[PATCH v3] xen/gnttab: fix gnttab_acquire_resource()

[Juergen Gross <jgross@xxxxxxxx>]

Commit 9dc46386d89d ("gnttab: work around "may be used uninitialized"
warning") was wrong, as vaddrs can legitimately be NULL in case
XENMEM_resource_grant_table_id_status was specified for a grant table
v1. This would result in crashes in debug builds due to
ASSERT_UNREACHABLE() triggering.

Check vaddrs only to be NULL in the rc == 0 case.

Expand the tests in tools/tests/resource to verify that using
XENMEM_resource_grant_table_id_status on a V1 grant table will result
in EINVAL.

Fixes: 9dc46386d89d ("gnttab: work around "may be used uninitialized" warning")
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx> # xen
Release-acked-by: Henry Wang <Henry.Wang@xxxxxxx>
---
V2:
- rework (Jan Beulich, Julien Grall)
V3:
- added test support (Andrew Cooper)
---
 tools/tests/resource/test-resource.c | 11 +++++++++++
 xen/common/grant_table.c             |  2 +-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/tools/tests/resource/test-resource.c 
b/tools/tests/resource/test-resource.c
index 189353ebcb..71a81f207e 100644
--- a/tools/tests/resource/test-resource.c
+++ b/tools/tests/resource/test-resource.c
@@ -106,6 +106,17 @@ static void test_gnttab(uint32_t domid, unsigned int 
nr_frames,
     if ( rc )
         return fail("    Fail: Unmap grant table %d - %s\n",
                     errno, strerror(errno));
+
+    /* Verify that the attempt to map the status frames is failing for V1. */
+    res = xenforeignmemory_map_resource(
+        fh, domid, XENMEM_resource_grant_table,
+        XENMEM_resource_grant_table_id_status, 0, 1,
+        (void **)&gnttab, PROT_READ | PROT_WRITE, 0);
+    if ( res || errno != EINVAL )
+        fail("    Fail: Map status not failing with EINVAL %d - %s\n",
+             res ? 0 : errno, res ? "no error" : strerror(errno));
+    if ( res )
+        xenforeignmemory_unmap_resource(fh, res);
 }
 
 static void test_domain_configurations(void)
diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index ad773a6996..fba329dcc2 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -4140,7 +4140,7 @@ int gnttab_acquire_resource(
      * on non-error paths, and hence it needs setting to NULL at the top of the
      * function.  Leave some runtime safety.
      */
-    if ( !vaddrs )
+    if ( !rc && !vaddrs )
     {
         ASSERT_UNREACHABLE();
         rc = -ENODATA;
-- 
2.35.3