x86: make traps on 'iret' be debuggable in user space

This makes a trap on the 'iret' that returns us to user space
cause a nice clean SIGSEGV, instead of just a hard (and silent)
exit.

That way a debugger can actually try to see what happened, and
we also properly notify everybody who might be interested about
us being gone.

This loses the error code, but tells the debugger what happened
with ILL_BADSTK in the siginfo.

arch/i386/kernel/entry.S

@@ -260,11 +260,9 @@ restore_nocheck:
 .section .fixup,"ax"
 iret_exc:
 	sti
-	movl $__USER_DS, %edx
-	movl %edx, %ds
-	movl %edx, %es
-	movl $11,%eax
-	call do_exit
+	pushl $0			# no error code
+	pushl $do_iret_error
+	jmp error_code
 .previous
 .section __ex_table,"a"
 	.align 4

arch/i386/kernel/traps.c

@@ -451,6 +451,7 @@ DO_ERROR(10, SIGSEGV, "invalid TSS", invalid_TSS)
 DO_ERROR(11, SIGBUS,  "segment not present", segment_not_present)
 DO_ERROR(12, SIGBUS,  "stack segment", stack_segment)
 DO_ERROR_INFO(17, SIGBUS, "alignment check", alignment_check, BUS_ADRALN, 0)
+DO_ERROR_INFO(32, SIGSEGV, "iret exception", iret_error, ILL_BADSTK, 0)
 
 fastcall void do_general_protection(struct pt_regs * regs, long error_code)
 {