[PATCH v2 3/6] xen: xenstore: add possibility to preserve owner

[Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>]

Add option to preserve owner when creating an entry in Xen Store. This
may be needed in cases when Qemu is working as device model in a
domain that is Domain-0, e.g. in driver domain.

"owner" parameter for qemu_xen_xs_create() function can have special
value XS_PRESERVE_OWNER, which will make specific implementation to
get original owner of an entry and pass it back to
set_permissions() call.

Please note, that XenStore inherits permissions, so even if entry is
newly created by, it already has the owner set to match owner of entry
at previous level.

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>

--

In v2:

 - Pass transaction to xs_get_permissions() in libxenstore_create()
 - Added comment before XS_PRESERVE_OWNER defintion
 - Extended the commit message
---
 hw/i386/kvm/xen_xenstore.c       | 18 ++++++++++++++++++
 hw/xen/xen-operations.c          | 12 ++++++++++++
 include/hw/xen/xen_backend_ops.h |  7 +++++++
 3 files changed, 37 insertions(+)

diff --git a/hw/i386/kvm/xen_xenstore.c b/hw/i386/kvm/xen_xenstore.c
index 6e651960b3..d0fd5d4681 100644
--- a/hw/i386/kvm/xen_xenstore.c
+++ b/hw/i386/kvm/xen_xenstore.c
@@ -1595,6 +1595,24 @@ static bool xs_be_create(struct qemu_xs_handle *h, 
xs_transaction_t t,
         return false;
     }
 
+    if (owner == XS_PRESERVE_OWNER) {
+        GList *prev_perms;
+        char letter;
+
+        err = xs_impl_get_perms(h->impl, 0, t, path, &prev_perms);
+        if (err) {
+            errno = err;
+            return false;
+        }
+
+        if (sscanf(prev_perms->data, "%c%u", &letter, &owner) != 2) {
+            errno = EFAULT;
+            g_list_free_full(prev_perms, g_free);
+            return false;
+        }
+        g_list_free_full(prev_perms, g_free);
+    }
+
     perms_list = g_list_append(perms_list,
                                xs_perm_as_string(XS_PERM_NONE, owner));
     perms_list = g_list_append(perms_list,
diff --git a/hw/xen/xen-operations.c b/hw/xen/xen-operations.c
index e00983ec44..ae8265635f 100644
--- a/hw/xen/xen-operations.c
+++ b/hw/xen/xen-operations.c
@@ -300,6 +300,18 @@ static bool libxenstore_create(struct qemu_xs_handle *h, 
xs_transaction_t t,
         return false;
     }
 
+    if (owner == XS_PRESERVE_OWNER) {
+        struct xs_permissions *tmp;
+        unsigned int num;
+
+        tmp = xs_get_permissions(h->xsh, t, path, &num);
+        if (tmp == NULL) {
+            return false;
+        }
+        perms_list[0].id = tmp[0].id;
+        free(tmp);
+    }
+
     return xs_set_permissions(h->xsh, t, path, perms_list,
                               ARRAY_SIZE(perms_list));
 }
diff --git a/include/hw/xen/xen_backend_ops.h b/include/hw/xen/xen_backend_ops.h
index 90cca85f52..79021538a3 100644
--- a/include/hw/xen/xen_backend_ops.h
+++ b/include/hw/xen/xen_backend_ops.h
@@ -266,6 +266,13 @@ typedef uint32_t xs_transaction_t;
 #define XS_PERM_READ  0x01
 #define XS_PERM_WRITE 0x02
 
+/*
+ * This is QEMU-specific special value used only by QEMU wrappers
+ * around XenStore. It can be passed to qemu_xen_xs_create() to
+ * inherit owner value from higher-level XS entry.
+ */
+#define XS_PRESERVE_OWNER        0xFFFE
+
 struct xenstore_backend_ops {
     struct qemu_xs_handle *(*open)(void);
     void (*close)(struct qemu_xs_handle *h);
-- 
2.42.0