Commit a64314e6 authored Sep 29, 2007 by Jan Lübbe Committed by Linus Torvalds Sep 29, 2007

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>

parent ed7fdff5

drivers/char/vt_ioctl.c

+5 −2

Original line number	Diff line number	Diff line
		@@ -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;
		}