x86 setup: don't recalculate ss:esp unless really necessary

In order to work around old LILO versions providing an invalid ss
register, the current setup code always sets up a new stack,
immediately following .bss and the heap. But this breaks LOADLIN.

This rewrite of the workaround checks for an invalid stack (ss!=ds)
first, and leaves ss:sp alone otherwise (apart from aligning esp).

[hpa note: LOADLIN has a number of arbitrary hard-coded limits that
are being pushed up against.  Without some major revision of LOADLIN
itself it will not be sustainable keeping it alive.  This gives it
another brief lease on life, however.  This patch also helps the
cmdline truncation problem with old versions of SYSLINUX.]

Signed-off-by: Jens Rottmann <JRottmann at LiPPERT-AT. de>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/boot/header.S

@@ -236,39 +236,30 @@ start_of_setup:
 	movw	%ax, %es
 	cld
 
-# Apparently some ancient versions of LILO invoked the kernel
-# with %ss != %ds, which happened to work by accident for the
-# old code.  If the CAN_USE_HEAP flag is set in loadflags, or
-# %ss != %ds, then adjust the stack pointer.
+# Apparently some ancient versions of LILO invoked the kernel with %ss != %ds,
+# which happened to work by accident for the old code.  Recalculate the stack
+# pointer if %ss is invalid.  Otherwise leave it alone, LOADLIN sets up the
+# stack behind its own code, so we can't blindly put it directly past the heap.
 
-	# Smallest possible stack we can tolerate
-	movw	$(_end+STACK_SIZE), %cx
-
-	movw	heap_end_ptr, %dx
-	addw	$512, %dx
-	jnc	1f
-	xorw	%dx, %dx	# Wraparound - whole segment available
-1:	testb	$CAN_USE_HEAP, loadflags
-	jnz	2f
-
-	# No CAN_USE_HEAP
 	movw	%ss, %dx
 	cmpw	%ax, %dx	# %ds == %ss?
 	movw	%sp, %dx
-	# If so, assume %sp is reasonably set, otherwise use
-	# the smallest possible stack.
-	jne	4f		# -> Smallest possible stack...
+	je	2f		# -> assume %sp is reasonably set
+
+	# Invalid %ss, make up a new stack
+	movw	$_end, %dx
+	testb	$CAN_USE_HEAP, loadflags
+	jz	1f
+	movw	heap_end_ptr, %dx
+1:	addw	$STACK_SIZE, %dx
+	jnc	2f
+	xorw	%dx, %dx	# Prevent wraparound
 
-	# Make sure the stack is at least minimum size.  Take a value
-	# of zero to mean "full segment."
-2:
+2:	# Now %dx should point to the end of our stack space
 	andw	$~3, %dx	# dword align (might as well...)
 	jnz	3f
 	movw	$0xfffc, %dx	# Make sure we're not zero
-3:	cmpw	%cx, %dx
-	jnb	5f
-4:	movw	%cx, %dx	# Minimum value we can possibly use
-5:	movw	%ax, %ss
+3:	movw	%ax, %ss
 	movzwl	%dx, %esp	# Clear upper half of %esp
 	sti			# Now we should have a working stack