[PATCH V2] xen/memory: Make resource_max_frames() to return 0 on unknown type

[Oleksandr Tyshchenko <olekstysh@xxxxxxxxx>]

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>

This is actually what the caller acquire_resource() expects on any kind
of error (the comment on top of resource_max_frames() also suggests that).
Otherwise, the caller will treat -errno as a valid value and propagate incorrect
nr_frames to the VM. As a possible consequence, a VM trying to query a resource
size of an unknown type will get the success result from the hypercall and 
obtain
nr_frames 4294967201.

Also, add an ASSERT_UNREACHABLE() in the default case of _acquire_resource(),
normally we won't get to this point, as an unknown type will always be rejected
earlier in resource_max_frames().

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
  V2:
   - add R-b
   - add ASSERT_UNREACHABLE() in the default case of _acquire_resource()
     and update commit desc regarding that
   - drop post-commit remark
---
---
 xen/common/memory.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/common/memory.c b/xen/common/memory.c
index a6f2f6d1b3..8ca4e1a842 100644
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -1157,7 +1157,7 @@ static unsigned int resource_max_frames(const struct 
domain *d,
         return d->vmtrace_size >> PAGE_SHIFT;
 
     default:
-        return -EOPNOTSUPP;
+        return 0;
     }
 }
 
@@ -1240,6 +1240,7 @@ static int _acquire_resource(
         return acquire_vmtrace_buf(d, id, frame, nr_frames, mfn_list);
 
     default:
+        ASSERT_UNREACHABLE();
         return -EOPNOTSUPP;
     }
 }
-- 
2.34.1