From: Jan Lübbe <jluebbe@lasnet.de>
Date: Sat, 29 Sep 2007 16:47:51 +0000 (+0200)
Subject: fix console change race exposed by CFS
X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=a64314e62d89562b6fc77593648bec3acc35bf61;p=GitHub%2Fmoto-9609%2Fandroid_kernel_motorola_exynos9610.git

fix console change race exposed by CFS

The new behaviour of CFS exposes a race which occurs if a switch is
requested when vt_mode.mode is VT_PROCESS.

The process with vc->vt_pid is signaled before vc->vt_newvt is set.
This causes the switch to fail when triggered by the monitoing process
because the target is still -1.

[ If the signal sending fails, the subsequent "reset_vc(vc)" will then
  reset vt_newvt to -1, so this works for that case too.   - Linus ]

Signed-off-by: Jan Lübbe <jluebbe@lasnet.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---

diff --git a/drivers/char/vt_ioctl.c b/drivers/char/vt_ioctl.c
index c6f6f4209739..045e6888d15b 100644
--- a/drivers/char/vt_ioctl.c
+++ b/drivers/char/vt_ioctl.c
@@ -1208,15 +1208,18 @@ void change_console(struct vc_data *new_vc)
 		/*
 		 * Send the signal as privileged - kill_pid() will
 		 * tell us if the process has gone or something else
-		 * is awry
+		 * is awry.
+		 *
+		 * We need to set vt_newvt *before* sending the signal or we
+		 * have a race.
 		 */
+		vc->vt_newvt = new_vc->vc_num;
 		if (kill_pid(vc->vt_pid, vc->vt_mode.relsig, 1) == 0) {
 			/*
 			 * It worked. Mark the vt to switch to and
 			 * return. The process needs to send us a
 			 * VT_RELDISP ioctl to complete the switch.
 			 */
-			vc->vt_newvt = new_vc->vc_num;
 			return;
 		}