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Re: [Xen-devel] [PATCH 1/2] xen/hypercall: fix hypercall fallback code for very old hypervisors



On Wed, Oct 31, 2012 at 08:55:54AM +0000, Jan Beulich wrote:
> >>> On 30.10.12 at 16:44, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> 
> >>> wrote:
> > On Mon, Oct 29, 2012 at 10:08:17AM -0400, Konrad Rzeszutek Wilk wrote:
> >> From: Jan Beulich <jbeulich@xxxxxxxx>
> >> 
> >> 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().]
> >> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > 
> > And it looks like I get 
> > 
> > ERROR: "HYPERVISOR_event_channel_op_compat" [drivers/xen/xen-evtchn.ko] 
> > undefined!
> > 
> > when I build xen-evtchn as module. Jan did you encounter this issue on 
> > 2.6.18?
> 
> No - the event channel driver there can't be built as a module, and
> for the forward ported kernels I apparently never tried to build with
> a compat setting of 3.0.2 or less (and I didn't care that much either
> because the oldest we're actually concerned about is 3.0.4 to cover
> some of those very old EC2 systems; I'll add the export at the right
> point nevertheless).

Ok, ended up with this version which I was thinking to for v3.7-rc5:


>From 4ae4c7658a7c0501521c422d618038587c3edeca Mon Sep 17 00:00:00 2001
From: Jan Beulich <jbeulich@xxxxxxxx>
Date: Fri, 19 Oct 2012 15:25:37 -0400
Subject: [PATCH] xen/hypercall: 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().]
[v3: Fix compile errors when modules use said hypercalls]
[v4: Add xen_ prefix to the HYPERCALL_..]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
 arch/x86/include/asm/xen/hypercall.h | 21 ++++------
 drivers/xen/Makefile                 |  2 +-
 drivers/xen/fallback.c               | 81 ++++++++++++++++++++++++++++++++++++
 3 files changed, 89 insertions(+), 15 deletions(-)
 create mode 100644 drivers/xen/fallback.c

diff --git a/arch/x86/include/asm/xen/hypercall.h 
b/arch/x86/include/asm/xen/hypercall.h
index 59c226d..4055421 100644
--- a/arch/x86/include/asm/xen/hypercall.h
+++ b/arch/x86/include/asm/xen/hypercall.h
@@ -359,18 +359,14 @@ HYPERVISOR_update_va_mapping(unsigned long va, pte_t 
new_val,
                return _hypercall4(int, update_va_mapping, va,
                                   new_val.pte, new_val.pte >> 32, flags);
 }
+extern int __must_check xen_HYPERVISOR_event_channel_op_compat(int, void *);
 
 static inline int
 HYPERVISOR_event_channel_op(int cmd, void *arg)
 {
        int rc = _hypercall2(int, event_channel_op, cmd, arg);
-       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 = xen_HYPERVISOR_event_channel_op_compat(cmd, arg);
        return rc;
 }
 
@@ -386,17 +382,14 @@ HYPERVISOR_console_io(int cmd, int count, char *str)
        return _hypercall3(int, console_io, cmd, count, str);
 }
 
+extern int __must_check xen_HYPERVISOR_physdev_op_compat(int, void *);
+
 static inline int
 HYPERVISOR_physdev_op(int cmd, void *arg)
 {
        int rc = _hypercall2(int, physdev_op, cmd, arg);
-       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 = xen_HYPERVISOR_physdev_op_compat(cmd, arg);
        return rc;
 }
 
diff --git a/drivers/xen/Makefile b/drivers/xen/Makefile
index 0e863703..46de6cd 100644
--- a/drivers/xen/Makefile
+++ b/drivers/xen/Makefile
@@ -2,7 +2,7 @@ ifneq ($(CONFIG_ARM),y)
 obj-y  += manage.o balloon.o
 obj-$(CONFIG_HOTPLUG_CPU)              += cpu_hotplug.o
 endif
-obj-y  += grant-table.o features.o events.o
+obj-y  += grant-table.o features.o events.o fallback.o
 obj-y  += xenbus/
 
 nostackp := $(call cc-option, -fno-stack-protector)
diff --git a/drivers/xen/fallback.c b/drivers/xen/fallback.c
new file mode 100644
index 0000000..b29ce32
--- /dev/null
+++ b/drivers/xen/fallback.c
@@ -0,0 +1,81 @@
+#include <linux/kernel.h>
+#include <linux/string.h>
+#include <linux/bug.h>
+#include <linux/export.h>
+#include <asm/hypervisor.h>
+#include <asm/xen/hypercall.h>
+
+int xen_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;
+}
+EXPORT_SYMBOL_GPL(xen_HYPERVISOR_event_channel_op_compat);
+
+int xen_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;
+}
+EXPORT_SYMBOL_GPL(xen_HYPERVISOR_physdev_op_compat);
-- 
1.7.11.7


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