davest%tektronix.csnet@CSNET-RELAY.arpa (David C. Stewart) (07/26/86)
Index: ucb/window/wwiomux.c 4.3BSD
Description:
Window uses fcntl(2) to set the FASYNC flag on fd 0. (FASYNC means
that pending I/O causes a SIGIO to get sent to the process group. This
is how window knows that there is keyboard input.) This causes the
FASYNC bit to get set in the open file structure flags for fd 0. Also,
an fioctl(fp, FIOASYNC, 1) is done to set the ASYNC bit in the t_state
field of the tty structure (because fioctl() calls ttioctl()). This
takes place in fset(), which was called by fcntl()
(sys/kern_descrip.c).
Wall comes along and opens each tty in the utmp file in turn and
sets the FNDELAY flag on the open tty using fcntl(). This does a
fioctl(fp, FIONBIO, 1) on the fd, as described above, but because
FNDELAY and FASYNC are in the same flags word, a fioctl(fp, FASYNC, 0)
call is also made, and the FASYNC flag is cleared in the tty. Window,
which is waiting for a SIGIO to tell it to read() from the tty,
will never get one and has wedged the poor luser's terminal. You
now need to login from another terminal and kill your window
process.
Repeat-By:
Run window. In a window, run wall. The wall output will write to
your tty and your tty will ignore you from then on. Note that
any output bound for a window still gets written, because window
uses a select() to wait for window output.
Fix:
There are many ways to change wall and/or window to prevent this
from happening. However, the real problem stems from the fact that
one ought to be able to determine the current value of the
t_state field of the tty structure without opening /dev/kmem. In
any case, my intuition is that the kernel should not allow this
to happen: if a tty needs to have FASYNC or FNDELAY set by a process,
this should occur without fouling up the settings that another process
has made. It is easy to blame the implementation of fcntl(), but
doing a raw FIONBIO or FIOASYNC ioctl() on the tty is almost as bad,
since there is still no way to tell whether or not you are messing
up the current settings. This is an appeal to you subscribers for a
better fix.
In the meantime, I offer the following modification to window which
is a gross hack, and is not elegant, but it should free up the
terminal. Instead of having window sleep in select() forever, I
have it timeout after SELTIMEOUT seconds, at which time it forces
FASYNC to be asserted on fd 0. (Since there is no way to read this
information besides opening kmem and I don't want to make window
setuid kmem, thank you, I am forced to set the flag every time - the
only way I can be *sure* it's set). The cost is that an idle
window session will accrue one system call every SELTIMEOUT seconds.
One more note: There can be no output to any window for SELTIMEOUT
seconds for the terminal to get unjammed.
Diffs follow:
RCS file: RCS/wwiomux.c,v
retrieving revision 1.0
retrieving revision 1.1
diff -c -r1.0 -r1.1
*** /tmp/,RCSt1026703 Fri Jul 25 17:59:12 1986
--- /tmp/,RCSt2026703 Fri Jul 25 17:59:13 1986
***************
*** 1,5
#ifndef lint
! static char *RCSid = "$Header: wwiomux.c,v 1.0 86/07/25 15:44:35 root Exp $";
#endif
#ifndef lint
static char sccsid[] = "@(#)wwiomux.c 3.14 4/24/85";
--- 1,5 -----
#ifndef lint
! static char *RCSid = "$Header: wwiomux.c,v 1.1 86/07/25 17:54:45 davest Exp $";
#endif
#ifndef lint
static char sccsid[] = "@(#)wwiomux.c 3.14 4/24/85";
***************
*** 13,18
#include "ww.h"
#include <sys/time.h>
/*
* Multiple window output handler.
--- 13,20 -----
#include "ww.h"
#include <sys/time.h>
+ #include <fcntl.h>
+ #define SELTIMEOUT 5
/*
* Multiple window output handler.
***************
*** 33,38
register char *p;
char c;
static struct timeval tv = { 0, 0 };
char noblock;
loop:
--- 35,41 -----
register char *p;
char c;
static struct timeval tv = { 0, 0 };
+ static struct timeval tmout = { SELTIMEOUT, 0 };
char noblock;
loop:
***************
*** 63,68
return;
}
}
wwnselect++;
n = select(wwdtablesize, &imask, (int *)0, (int *)0,
noblock ? &tv : (struct timeval *)0);
--- 66,72 -----
return;
}
}
+ doselect:;
wwnselect++;
n = select(wwdtablesize, &imask, (int *)0, (int *)0,
noblock ? &tv : &tmout);
***************
*** 65,71
}
wwnselect++;
n = select(wwdtablesize, &imask, (int *)0, (int *)0,
! noblock ? &tv : (struct timeval *)0);
wwsetjmp = 0;
if (n < 0)
--- 69,75 -----
doselect:;
wwnselect++;
n = select(wwdtablesize, &imask, (int *)0, (int *)0,
! noblock ? &tv : &tmout);
wwsetjmp = 0;
if (n < 0)
***************
*** 70,76
if (n < 0)
wwnselecte++;
! else if (n == 0)
wwnselectz++;
else
for (w = wwhead.ww_forw; w != &wwhead; w = w->ww_forw) {
--- 74,80 -----
if (n < 0)
wwnselecte++;
! else if (n == 0) {
wwnselectz++;
if (!noblock) { /* timeout, FASYNC may have cleared */
fcntl(0, F_SETFL, wwnewtty.ww_fflags|FASYNC);
***************
*** 72,77
wwnselecte++;
else if (n == 0)
wwnselectz++;
else
for (w = wwhead.ww_forw; w != &wwhead; w = w->ww_forw) {
if (w->ww_pty < 0 || (imask & 1 << w->ww_pty) == 0)
--- 76,87 -----
wwnselecte++;
else if (n == 0) {
wwnselectz++;
+ if (!noblock) { /* timeout, FASYNC may have cleared */
+ fcntl(0, F_SETFL, wwnewtty.ww_fflags|FASYNC);
+ wwsetjmp = 1;
+ goto doselect;
+ }
+ }
else
for (w = wwhead.ww_forw; w != &wwhead; w = w->ww_forw) {
if (w->ww_pty < 0 || (imask & 1 << w->ww_pty) == 0)