spurious illegal instructions under 4.2

[Robert W. Scheifler]

Index:	sys/vax/machdep.c 4.2BSD

Description:
	The checks for growing P1 space in sendsig() are inadequate.
	The code assumes that, if a signal stack is used, the signal stack
	will always be in P0 space.  This is quite likely, but it is a bit
	bogus to make that restriction.  The code also assumes that, if
	a signal stack is used, the signal context will always go on the
	signal stack.  Unfortunately, the signal context goes on the
	current stack, not the new stack.  If the current stack is in P1
	space, and the current SP is close to the current end of P1 space,
	P1 space will not be grown and a subsequent check for write access
	will fail, causing your program to get a special SIGILL which cannot
	be caught (which made this bug a bit hard to track down).
Repeat-By:
	Generate a signal that will use a signal stack in previously
	untouched P1 space.

	Generate a signal that will use a signal stack in P0 space when the
	current SP is within a sigcontext's length of the current end of P1
	space.
Fix:
	In sendsig(), change
		if (!oonstack && (int)fp <= USRSTACK - ctob(u.u_ssize))
			grow((unsigned)fp);
	to be
		if ((int)fp <= USRSTACK - ctob(u.u_ssize))
			grow((unsigned)fp);
	and change
		if (!u.u_onstack && (int)scp <= USRSTACK - ctob(u.u_ssize))
			grow((unsigned)scp);
	to be
		if ((int)scp <= USRSTACK - ctob(u.u_ssize))
			grow((unsigned)scp);
	Note that when fp (scp) is P0 space, grow will be called gratuitously.
	There should probably be additional checks to see if the address is
	in P1 space before calling grow.