[PATCH] fix broken vm86 interrupt/signal handling

Commit c3ff8ec31c1249d268cd11390649768a12bec1b9 ("[PATCH] i386: Don't
miss pending signals returning to user mode after signal processing")
meant that vm86 interrupt/signal handling got broken for the case when
vm86 is called from kernel space.

In this scenario, if signal is pending because of vm86 interrupt,
do_notify_resume/do_signal exits immediately due to user_mode() check,
without processing any signals.  Thus, resume_userspace handler is spinning
in a tight loop with signal pending and TIF_SIGPENDING is set.  Previously
everything worked Ok.

No in-tree usage of vm86() from kernel space exists, but I've heard
about a number of projects out there which use vm86 calls from kernel,
one of them being this, for instance:

	http://dev.gentoo.org/~spock/projects/vesafb-tng/

The following patch fixes the issue.

Signed-off-by: Aleksey Gorelov <aleksey_gorelov@phoenix.com>
Cc: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/i386/kernel/entry.S

@@ -83,6 +83,12 @@ VM_MASK		= 0x00020000
 #define resume_kernel		restore_nocheck
 #endif
 
+#ifdef CONFIG_VM86
+#define resume_userspace_sig	check_userspace
+#else
+#define resume_userspace_sig	resume_userspace
+#endif
+
 #define SAVE_ALL \
 	cld; \
 	pushl %es; \
@@ -211,6 +217,7 @@ ret_from_exception:
 	preempt_stop
 ret_from_intr:
 	GET_THREAD_INFO(%ebp)
+check_userspace:
 	movl EFLAGS(%esp), %eax		# mix EFLAGS and CS
 	movb CS(%esp), %al
 	testl $(VM_MASK | 3), %eax
@@ -415,7 +422,7 @@ work_notifysig:				# deal with pending signals and
 					# vm86-space
 	xorl %edx, %edx
 	call do_notify_resume
-	jmp resume_userspace
+	jmp resume_userspace_sig
 
 	ALIGN
 work_notifysig_v86:
@@ -428,7 +435,7 @@ work_notifysig_v86:
 	movl %eax, %esp
 	xorl %edx, %edx
 	call do_notify_resume
-	jmp resume_userspace
+	jmp resume_userspace_sig
 #endif
 
 	# perform syscall exit tracing