x86: make certain memory sub-ops return valid values

When a domain's shared info field "max_pfn" is zero,
domain_get_maximum_gpfn() so far returned ULONG_MAX, which
do_memory_op() in turn converted to -1 (i.e. -EPERM). Make the former
always return a sensible number (i.e. zero if the field was zero) and
have the latter no longer truncate return values.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -433,7 +433,7 @@ unsigned long domain_get_maximum_gpfn(st
     if ( is_hvm_domain(d) )
         return p2m_get_hostp2m(d)->max_mapped_pfn;
     /* NB. PV guests specify nr_pfns rather than max_pfn so we adjust here. */
-    return arch_get_max_pfn(d) - 1;
+    return (arch_get_max_pfn(d) ?: 1) - 1;
 }
 
 void share_xen_page_with_guest(
--- a/xen/common/compat/memory.c
+++ b/xen/common/compat/memory.c
@@ -15,7 +15,8 @@ CHECK_TYPE(domid);
 
 int compat_memory_op(unsigned int cmd, XEN_GUEST_HANDLE_PARAM(void) compat)
 {
-    int rc, split, op = cmd & MEMOP_CMD_MASK;
+    int split, op = cmd & MEMOP_CMD_MASK;
+    long rc;
     unsigned int start_extent = cmd >> MEMOP_EXTENT_SHIFT;
 
     do
@@ -204,7 +205,7 @@ int compat_memory_op(unsigned int cmd, X
 
         rc = do_memory_op(cmd, nat.hnd);
         if ( rc < 0 )
-            return rc;
+            break;
 
         cmd = 0;
         if ( hypercall_xlat_continuation(&cmd, 0x02, nat.hnd, compat) )
@@ -326,5 +327,11 @@ int compat_memory_op(unsigned int cmd, X
                 __HYPERVISOR_memory_op, "ih", cmd, compat);
     } while ( split > 0 );
 
+    if ( unlikely(rc > INT_MAX) )
+        return INT_MAX;
+
+    if ( unlikely(rc < INT_MIN) )
+        return INT_MIN;
+
     return rc;
 }
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -545,14 +545,13 @@ static long memory_exchange(XEN_GUEST_HA
 long do_memory_op(unsigned long cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
 {
     struct domain *d;
-    int rc, op;
+    long rc;
     unsigned int address_bits;
     unsigned long start_extent;
     struct xen_memory_reservation reservation;
     struct memop_args args;
     domid_t domid;
-
-    op = cmd & MEMOP_CMD_MASK;
+    int op = cmd & MEMOP_CMD_MASK;
 
     switch ( op )
     {