[PATCH] xen/gnttab: fix gnttab_acquire_resource()

[Juergen Gross <jgross@xxxxxxxx>]

Commit 9dc46386d89d ("gnttab: work around "may be used uninitialized"
warning") was wrong, as vaddr 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.

Basically revert said commit, but keep returning -ENODATA in that case.

Fixes: 9dc46386d89d ("gnttab: work around "may be used uninitialized" warning")
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
Might be considered for 4.17 and for backporting
---
 xen/common/grant_table.c | 14 +++-----------
 1 file changed, 3 insertions(+), 11 deletions(-)

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index ad773a6996..68e7f1df38 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -4125,7 +4125,10 @@ int gnttab_acquire_resource(
 
     case XENMEM_resource_grant_table_id_status:
         if ( gt->gt_version != 2 )
+        {
+            rc = -ENODATA;
             break;
+        }
 
         /* Check that void ** is a suitable representation for gt->status. */
         BUILD_BUG_ON(!__builtin_types_compatible_p(
@@ -4135,17 +4138,6 @@ int gnttab_acquire_resource(
         break;
     }
 
-    /*
-     * Some older toolchains can't spot that vaddrs won't remain uninitialized
-     * on non-error paths, and hence it needs setting to NULL at the top of the
-     * function.  Leave some runtime safety.
-     */
-    if ( !vaddrs )
-    {
-        ASSERT_UNREACHABLE();
-        rc = -ENODATA;
-    }
-
     /* Any errors?  Bad id, or from growing the table? */
     if ( rc )
         goto out;
-- 
2.35.3