run a particular handling
function.
</p><!-- l. 1478 --><p class='indent'> Here is an example where buttons are connected to GPIO numbers 17 and 18 and
</p><!-- l. 1480 --><p class='indent'> Here is an example where buttons are connected to GPIO numbers 17 and 18 and
an LED is connected to GPIO 4. You can change those numbers to whatever is
appropriate for your board.
</p><!-- l. 1 --><p class='indent'>
@ -4452,14 +4452,14 @@ appropriate for your board.
<a id='x1-57286r143'></a><span class='ecrm-0500'>143</span>
<a id='x1-57288r144'></a><span class='ecrm-0500'>144</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2290'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span>
<a id='x1-57290r145'></a><span class='ecrm-0500'>145</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2291'><span class='ectt-0800'>"Handle some GPIO interrupts"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1483 --><p class='noindent'>
<!-- l. 1485 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='bottom-half'><span class='titlemark'>0.15.3 </span> <a id='x1-580000.15.3'></a>Bottom Half</h4>
<!-- l. 1485 --><p class='noindent'>Suppose you want to do a bunch of stuff inside of an interrupt routine. A common
<!-- l. 1487 --><p class='noindent'>Suppose you want to do a bunch of stuff inside of an interrupt routine. A common
way to do that without rendering the interrupt unavailable for a significant duration
is to combine it with a tasklet. This pushes the bulk of the work off into the
scheduler.
</p><!-- l. 1489 --><p class='indent'> The example below modifies the previous example to also run an additional task
</p><!-- l. 1491 --><p class='indent'> The example below modifies the previous example to also run an additional task
when an interrupt is triggered.
</p><!-- l. 1 --><p class='indent'>
</p>
@ -4624,22 +4624,22 @@ when an interrupt is triggered.
<a id='x1-58318r159'></a><span class='ecrm-0500'>159</span>
<a id='x1-58320r160'></a><span class='ecrm-0500'>160</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2412'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span>
<a id='x1-58322r161'></a><span class='ecrm-0500'>161</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2413'><span class='ectt-0800'>"Interrupt with top and bottom half"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1493 --><p class='noindent'>
<!-- l. 1495 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='crypto'><span class='titlemark'>0.16 </span> <a id='x1-590000.16'></a>Crypto</h3>
<!-- l. 1495 --><p class='noindent'>At the dawn of the internet everybody trusted everybody completely…but that did
not work out so well. When this guide was originally written it was a more innocent
<!-- l. 1497 --><p class='noindent'>At the dawn of the internet, everybody trusted everybody completely…but that did
not work out so well. When this guide was originally written, it was a more innocent
era in which almost nobody actually gave a damn about crypto - least of all kernel
developers. That is certainly no longer the case now. To handle crypto stuff the
developers. That is certainly no longer the case now. To handle crypto stuff, the
kernel has its own API enabling common methods of encryption, decryption and your
favourite hash functions.
</p><!-- l. 1500 --><p class='noindent'>
</p><!-- l. 1502 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='hash-functions'><span class='titlemark'>0.16.1 </span> <a id='x1-600000.16.1'></a>Hash functions</h4>
<!-- l. 1503 --><p class='noindent'>Calculating and checking the hashes of things is a common operation. Here is a
<!-- l. 1505 --><p class='noindent'>Calculating and checking the hashes of things is a common operation. Here is a
demonstration of how to calculate a sha256 hash within a kernel module.
</p><!-- l. 1 --><p class='indent'>
</p>
@ -4705,21 +4705,21 @@ demonstration of how to calculate a sha256 hash within a kernel module.
<a id='x1-60120r60'></a><span class='ecrm-0500'>60</span>
<a id='x1-60122r61'></a><span class='ecrm-0500'>61</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2463'><span class='ectt-0800'>"sha256 hash test"</span></span><span class='ectt-0800'>);</span>
<a id='x1-60124r62'></a><span class='ecrm-0500'>62</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2464'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1508 --><p class='indent'> Make and install the module:
<!-- l. 1510 --><p class='indent'> Make and install the module:
</p><!-- l. 1 --><p class='indent'>
</p>
<pre class='fancyvrb' id='fancyvrb68'><a id='x1-60129r1'></a><span class='ecrm-0500'>1</span><span class='ectt-1000'>make</span>
<a id='x1-60131r2'></a><span class='ecrm-0500'>2</span><span class='ectt-1000'>sudo insmod cryptosha256.ko</span>
<a id='x1-60133r3'></a><span class='ecrm-0500'>3</span><span class='ectt-1000'>dmesg</span></pre>
<!-- l. 1516 --><p class='indent'> And you should see that the hash was calculated for the test string.
</p><!-- l. 1518 --><p class='indent'> Finally, remove the test module:
<!-- l. 1518 --><p class='indent'> And you should see that the hash was calculated for the test string.
</p><!-- l. 1520 --><p class='indent'> Finally, remove the test module:
</p><!-- l. 1 --><p class='indent'>
</p>
<pre class='fancyvrb' id='fancyvrb69'><a id='x1-60136r1'></a><span class='ecrm-0500'>1</span><span class='ectt-1000'>sudo rmmod cryptosha256</span></pre>
<!-- l. 1524 --><p class='noindent'>
<!-- l. 1526 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='symmetric-key-encryption'><span class='titlemark'>0.16.2 </span> <a id='x1-610000.16.2'></a>Symmetric key encryption</h4>
<!-- l. 1526 --><p class='noindent'>Here is an example of symmetrically encrypting a string using the AES algorithm
<!-- l. 1528 --><p class='noindent'>Here is an example of symmetrically encrypting a string using the AES algorithm
and a password.
</p><!-- l. 1 --><p class='indent'>
</p>
@ -4920,10 +4920,10 @@ and a password.
<a id='x1-61390r195'></a><span class='ecrm-0500'>195</span>
<a id='x1-61392r196'></a><span class='ecrm-0500'>196</span><span class='ectt-0800'>MODULE_DESCRIPTION(</span><span id='textcolor2610'><span class='ectt-0800'>"Symmetric key encryption example"</span></span><span class='ectt-0800'>);</span>
<a id='x1-61394r197'></a><span class='ecrm-0500'>197</span><span class='ectt-0800'>MODULE_LICENSE(</span><span id='textcolor2611'><span class='ectt-0800'>"GPL"</span></span><span class='ectt-0800'>);</span></pre>
<!-- l. 1530 --><p class='noindent'>
<!-- l. 1532 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='standardizing-the-interfaces-the-device-model'><span class='titlemark'>0.17 </span> <a id='x1-620000.17'></a>Standardizing the interfaces: The Device Model</h3>
<!-- l. 1532 --><p class='noindent'>Up to this point we have seen all kinds of modules doing all kinds of things, but there
<!-- l. 1534 --><p class='noindent'>Up to this point we have seen all kinds of modules doing all kinds of things, but there
was no consistency in their interfaces with the rest of the kernel. To impose some
consistency such that there is at minimum a standardized way to start, suspend and
resume a device a device model was added. An example is show below, and you can
@ -5033,13 +5033,13 @@ functions.
<!-- l. 1538 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='optimizations'><span class='titlemark'>0.18 </span> <a id='x1-630000.18'></a>Optimizations</h3>
<!-- l. 1540 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='optimizations'><span class='titlemark'>0.18 </span> <a id='x1-630000.18'></a>Optimizations</h3>
<!-- l. 1542 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='likely-and-unlikely-conditions'><span class='titlemark'>0.18.1 </span> <a id='x1-640000.18.1'></a>Likely and Unlikely conditions</h4>
<!-- l. 1542 --><p class='noindent'>Sometimes you might want your code to run as quickly as possible,
<!-- l. 1544 --><p class='noindent'>Sometimes you might want your code to run as quickly as possible,
especially if it is handling an interrupt or doing something which might
cause noticible latency. If your code contains boolean conditions and if you
know that the conditions are almost always likely to evaluate as either
@ -5049,7 +5049,7 @@ know that the conditions are almost always likely to evaluate as either
<code> <span class='ectt-1000'>likely</span>
</code> and <code> <span class='ectt-1000'>unlikely</span>
</code> macros.
</p><!-- l. 1546 --><p class='indent'> For example, when allocating memory you are almost always expecting this to
</p><!-- l. 1548 --><p class='indent'> For example, when allocating memory you are almost always expecting this to
succeed.
</p><!-- l. 1 --><p class='indent'>
</p>
@ -5059,50 +5059,50 @@ succeed.
<a id='x1-64018r4'></a><span class='ecrm-0500'>4</span><span class='ectt-0800'>    bio = NULL;</span>
<a id='x1-64020r5'></a><span class='ecrm-0500'>5</span><span class='ectt-0800'>    </span><span id='textcolor2689'><span class='ectt-0800'>goto</span></span><span class='ectt-0800'> out;</span>
<a id='x1-64022r6'></a><span class='ecrm-0500'>6</span><span class='ectt-0800'>}</span></pre>
<!-- l. 1557 --><p class='indent'> When the <code> <span class='ectt-1000'>unlikely</span>
<!-- l. 1559 --><p class='indent'> When the <code> <span class='ectt-1000'>unlikely</span>
</code> macro is used, the compiler alters its machine instruction output, so that it
continues along the false branch and only jumps if the condition is true. That
avoids flushing the processor pipeline. The opposite happens if you use the
<code> <span class='ectt-1000'>likely</span>
</code> macro.
</p><!-- l. 1561 --><p class='noindent'>
</p><!-- l. 1563 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='common-pitfalls'><span class='titlemark'>0.19 </span> <a id='x1-650000.19'></a>Common Pitfalls</h3>
<!-- l. 1564 --><p class='noindent'>
<!-- l. 1566 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='using-standard-libraries'><span class='titlemark'>0.19.1 </span> <a id='x1-660000.19.1'></a>Using standard libraries</h4>
<!-- l. 1566 --><p class='noindent'>You can not do that. In a kernel module you can only use kernel functions, which are
<!-- l. 1568 --><p class='noindent'>You can not do that. In a kernel module, you can only use kernel functions which are
the functions you can see in <span class='obeylines-h'><span class='verb'><span class='ectt-1000'>/proc/kallsyms</span></span></span>.
</p><!-- l. 1569 --><p class='noindent'>
</p><!-- l. 1571 --><p class='noindent'>
</p>
<h4 class='subsectionHead' id='disabling-interrupts'><span class='titlemark'>0.19.2 </span> <a id='x1-670000.19.2'></a>Disabling interrupts</h4>
<!-- l. 1571 --><p class='noindent'>You might need to do this for a short time and that is OK, but if you do not enable
<!-- l. 1573 --><p class='noindent'>You might need to do this for a short time and that is OK, but if you do not enable
them afterwards, your system will be stuck and you will have to power it
off.
</p><!-- l. 1573 --><p class='noindent'>
</p><!-- l. 1575 --><p class='noindent'>
</p>
<h3 class='sectionHead' id='where-to-go-from-here'><span class='titlemark'>0.20 </span> <a id='x1-680000.20'></a>Where To Go From Here?</h3>
<!-- l. 1575 --><p class='noindent'>For people seriously interested in kernel programming, I recommend <a href='https://kernelnewbies.org'>kernelnewbies.org</a>
<!-- l. 1577 --><p class='noindent'>For people seriously interested in kernel programming, I recommend <a href='https://kernelnewbies.org'>kernelnewbies.org</a>
and the <a href='https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/Documentation'>Documentation</a> subdirectory within the kernel source code which is not
always easy to understand but can be a starting point for further investigation. Also,
as Linus Torvalds said, the best way to learn the kernel is to read the source code
yourself.
</p><!-- l. 1578 --><p class='indent'> If you are interested in more examples of short kernel modules then searching on
</p><!-- l. 1580 --><p class='indent'> If you are interested in more examples of short kernel modules then searching on
sites such as Github and Gitlab is a good way to start, although there is a lot of
duplication of older LKMPG examples which may not compile with newer kernel
versions. You will also be able to find examples of the use of kernel modules to attack
or compromise systems or exfiltrate data and those can be useful for thinking about
how to defend systems and learning about existing security mechanisms within the
kernel.
</p><!-- l. 1581 --><p class='indent'> I hope I have helped you in your quest to become a better programmer, or at
</p><!-- l. 1583 --><p class='indent'> I hope I have helped you in your quest to become a better programmer, or at
least to have fun through technology. And, if you do write useful kernel modules, I
hope you publish them under the GPL, so I can use them too.
</p><!-- l. 1584 --><p class='indent'> If you would like to contribute to this guide or notice anything glaringly wrong,
</p><!-- l. 1586 --><p class='indent'> If you would like to contribute to this guide or notice anything glaringly wrong,
please create an issue at <a class='url' href='https://github.com/sysprog21/lkmpg'><span class='ectt-1000'>https://github.com/sysprog21/lkmpg</span></a>.
</p><!-- l. 1586 --><p class='indent'> Happy hacking!
</p><!-- l. 1588 --><p class='indent'> Happy hacking!
</p>
</body>