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   <section id="stm32-null-pointer-detection">
 <h1>STM32 Null Pointer Detection<a class="headerlink" href="#stm32-null-pointer-detection" title="Permalink to this heading"></a></h1>
 <section id="the-null-pointer-problem">
 <h2>The NULL Pointer Problem<a class="headerlink" href="#the-null-pointer-problem" title="Permalink to this heading"></a></h2>
 <p>A common cause of software bugs is null pointers. Pointers may be NULL if they
 are un-initialized and un-checked. The use of NULL pointers almost always results
 in something bad happening. Often, NULL pointer access can cause error exceptions
 and or diagnostic crashes. But on MCUs that have valid address decoding at address
 0x0000:0000, the use of NULL pointers may not cause a crash at all but may, instead,
 cause strange behaviors that can sometimes be difficult to debug.</p>
 </section>
 <section id="cortex-m-memory">
 <h2>Cortex-M Memory<a class="headerlink" href="#cortex-m-memory" title="Permalink to this heading"></a></h2>
 <p>The Cortex-M family (Cortex-M0, M3, and M4) are such MCUs. They have their
 interrupt vectors positioned at address zero. Because of this, NULL pointer
 accesses will not necessarily cause crashes. Instead, the NULL pointers will
 access memory in the vicinity of the vector table and who knows what will happen
 next?</p>
 </section>
 <section id="stm32-memory-aliasing">
 <h2>STM32 Memory Aliasing<a class="headerlink" href="#stm32-memory-aliasing" title="Permalink to this heading"></a></h2>
 <p>The STMicro STM32 family of Cortex-M3/4 MCUs do things a little differently.
 FLASH is physically addressed at address 0x0800:0000; the STM32 vector table
 is then physically located at 0x0800:0000 instead of 0x0000:0000. If the STM32
 hardware is configured to boot from FLASH, then the the STM32 will remap the
 FLASH memory so that is aliased at address 0x0000:00000. In that way, the STM32
 can boot from FLASH or external memory or any other memory region that it is
 capable of mapping.</p>
 <p>In the NuttX linker scripts, the applications are linked to execute from the
 physical FLASH region at address 0x0800:0000. All valid FLASH memory access
 will then access memory in the 0x0800:0000 FLASH address range. But illegal
 NULL pointer access will access the aliased copy of FLASH beginning at 0x0000:0000.
 So we still have the problem.</p>
 </section>
 <section id="the-cortex-m-memory-protection-unit">
 <h2>The Cortex-M Memory Protection Unit<a class="headerlink" href="#the-cortex-m-memory-protection-unit" title="Permalink to this heading"></a></h2>
 <p>The Memory Protection Unit (MPU) is an optional component of a Cortex-M implementation.
 Most popular Cortex-M3/4 MCUs do support the MPU. The MPU can be used to protect regions
 of memory so that if there is any attempted, unauthorized access to certain memory
 regions, then a memory protection violation exception will occur and the system will
 detect the illegal access.</p>
 <p>See the ARM website for more information about the Cortex-M3/4 families and the
 Cortex-M3/4 MPU. See, for example
 <a class="reference external" href="http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dai0179b/CHDFDFIG.html">2.2. Memory Protection Unit (MPU)</a>.</p>
 </section>
 <section id="using-the-mpu-to-detect-null-pointer-usage">
 <h2>Using the MPU to Detect Null Pointer Usage<a class="headerlink" href="#using-the-mpu-to-detect-null-pointer-usage" title="Permalink to this heading"></a></h2>
 <p>So, for the STM32, one thing that we can do is to program the MPU to prohibit software
 access to the memory region beginning at address 0x0000:0000. Petteri Aimonen posted a code
 snippet on the NuttX Forum showing how to do this. Here is Petteri’s post:</p>
 <div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="cm">/* Catch any null pointer dereferences */</span>

 <span class="kt">int</span><span class="w"> </span><span class="n">region</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>

 <span class="n">putreg32</span><span class="p">(</span><span class="n">region</span><span class="p">,</span><span class="w"> </span><span class="n">MPU_RNR</span><span class="p">);</span>
 <span class="n">putreg32</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="n">MPU_RBAR</span><span class="p">);</span>
 <span class="n">putreg32</span><span class="p">(</span><span class="n">MPU_RASR_ENABLE</span><span class="w"> </span><span class="o">|</span><span class="w"> </span><span class="n">MPU_RASR_SIZE_LOG2</span><span class="p">(</span><span class="mi">20</span><span class="p">)</span><span class="w"> </span><span class="o">|</span><span class="w"> </span><span class="p">(</span><span class="mh">0xFF</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">MPU_RASR_SRD_SHIFT</span><span class="p">)</span><span class="w"> </span><span class="o">|</span><span class="w"> </span><span class="n">MPU_RASR_AP_NONO</span><span class="p">,</span><span class="w"> </span><span class="n">MPU_RASR</span><span class="p">);</span>
 <span class="n">mpu_control</span><span class="p">(</span><span class="nb">true</span><span class="p">,</span><span class="w"> </span><span class="nb">false</span><span class="p">,</span><span class="w"> </span><span class="nb">true</span><span class="p">);</span>
 </pre></div>
 </div>
 </section>
 </section>


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	<section id="stm32-null-pointer-detection">
	<h1>STM32 Null Pointer Detection<a class="headerlink" href="#stm32-null-pointer-detection" title="Permalink to this heading"></a></h1>
	<section id="the-null-pointer-problem">
	<h2>The NULL Pointer Problem<a class="headerlink" href="#the-null-pointer-problem" title="Permalink to this heading"></a></h2>
	<p>A common cause of software bugs is null pointers. Pointers may be NULL if they
	are un-initialized and un-checked. The use of NULL pointers almost always results
	in something bad happening. Often, NULL pointer access can cause error exceptions
	and or diagnostic crashes. But on MCUs that have valid address decoding at address
	0x0000:0000, the use of NULL pointers may not cause a crash at all but may, instead,
	cause strange behaviors that can sometimes be difficult to debug.</p>
	</section>
	<section id="cortex-m-memory">
	<h2>Cortex-M Memory<a class="headerlink" href="#cortex-m-memory" title="Permalink to this heading"></a></h2>
	<p>The Cortex-M family (Cortex-M0, M3, and M4) are such MCUs. They have their
	interrupt vectors positioned at address zero. Because of this, NULL pointer
	accesses will not necessarily cause crashes. Instead, the NULL pointers will
	access memory in the vicinity of the vector table and who knows what will happen
	next?</p>
	</section>
	<section id="stm32-memory-aliasing">
	<h2>STM32 Memory Aliasing<a class="headerlink" href="#stm32-memory-aliasing" title="Permalink to this heading"></a></h2>
	<p>The STMicro STM32 family of Cortex-M3/4 MCUs do things a little differently.
	FLASH is physically addressed at address 0x0800:0000; the STM32 vector table
	is then physically located at 0x0800:0000 instead of 0x0000:0000. If the STM32
	hardware is configured to boot from FLASH, then the the STM32 will remap the
	FLASH memory so that is aliased at address 0x0000:00000. In that way, the STM32
	can boot from FLASH or external memory or any other memory region that it is
	capable of mapping.</p>
	<p>In the NuttX linker scripts, the applications are linked to execute from the
	physical FLASH region at address 0x0800:0000. All valid FLASH memory access
	will then access memory in the 0x0800:0000 FLASH address range. But illegal
	NULL pointer access will access the aliased copy of FLASH beginning at 0x0000:0000.
	So we still have the problem.</p>
	</section>
	<section id="the-cortex-m-memory-protection-unit">
	<h2>The Cortex-M Memory Protection Unit<a class="headerlink" href="#the-cortex-m-memory-protection-unit" title="Permalink to this heading"></a></h2>
	<p>The Memory Protection Unit (MPU) is an optional component of a Cortex-M implementation.
	Most popular Cortex-M3/4 MCUs do support the MPU. The MPU can be used to protect regions
	of memory so that if there is any attempted, unauthorized access to certain memory
	regions, then a memory protection violation exception will occur and the system will
	detect the illegal access.</p>
	<p>See the ARM website for more information about the Cortex-M3/4 families and the
	Cortex-M3/4 MPU. See, for example
	<a class="reference external" href="http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dai0179b/CHDFDFIG.html">2.2. Memory Protection Unit (MPU)</a>.</p>
	</section>
	<section id="using-the-mpu-to-detect-null-pointer-usage">
	<h2>Using the MPU to Detect Null Pointer Usage<a class="headerlink" href="#using-the-mpu-to-detect-null-pointer-usage" title="Permalink to this heading"></a></h2>
	<p>So, for the STM32, one thing that we can do is to program the MPU to prohibit software
	access to the memory region beginning at address 0x0000:0000. Petteri Aimonen posted a code
	snippet on the NuttX Forum showing how to do this. Here is Petteri’s post:</p>
	<div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="cm">/* Catch any null pointer dereferences */</span>

	<span class="kt">int</span><span class="w"> </span><span class="n">region</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>

	<span class="n">putreg32</span><span class="p">(</span><span class="n">region</span><span class="p">,</span><span class="w"> </span><span class="n">MPU_RNR</span><span class="p">);</span>
	<span class="n">putreg32</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="n">MPU_RBAR</span><span class="p">);</span>
	<span class="n">putreg32</span><span class="p">(</span><span class="n">MPU_RASR_ENABLE</span><span class="w"> </span><span class="o">\|</span><span class="w"> </span><span class="n">MPU_RASR_SIZE_LOG2</span><span class="p">(</span><span class="mi">20</span><span class="p">)</span><span class="w"> </span><span class="o">\|</span><span class="w"> </span><span class="p">(</span><span class="mh">0xFF</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">MPU_RASR_SRD_SHIFT</span><span class="p">)</span><span class="w"> </span><span class="o">\|</span><span class="w"> </span><span class="n">MPU_RASR_AP_NONO</span><span class="p">,</span><span class="w"> </span><span class="n">MPU_RASR</span><span class="p">);</span>
	<span class="n">mpu_control</span><span class="p">(</span><span class="nb">true</span><span class="p">,</span><span class="w"> </span><span class="nb">false</span><span class="p">,</span><span class="w"> </span><span class="nb">true</span><span class="p">);</span>
	</pre></div>
	</div>
	</section>
	</section>


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