diff --git a/src/pkg/runtime/386/asm.s b/src/pkg/runtime/386/asm.s
index bd88f0fdc5..7ec62161d9 100644
--- a/src/pkg/runtime/386/asm.s
+++ b/src/pkg/runtime/386/asm.s
@@ -323,6 +323,17 @@ TEXT	runcgo(SB),7,$16
 	MOVL	4(SP), SP
 	RET
 
+// check that SP is in range [g->stackbase, g->stackguard)
+TEXT stackcheck(SB), 7, $0
+	MOVL g, AX
+	CMPL g_stackbase(AX), SP
+	JHI 2(PC)
+	INT $3
+	CMPL SP, g_stackguard(AX)
+	JHI 2(PC)
+	INT $3
+	RET
+
 
 GLOBL m0(SB), $1024
 GLOBL g0(SB), $1024
diff --git a/src/pkg/runtime/amd64/asm.s b/src/pkg/runtime/amd64/asm.s
index aee4e9a3f7..3bd63ad15b 100644
--- a/src/pkg/runtime/amd64/asm.s
+++ b/src/pkg/runtime/amd64/asm.s
@@ -301,3 +301,13 @@ TEXT runcgo(SB),7,$32
 	MOVQ	8(SP), SP
 	RET
 
+// check that SP is in range [g->stackbase, g->stackguard)
+TEXT stackcheck(SB), 7, $0
+	CMPQ g_stackbase(g), SP
+	JHI 2(PC)
+	INT $3
+	CMPQ SP, g_stackguard(g)
+	JHI 2(PC)
+	INT $3
+	RET
+
diff --git a/src/pkg/runtime/darwin/386/sys.s b/src/pkg/runtime/darwin/386/sys.s
index 445f530028..38459447f8 100644
--- a/src/pkg/runtime/darwin/386/sys.s
+++ b/src/pkg/runtime/darwin/386/sys.s
@@ -153,6 +153,7 @@ TEXT bsdthread_start(SB),7,$0
 	MOVL	AX, g
 	MOVL	DX, m
 	MOVL	BX, m_procid(DX)	// m->procid = thread port (for debuggers)
+	CALL	stackcheck(SB)		// smashes AX
 	CALL	CX	// fn()
 	CALL	exit1(SB)
 	RET
diff --git a/src/pkg/runtime/freebsd/386/sys.s b/src/pkg/runtime/freebsd/386/sys.s
index 1c0eaead5b..651ccb2348 100644
--- a/src/pkg/runtime/freebsd/386/sys.s
+++ b/src/pkg/runtime/freebsd/386/sys.s
@@ -9,13 +9,13 @@
 #include "386/asm.h"
 	
 TEXT sys_umtx_op(SB),7,$-4
-	MOVL $454, AX
-	INT $0x80
+	MOVL	$454, AX
+	INT	$0x80
 	RET
 
 TEXT thr_new(SB),7,$-4
-	MOVL $455, AX
-	INT $0x80
+	MOVL	$455, AX
+	INT	$0x80
 	RET
 
 TEXT thr_start(SB),7,$0
@@ -33,10 +33,11 @@ TEXT thr_start(SB),7,$0
 	POPL	AX
 	POPL	AX
 	POPAL
-	MOVL BX, g
-	MOVL AX, m
-	CALL mstart(SB)
-	MOVL 0, AX			// crash (not reached)
+	MOVL	BX, g
+	MOVL	AX, m
+	CALL	stackcheck(SB)		// smashes AX
+	CALL	mstart(SB)
+	MOVL	0, AX			// crash (not reached)
 
 // Exit the entire program (like C exit)
 TEXT exit(SB),7,$-4
diff --git a/src/pkg/runtime/freebsd/amd64/sys.s b/src/pkg/runtime/freebsd/amd64/sys.s
index 1b62468105..30184e297c 100644
--- a/src/pkg/runtime/freebsd/amd64/sys.s
+++ b/src/pkg/runtime/freebsd/amd64/sys.s
@@ -28,6 +28,7 @@ TEXT thr_new(SB),7,$0
 TEXT thr_start(SB),7,$0
 	MOVQ DI, m
 	MOVQ m_g0(m), g
+	CALL stackcheck(SB)
 	CALL mstart(SB)
 	MOVQ 0, AX			// crash (not reached)
 
diff --git a/src/pkg/runtime/freebsd/thread.c b/src/pkg/runtime/freebsd/thread.c
index 5f44022365..bf891e9804 100644
--- a/src/pkg/runtime/freebsd/thread.c
+++ b/src/pkg/runtime/freebsd/thread.c
@@ -139,8 +139,8 @@ newosproc(M *m, G *g, void *stk, void (*fn)(void))
 
 	param.start_func = thr_start;
 	param.arg = m;
-	param.stack_base = stk;
-	param.stack_size = g->stackbase - g->stackguard + 256;
+	param.stack_base = (int8*)g->stackbase;
+	param.stack_size = (byte*)stk - (byte*)g->stackbase;
 	param.child_tid = (intptr*)&m->procid;
 	param.parent_tid = nil;
 	param.tls_base = (int8*)&m->tls[0];
diff --git a/src/pkg/runtime/linux/386/sys.s b/src/pkg/runtime/linux/386/sys.s
index 097dfe9155..72882cb9dc 100644
--- a/src/pkg/runtime/linux/386/sys.s
+++ b/src/pkg/runtime/linux/386/sys.s
@@ -152,6 +152,7 @@ TEXT clone(SB),7,$0
 	MOVL	DX, g
 	MOVL	BX, m
 
+	CALL	stackcheck(SB)	// smashes AX
 	MOVL	0(DX), DX	// paranoia; check they are not nil
 	MOVL	0(BX), BX
 
diff --git a/src/pkg/runtime/linux/amd64/sys.s b/src/pkg/runtime/linux/amd64/sys.s
index 238a423b13..6565d86de2 100644
--- a/src/pkg/runtime/linux/amd64/sys.s
+++ b/src/pkg/runtime/linux/amd64/sys.s
@@ -149,6 +149,7 @@ TEXT clone(SB),7,$0
 	MOVQ	SI, SP
 	MOVQ	R8, m
 	MOVQ	R9, g
+	CALL	stackcheck(SB)
 
 	// Initialize m->procid to Linux tid
 	MOVL	$186, AX	// gettid