Bourne shell's wacky memory allocation

[Guy Harris]

> This is real easy to fix.

I know - I had to fix it here; I did so by doing the "setbrk" directly
instead of faking a SIGSEGV.  (The code in question - added to the shell -
was allocating some BMF structures, and I didn't feel like having the shell
keep bumping its data space up until it got big enough).

Your line numbers may differ.  This is for the S5R2 shell.  It's probably
similar for others.

*** /tmp/da6483	Sat Oct 26 15:17:15 1985
--- blok.c	Thu Oct 10 18:20:49 1985
***************
*** 87,92
  	reqd &= ~(brkincr - 1);
  	blokp = bloktop;
  	bloktop = bloktop->word = (struct blk *)(Rcheat(bloktop) + reqd);
  	bloktop->word = (struct blk *)(brkbegin + 1);
  	{
  		register char *stakadr = (char *)(bloktop + 2);

--- 87,95 -----
  	reqd &= ~(brkincr - 1);
  	blokp = bloktop;
  	bloktop = bloktop->word = (struct blk *)(Rcheat(bloktop) + reqd);
+ 	reqd = (char *)(&bloktop->word) - brkend;
+ 	if (reqd > 0 && setbrk(reqd) == (char *)-1)
+ 		error(nospace);
  	bloktop->word = (struct blk *)(brkbegin + 1);
  	{
  		register char *stakadr = (char *)(bloktop + 2);

The reason why this causes problems on 68010s is not that you can't continue
from an instruction.  The problem is that you can't do so *from user mode*
without kernel hacks.  In order to continue the instruction, the "stack
puke" (as Henry Spencer described it) must be on the stack when the RTE is
done.  The RTE must be done from the kernel, since it's privileged.  In some
UNIXes, returning from a signal doesn't involve the kernel (you can do an
RTR to reload the condition codes) - you're out of luck there.  In others
(like 4.2BSD, which has to reenable the signal in question) returning from a
signal requires you to pass through the kernel.  However, the bus error
frame isn't likely to be on the kernel stack anymore.  You'd have to
squirrel it away somewhere - somewhere inaccessible to the user, because I
don't think Motorola tells you 1) that it won't kill the system if you RTE
with arbitrary bits or 2) how to make sure that a bus error frame is safe.
Sun UNIX doesn't do this, and other 68010 UNIXes may not either.

	Guy Harris