Fixed the horrible 'nofoo' bug!

When compiled with gcc 4.6.1, 32-bit Sortix would triple fault during
early boot: When the TLB is being flushed, somehow a garbage value had
sneaked into Sortix::Memory::currentdir, and a non-page aligned (and
garbage) page directory is loaded. (Triple fault, here we come!)

However, adding a volatile addr_t foo after the currentdir variable
actually caused the system to boot correctly - the garbage was written
into that variable instead. To debug the problem, I set the foo value
to 0: as long as !foo (hence the name nofoo) everything was alright.

After closer examination I found that the initrd open code wrote to a
pointer supplied by kernel.cpp. The element pointed to was on the
stack. Worse, its address was the same as currentdir (now foo).

Indeed, the stack had gone into the kernel's data segment!

Turns out that this gcc configuration stores variables in the data
segment in the reverse order they are defined in, whereas previous
compilers did the opposite. The hack used to set up the stack during
early boot relied on this (now obviously incorrect) fact.

In effect, the stack was initialized to the end of the stack, not
the start of it: completely ignoring all the nice stack space
allocated in kernel.cpp.

I did not see that one coming.

sortix/kernel.cpp

@@ -56,8 +56,8 @@ using namespace Maxsi;
 
 void* RunApplication(void* Parameter);
 
+// Keep the stack size aligned with $CPU/base.s
 extern "C" { size_t stack[64*1024] = {0}; }
-extern "C" { size_t stackend = 0; }
 
 namespace Sortix
 {

sortix/x64/base.s

@@ -41,7 +41,8 @@ _beginkernel:
 	movw $0x758, 0xB83F8
 
 	# Initialize the stack pointer.
-	movq $stackend, %rsp
+	movq $stack, %rsp
+	addq $65536, %rsp # 64 KiB, see kernel.cpp
 
 	# Reset EFLAGS.
 	# pushl	$0

sortix/x86-family/memorymanagement.cpp

@@ -291,6 +291,12 @@ namespace Sortix
 
 		addr_t SwitchAddressSpace(addr_t addrspace)
 		{
+			if ( currentdir != Page::AlignDown(currentdir) )
+			{
+				PanicF("Attempted to switch to address space 0x%zx, but it "
+				       "wasn't page aligned!", currentdir);
+			}
+
 			// Don't switch if we are already there.
 			if ( addrspace == currentdir ) { return currentdir; }
 

sortix/x86/base.s

@@ -31,8 +31,9 @@
 .type _beginkernel, @function
 beginkernel:
 _beginkernel:
-	# Initialize the stack pointer.
-	mov	$stackend, %esp
+	# Initialize the stack pointer. The magic value is from kernel.cpp.
+	movl $stack, %esp
+	addl $65536, %esp # 64 KiB, see kernel.cpp
 
 	# Reset EFLAGS.
 	# pushl	$0