Some syscalls using set_mcontext can sneakily change
parameters and later when those syscalls references parameters,
they will wrongly use register values in mcontext_t.

	This problem is critical for thread related syscalls!

Fix: Simple patch. See also amd64/amd64/trap.c revision 1.278.

<http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/amd64/amd64/trap.c.diff?r1=1.277&r2=1.278>
How-To-Repeat: 
	Use following changes in kernel:

--- debug.patch begins here ---
--- sys/kern/kern_kse.c.orig	Fri Oct 22 14:47:07 2004
+++ sys/kern/kern_kse.c	Fri Oct 22 14:46:13 2004
@@ -140,8 +140,12 @@ kse_switchin(struct thread *td, struct k
 	if (!error && (uap->flags & KSE_SWITCHIN_SETTMBX))
 		error = (suword(&ku->ku_mailbox->km_curthread,
 			 (long)uap->tmbx) != 0 ? EINVAL : 0);
+	TR2("thread_schedule_upcall: before set_mcontext: tmbx %p flags %d",
+	     uap->tmbx, uap->flags);
 	if (!error)
 		error = set_mcontext(td, &tmbx.tm_context.uc_mcontext);
+	TR2("thread_schedule_upcall: after set_mcontext: tmbx %p flags %d",
+	     uap->tmbx, uap->flags);
 	if (!error) {
 		suword32(&uap->tmbx->tm_lwp, td->td_tid);
 		if (uap->flags & KSE_SWITCHIN_SETTMBX) {
--- sys/sparc64/conf/GENERIC.orig	Wed Sep 22 23:23:39 2004
+++ sys/sparc64/conf/GENERIC	Fri Oct 22 15:00:49 2004
@@ -201,3 +201,7 @@
 device		firewire	# FireWire bus code
 device		sbp		# SCSI over FireWire (Requires scbus and da)
 device		fwe		# Ethernet over FireWire (non-standard!)
+
+options		KDB		#Enable kernel debugger support
+options		DDB		#Enable the kernel debugger
+options		GDB		#Support remote GDB
+options		KTR
+options		KTR_COMPILE=(KTR_GEN|KTR_SMP|KTR_TRAP|KTR_INTR|KTR_SIG|KTR_PROC|KTR_SYSC)
+options		KTR_ENTRIES=8192
--- debug.patch ends here ---

	Run any simple threated program compiled with libkse. For example:

--- hello_d.c begins here ---
/****************************************************************************
 *
 * Simple diff mode test.
 *
 * $FreeBSD: src/lib/libpthread/test/hello_d.c,v 1.1 2000/04/24 21:07:37 jasone Exp $
 *
 ****************************************************************************/

#include <stdio.h>
#include <string.h>
#include <pthread.h>

extern int _libkse_debug;

void *
entry(void * a_arg)
{
	char *str = NULL;
	fprintf(stderr, "Hello world\n");
#if 0
	*str = 'a';
#endif

	return NULL;
}

int
main()
{
	pthread_t thread;
	int error;
	pthread_attr_t attr;
	int scope;

/*
	_libkse_debug = 1;
 */
	error = pthread_attr_init(&attr);
	if (error)
		fprintf(stderr, "Error in pthread_attr_init(): %s\n",
			strerror(error));

	error = pthread_attr_getscope(&attr, &scope);
	if (error)
		fprintf(stderr, "Error in pthread_attr_getscope(): %s\n",
			strerror(error));

	printf("Default scope: %d\n", scope);
#if 1
	scope |= PTHREAD_SCOPE_SYSTEM;
#endif
	printf("Set scope: %d\n", scope);

	error = pthread_attr_setscope(&attr, scope);
	if (error)
		fprintf(stderr, "Error in pthread_attr_setscope(): %s\n",
			strerror(error));

	error = pthread_create(&thread, &attr, entry, NULL);
	if (error)
		fprintf(stderr, "Error in pthread_create(): %s\n",
			strerror(error));

	error = pthread_join(thread, NULL);
	if (error)
		fprintf(stderr, "Error in pthread_join(): %s\n",
			strerror(error));

	error = pthread_attr_destroy(&attr);
	if (error)
		fprintf(stderr, "Error in pthread_attr_destroy(): %s\n",
			strerror(error));

	return 0;
}
--- hello_d.c ends here ---

	Go to kernel debugger by pressing Control-Alt-Esc and "show ktr".
Compare arguments before and after set_mcontext.