Bug fix to crash handler (still 3.3RC3 candidate)

Tyhe crash handler was allocating the buffer for reading/writing flash
memory on the stack. When the crash handler was called in-process (e.g.
due to an assertion failure or a memory allocation failure), this
typically corrupted the task details.

3 files changed, 31 insertions, 3 deletions

diff --git a/src/Hardware/ExceptionHandlers.cpp b/src/Hardware/ExceptionHandlers.cpp
index 7253f5cf..640c0a41 100644
--- a/src/Hardware/ExceptionHandlers.cpp
+++ b/src/Hardware/ExceptionHandlers.cpp
@@ -8,6 +8,7 @@
 #include "ExceptionHandlers.h"
 #include <Platform/RepRap.h>
 #include <Platform/Platform.h>
+#include <Platform/Tasks.h>
 #include <Hardware/NonVolatileMemory.h>
 #include <Cache.h>
 #if SAME70 || SAM4S || SAM4E
@@ -62,10 +63,10 @@
 		}
 
 		// Record the reason for the software reset
-		NonVolatileMemory mem;
-		SoftwareResetData * const srd = mem.AllocateResetDataSlot();
+		NonVolatileMemory * const mem = new(Tasks::GetNVMBuffer(stk)) NonVolatileMemory;
+		SoftwareResetData * const srd = mem->AllocateResetDataSlot();
         srd->Populate(fullReason, stk);
-        mem.EnsureWritten();
+        mem->EnsureWritten();
 	}
 
 #if defined(__LPC17xx__)
diff --git a/src/Platform/Tasks.cpp b/src/Platform/Tasks.cpp
index e5b1331c..e433b8d8 100644
--- a/src/Platform/Tasks.cpp
+++ b/src/Platform/Tasks.cpp
@@ -11,6 +11,7 @@
 #include <Cache.h>
 #include <Platform/TaskPriorities.h>
 #include <Hardware/SoftwareReset.h>
+#include <Hardware/NonVolatileMemory.h>
 #include <Storage/CRC32.h>
 #include <Movement/StepTimer.h>
 
@@ -102,6 +103,31 @@ extern "C" void ReleaseMallocMutex() noexcept
 	}
 }
 
+// Get a 4-byte aligned NonVolatileMemory buffer suitable for the crash handler to use for reading/writing flash memory.
+// We don't want to use a static buffer because that is wasteful of RAM, given that only the crash handler uses it, we have interrupts disabled while we use it,
+// and we reset immediately afterwards.
+// Instead we use either the bottom or top of the main task stack.
+// Parameter 'stk' is the stack we are interested in, which we must not overwrite. The caller is either using the same stack a little lower, or the exception stack.
+void *Tasks::GetNVMBuffer(const uint32_t *stk) noexcept
+{
+	constexpr size_t stackAllowance = 128;
+	static_assert((sizeof(NonVolatileMemory) & 3) == 0);
+	static_assert(MainTaskStackWords * 4 >= 2 * sizeof(NonVolatileMemory) + stackAllowance + 4);
+	const char * const cStack = reinterpret_cast<const char*>(stk);
+
+	// See if we can use the bottom of the main task stack
+	char *ret = (char *)&mainTask + sizeof(TaskBase);
+	if (cStack > ret + (sizeof(NonVolatileMemory) + stackAllowance + 4))	// allow space for the buffer + 128b in case we are on that stack
+	{
+		ret += 4;															// the +4 is so that we leave the stack marker alone in case the main task raised the exception
+	}
+	else
+	{
+		ret += (MainTaskStackWords * 4) - sizeof(NonVolatileMemory);		// use the top area instead
+	}
+	return ret;
+}
+
 // Application entry point
 [[noreturn]] void AppMain() noexcept
 {
diff --git a/src/Platform/Tasks.h b/src/Platform/Tasks.h
index aff42e38..f3f8b878 100644
--- a/src/Platform/Tasks.h
+++ b/src/Platform/Tasks.h
@@ -21,6 +21,7 @@ namespace Tasks
 	const char* GetHeapTop() noexcept;
 	Mutex *GetI2CMutex() noexcept;
 	Mutex *GetFilamentsMutex() noexcept;
+	void *GetNVMBuffer(const uint32_t *stk) noexcept;
 }
 
 #if SUPPORT_CAN_EXPANSION