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f6ecf80ebb
The documentation grew one page at a time, so the tree follows the
history of who wrote what and not the shape of NuttX. Scheduling is
spread over three places, a driver page can sit above the subsystem
that owns it, and the front page lists everything at the same level.
That is a lot to face when all you want to know is where the scheduler
lives.
This change files every page under the code it describes. It is a move,
not a rewrite: outside the ten pages named below, every page keeps the
text that is already in master, and no page's text is deleted.
What it does:
* Groups the table of contents into nine chapters.
* Moves the OS subsystems under os/: scheduling, memory, drivers,
filesystem, networking, IPC, interrupts, libs, time.
* Renames the platform pages to the names the source tree uses, and
derives their tags from the tree instead of by hand.
* Splits guides/ by subject.
* Adds Documentation/redirects.py, with a rule for every page that left
its old path, so old URLs keep working. The redirect page also carries
a link's #anchor across to the new page.
Ten pages have text that is new or rewritten. Nine of them are the
landing page of a chapter, which has to exist for the new structure:
index the front page
os/index OS Design
os/scheduling/index Scheduling
os/interrupts/index Interrupts
os/ipc/index IPC
os/time/index Time and timers
about/index About
developing/index Developing NuttX
ReleaseNotes/index Release notes
The tenth is os/libs/libbuiltin, the only page here with technical
content: libs/libbuiltin/ had no page at all. Five SVG diagrams come
with these pages, hand-written XML with no editor metadata.
Nothing outside Documentation/ is touched.
How it was checked:
* Sphinx builds with -W: no warnings, and no document left outside a
toctree.
* A script, offered in the PR, proves the narrow claim this rests on.
For every page outside the ten named above it erases what a move
touches -- link target, path, tag line, toctree block, table border --
from the whole old text and the whole new text, and requires the two
to be byte for byte identical. It also requires every sentence of a
deleted page to turn up somewhere, and every page that left its old
path to have a redirect, from a URL that existed, to where its content
went. It exits non-zero and names the page if any of that is not true,
and it tests added pages too, so forgetting to declare one cannot make
it pass.
* An independent audit checked 133 factual claims on these ten pages
against the tree, one shell command per claim: 130 confirmed, 1
refuted and fixed here, 2 not checkable.
* tools/checkpatch.sh is clean over the range.
The diff is large because moving a page changes every link that points
to it. Most of it is pure renames, and board pages that gained one tag
line.
Assisted-by: Claude:claude-opus-5
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3.1 KiB
ReStructuredText
70 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 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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