Fix for infrequent failures of the x86 "bt" command to handle cases

where a user space task with "resume_userspace" or "entry_INT80_32"
at the top of the stack, or which was interrupted by the crash NMI
while handling a timer interrupt.  Without the patch, the backtrace
would be proceeded with the error message "bt: cannot resolve stack
trace", and then dump the text symbols found on the stack and all
possible exception frames.
([email protected])

Author: Dave Anderson

lkcd_x86_trace.c

@@ -1953,13 +1953,24 @@ find_trace(
 static int 
 kernel_entry_from_user_space(sframe_t *curframe, struct bt_info *bt)
 {
+	ulong stack_segment;
+
 	if (is_kernel_thread(bt->tc->task))
 		return FALSE;
 
-	if (((curframe->fp + 4 + SIZE(pt_regs)) == GET_STACKTOP(bt->task)) &&
-	    !is_kernel_thread(bt->tc->task))
-		return TRUE;
-	else if (userspace_return(curframe->fp+4, bt))
+	stack_segment = GET_STACK_ULONG(curframe->fp + 4 + SIZE(pt_regs) - sizeof(kaddr_t));
+
+	if ((curframe->fp + 4 + SIZE(pt_regs)) == GET_STACKTOP(bt->task)) {
+		if ((stack_segment == 0x7b) || (stack_segment == 0x2b))
+			return TRUE;
+	}
+
+	if ((curframe->fp + 4 + SIZE(pt_regs) + 8) == GET_STACKTOP(bt->task)) {
+		if ((stack_segment == 0x7b) || (stack_segment == 0x2b))
+			return TRUE;
+	}
+
+	if (userspace_return(curframe->fp+4, bt))
 		return TRUE;
 	else
 		return FALSE;