4.1BSD -ljobs signal bug (with fix) (4.[23]BSD folks--please skip this)

[Arthur David Olson]

Index:	/usr/src/lib/libjobs/sigset.c 4.1 Fix
	N.B.:  This bug does *not* exist in 4.[23]BSD.  If you're running
	one of those, and are not obsessively interested in potential pits that
	code you write may fall into when it's moved to other systems,
	bail out of this article right now.

Description:
	Programs that use the -ljobs library and call "signal"
	("vnews", for example) can dump core mysteriously.

Repeat-By:
	Compile the program

		#include "signal.h"

		catcher(){}

		main()
		{
			int	pid;

			switch (pid = fork()) {
				case 0:		/* child */
					for ( ; ; ) {
						signal(SIGTSTP, catcher);
						signal(SIGTSTP, SIG_HOLD);
					}
				default:	/* parent */
					sleep(1);
					while (kill(pid, SIGTSTP) == 0)
						;
			}
		}

	with the "-ljobs" library and run it.  Sit back.  Have a jug of coffee.
	Or, if you're as impatient as I am, compile the above with a version
	of "sigset.c" where the function "signal" has been modified this way:

> ...
> 	cactions[signum] = action;
> #ifndef OLDVERSION
> 	if (action == SIG_HOLD) {
> 		register int	i;
> 
> 		for (i = 0; i < 100000; ++i)
> 			;
> 	}
> #endif
> 	if (action != SIG_HOLD && action != SIG_DFL && action != SIG_HOLD)
> ...

	In either event, you should eventually get a core dump.  Why?
	The "signal" function sets
		cactions[signum] = action;
	slightly before it does a subsequent
		action = sigsys(signum, action);
	call that tells the kernel to change the signal's handling.
	If a signal catcher has previously been set up, and a signal hits
	between the
		cactions[signum] = action;
	and the
		action = sigsys(signum, action);
	it gets processed by the function _sigcatch, which calls the
	function whose address is stored in cactions[signum].  So if you've
	just changed cactions[signum] to SIG_HOLD (for example), you end up at
	a bad place.

Fix:
	There's surely a better fix, but this is no time for a treasure hunt.
	As usual, the fix is conditioned on OLDVERSION.

	...
		_sigcatch(signum, code, xx, ps, pc)
		register signum;
		int code;
		{
			register int (*act)() = cactions[signum];

		#ifndef OLDVERSION
			if (act != SIG_IGN && act != SIG_DFL && act != SIG_HOLD)
		#endif
			{ asm("callg (ap),(r10)"); }
		#ifndef OLDVERSION
			/* watch out for asm bug! */
				;
		#endif
			if (setflg[signum]) {
	...
--
Bug/s is a Volkswagen/Warner Brothers trademark.
ljobs may be what folks at Next call el presidente.
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