[PATCH] x86: privilege cleanup

Privilege checking cleanup.  Originally, these diffs were much greater, but
recent cleanups in Linux have already done much of the cleanup.  I added
some explanatory comments in places where the reasoning behind certain
tests is rather subtle.

Also, in traps.c, we can skip the user_mode check in handle_BUG().  The
reason is, there are only two call chains - one via die_if_kernel() and one
via do_page_fault(), both entering from die().  Both of these paths already
ensure that a kernel mode failure has happened.  Also, the original check
here, if (user_mode(regs)) was insufficient anyways, since it would not
rule out BUG faults from V8086 mode execution.

Saving the %ss segment in show_regs() rather than assuming a fixed value
also gives better information about the current kernel state in the
register dump.

Signed-off-by: Zachary Amsden <zach@vmware.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/i386/kernel/signal.c

@@ -604,7 +604,9 @@ int fastcall do_signal(struct pt_regs *regs, sigset_t *oldset)
 	 * We want the common case to go fast, which
 	 * is why we may in certain cases get here from
 	 * kernel mode. Just return without doing anything
-	 * if so.
+ 	 * if so.  vm86 regs switched out by assembly code
+ 	 * before reaching here, so testing against kernel
+ 	 * CS suffices.
 	 */
 	if (!user_mode(regs))
 		return 1;