fix hypercall fallback code for very old hypervisors

While copying the argument structures in HYPERVISOR_event_channel_op()
and HYPERVISOR_physdev_op() into the local variable is sufficiently
safe even if the actual structure is smaller than the container one,
copying back eventual output values the same way isn't: This may
collide with on-stack variables (particularly "rc") which may change
between the first and second memcpy() (i.e. the second memcpy() could
discard that change).

Move the fallback code into out-of-line functions, and handle all of
the operations known by this old a hypervisor individually: Some don't
require copying back anything at all, and for the rest use the
individual argument structures' sizes rather than the container's.

Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v2: Reduce #define/#undef usage in HYPERVISOR_physdev_op_compat().

--- a/drivers/xen/core/Makefile
+++ b/drivers/xen/core/Makefile
@@ -2,7 +2,7 @@
 # Makefile for the linux kernel.
 #
 
-obj-y := evtchn.o gnttab.o features.o reboot.o machine_reboot.o
+obj-y := evtchn.o gnttab.o features.o reboot.o machine_reboot.o fallback.o
 
 obj-$(CONFIG_PCI)		+= pci.o
 obj-$(CONFIG_XEN_PRIVILEGED_GUEST) += firmware.o
--- /dev/null
+++ b/drivers/xen/core/fallback.c
@@ -0,0 +1,84 @@
+#include <linux/kernel.h>
+#include <linux/string.h>
+#include <asm/bug.h>
+#include <asm/hypervisor.h>
+
+#if 1//temp CONFIG_XEN_COMPAT <= 0x030002
+
+#include <xen/interface/event_channel.h>
+#include <xen/interface/physdev.h>
+
+int HYPERVISOR_event_channel_op_compat(int cmd, void *arg)
+{
+	struct evtchn_op op;
+	int rc;
+
+	op.cmd = cmd;
+	memcpy(&op.u, arg, sizeof(op.u));
+	rc = _hypercall1(int, event_channel_op_compat, &op);
+
+	switch (cmd) {
+	case EVTCHNOP_close:
+	case EVTCHNOP_send:
+	case EVTCHNOP_bind_vcpu:
+	case EVTCHNOP_unmask:
+		/* no output */
+		break;
+
+#define COPY_BACK(eop) \
+	case EVTCHNOP_##eop: \
+		memcpy(arg, &op.u.eop, sizeof(op.u.eop)); \
+		break
+
+	COPY_BACK(bind_interdomain);
+	COPY_BACK(bind_virq);
+	COPY_BACK(bind_pirq);
+	COPY_BACK(status);
+	COPY_BACK(alloc_unbound);
+	COPY_BACK(bind_ipi);
+#undef COPY_BACK
+
+	default:
+		WARN_ON(rc != -ENOSYS);
+		break;
+	}
+
+	return rc;
+}
+
+int HYPERVISOR_physdev_op_compat(int cmd, void *arg)
+{
+	struct physdev_op op;
+	int rc;
+
+	op.cmd = cmd;
+	memcpy(&op.u, arg, sizeof(op.u));
+	rc = _hypercall1(int, physdev_op_compat, &op);
+
+	switch (cmd) {
+	case PHYSDEVOP_IRQ_UNMASK_NOTIFY:
+	case PHYSDEVOP_set_iopl:
+	case PHYSDEVOP_set_iobitmap:
+	case PHYSDEVOP_apic_write:
+		/* no output */
+		break;
+
+#define COPY_BACK(pop, fld) \
+	case PHYSDEVOP_##pop: \
+		memcpy(arg, &op.u.fld, sizeof(op.u.fld)); \
+		break
+
+	COPY_BACK(irq_status_query, irq_status_query);
+	COPY_BACK(apic_read, apic_op);
+	COPY_BACK(ASSIGN_VECTOR, irq_op);
+#undef COPY_BACK
+
+	default:
+		WARN_ON(rc != -ENOSYS);
+		break;
+	}
+
+	return rc;
+}
+
+#endif /* CONFIG_XEN_COMPAT <= 0x030002 */
--- a/include/asm-i386/mach-xen/asm/hypercall.h
+++ b/include/asm-i386/mach-xen/asm/hypercall.h
@@ -33,7 +33,6 @@
 #ifndef __HYPERCALL_H__
 #define __HYPERCALL_H__
 
-#include <linux/string.h> /* memcpy() */
 #include <linux/stringify.h>
 
 #ifndef __HYPERVISOR_H__
@@ -128,6 +127,11 @@
 	__res;							\
 })
 
+#if CONFIG_XEN_COMPAT <= 0x030002
+int __must_check HYPERVISOR_event_channel_op_compat(int, void *);
+int __must_check HYPERVISOR_physdev_op_compat(int, void *);
+#endif
+
 static inline int __must_check
 HYPERVISOR_set_trap_table(
 	const trap_info_t *table)
@@ -267,13 +271,8 @@ HYPERVISOR_event_channel_op(
 	int rc = _hypercall2(int, event_channel_op, cmd, arg);
 
 #if CONFIG_XEN_COMPAT <= 0x030002
-	if (unlikely(rc == -ENOSYS)) {
-		struct evtchn_op op;
-		op.cmd = cmd;
-		memcpy(&op.u, arg, sizeof(op.u));
-		rc = _hypercall1(int, event_channel_op_compat, &op);
-		memcpy(arg, &op.u, sizeof(op.u));
-	}
+	if (unlikely(rc == -ENOSYS))
+		rc = HYPERVISOR_event_channel_op_compat(cmd, arg);
 #endif
 
 	return rc;
@@ -300,13 +299,8 @@ HYPERVISOR_physdev_op(
 	int rc = _hypercall2(int, physdev_op, cmd, arg);
 
 #if CONFIG_XEN_COMPAT <= 0x030002
-	if (unlikely(rc == -ENOSYS)) {
-		struct physdev_op op;
-		op.cmd = cmd;
-		memcpy(&op.u, arg, sizeof(op.u));
-		rc = _hypercall1(int, physdev_op_compat, &op);
-		memcpy(arg, &op.u, sizeof(op.u));
-	}
+	if (unlikely(rc == -ENOSYS))
+		rc = HYPERVISOR_physdev_op_compat(cmd, arg);
 #endif
 
 	return rc;
--- a/include/asm-i386/mach-xen/asm/hypervisor.h
+++ b/include/asm-i386/mach-xen/asm/hypervisor.h
@@ -39,8 +39,6 @@
 #include <linux/errno.h>
 #include <xen/interface/xen.h>
 #include <xen/interface/platform.h>
-#include <xen/interface/event_channel.h>
-#include <xen/interface/physdev.h>
 #include <xen/interface/sched.h>
 #include <xen/interface/nmi.h>
 #include <xen/interface/tmem.h>