71 lines
3.1 KiB
ReStructuredText
71 lines
3.1 KiB
ReStructuredText
============================
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STM32 Null Pointer Detection
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============================
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The NULL Pointer Problem
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========================
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A common cause of software bugs is null pointers. Pointers may be NULL if they
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are un-initialized and un-checked. The use of NULL pointers almost always results
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in something bad happening. Often, NULL pointer access can cause error exceptions
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and or diagnostic crashes. But on MCUs that have valid address decoding at address
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0x0000:0000, the use of NULL pointers may not cause a crash at all but may, instead,
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cause strange behaviors that can sometimes be difficult to debug.
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Cortex-M Memory
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===============
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The Cortex-M family (Cortex-M0, M3, and M4) are such MCUs. They have their
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interrupt vectors positioned at address zero. Because of this, NULL pointer
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accesses will not necessarily cause crashes. Instead, the NULL pointers will
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access memory in the vicinity of the vector table and who knows what will happen
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next?
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STM32 Memory Aliasing
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=====================
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The STMicro STM32 family of Cortex-M3/4 MCUs do things a little differently.
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FLASH is physically addressed at address 0x0800:0000; the STM32 vector table
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is then physically located at 0x0800:0000 instead of 0x0000:0000. If the STM32
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hardware is configured to boot from FLASH, then the the STM32 will remap the
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FLASH memory so that is aliased at address 0x0000:00000. In that way, the STM32
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can boot from FLASH or external memory or any other memory region that it is
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capable of mapping.
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In the NuttX linker scripts, the applications are linked to execute from the
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physical FLASH region at address 0x0800:0000. All valid FLASH memory access
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will then access memory in the 0x0800:0000 FLASH address range. But illegal
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NULL pointer access will access the aliased copy of FLASH beginning at 0x0000:0000.
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So we still have the problem.
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The Cortex-M Memory Protection Unit
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===================================
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The Memory Protection Unit (MPU) is an optional component of a Cortex-M implementation.
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Most popular Cortex-M3/4 MCUs do support the MPU. The MPU can be used to protect regions
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of memory so that if there is any attempted, unauthorized access to certain memory
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regions, then a memory protection violation exception will occur and the system will
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detect the illegal access.
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See the ARM website for more information about the Cortex-M3/4 families and the
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Cortex-M3/4 MPU. See, for example
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`2.2. Memory Protection Unit (MPU) <http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dai0179b/CHDFDFIG.html>`_.
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Using the MPU to Detect Null Pointer Usage
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==========================================
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So, for the STM32, one thing that we can do is to program the MPU to prohibit software
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access to the memory region beginning at address 0x0000:0000. Petteri Aimonen posted a code
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snippet on the NuttX Forum showing how to do this. Here is Petteri's post:
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.. code-block:: C
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/* Catch any null pointer dereferences */
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int region = 0;
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putreg32(region, MPU_RNR);
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putreg32(0, MPU_RBAR);
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putreg32(MPU_RASR_ENABLE | MPU_RASR_SIZE_LOG2(20) | (0xFF << MPU_RASR_SRD_SHIFT) | MPU_RASR_AP_NONO, MPU_RASR);
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mpu_control(true, false, true);
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