tools/libxc: fix error handling in xc_mem_paging_load

xc_mem_paging_load() does not pass errors in errno and the actual
errno from xc_mem_event_control() is overwritten by munlock().
xenpaging_populate_page() needs to check errno, but with the switch to
xc_mem_paging_load() it could not receive ENOMEM anymore.

Update xc_mem_paging_load() to return -1 and preserve errno during
munlock().

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Committed-by: Keir Fraser <keir@xen.org>

tools/libxc/xc_mem_paging.c

@@ -68,23 +68,28 @@ int xc_mem_paging_prep(xc_interface *xch, domid_t domain_id, unsigned long gfn)
 int xc_mem_paging_load(xc_interface *xch, domid_t domain_id, 
                                 unsigned long gfn, void *buffer)
 {
-    int rc;
+    int rc, old_errno;
+
+    errno = -EINVAL;
 
     if ( !buffer )
-        return -EINVAL;
+        return -1;
 
     if ( ((unsigned long) buffer) & (XC_PAGE_SIZE - 1) )
-        return -EINVAL;
+        return -1;
 
     if ( mlock(buffer, XC_PAGE_SIZE) )
-        return -errno;
+        return -1;
         
     rc = xc_mem_event_control(xch, domain_id,
                                 XEN_DOMCTL_MEM_EVENT_OP_PAGING_PREP,
                                 XEN_DOMCTL_MEM_EVENT_OP_PAGING,
                                 buffer, NULL, gfn);
 
-    (void)munlock(buffer, XC_PAGE_SIZE);
+    old_errno = errno;
+    munlock(buffer, XC_PAGE_SIZE);
+    errno = old_errno;
+
     return rc;
 }