xen/public: fix flexible array definitions

Flexible arrays in public headers can be problematic with some
compilers.

With XEN_FLEX_ARRAY_DIM there is a mechanism available to deal with
this issue, but care must be taken to not change the affected structs
in an incompatible way.

So bump __XEN_LATEST_INTERFACE_VERSION__ and introduce a new macro
XENPV_FLEX_ARRAY_DIM which will be XENPV_FLEX_ARRAY_DIM with the
interface version being new enough and "1" (the value used today in
the affected headers) when the interface version is an old one.

Replace the arr[1] instances (this includes the ones seen to be
problematic in recent Linux kernels [1]) with arr[XENPV_FLEX_ARRAY_DIM]
in order to avoid compilation errors.

[1]: https://bugzilla.kernel.org/show_bug.cgi?id=217693

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Henry Wang <Henry.Wang@arm.com> # CHANGELOG
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>

CHANGELOG.md

@@ -7,6 +7,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 ## [4.19.0 UNRELEASED](https://xenbits.xenproject.org/gitweb/?p=xen.git;a=shortlog;h=staging) - TBD
 
 ### Changed
+ - Changed flexible array definitions in public I/O interface headers to not
+   use "1" as the number of array elements.
 
 ### Added
  - On x86:

xen/include/public/io/cameraif.h

@@ -763,7 +763,7 @@ struct xencamera_buf_create_req {
  */
 struct xencamera_page_directory {
     grant_ref_t gref_dir_next_page;
-    grant_ref_t gref[1]; /* Variable length */
+    grant_ref_t gref[XENPV_FLEX_ARRAY_DIM];
 };
 
 /*

xen/include/public/io/displif.h

@@ -537,7 +537,7 @@ struct xendispl_dbuf_create_req {
 
 struct xendispl_page_directory {
     grant_ref_t gref_dir_next_page;
-    grant_ref_t gref[1]; /* Variable length */
+    grant_ref_t gref[XENPV_FLEX_ARRAY_DIM];
 };
 
 /*

xen/include/public/io/fsif.h

@@ -40,7 +40,7 @@ struct fsif_read_request {
     int32_t pad;
     uint64_t len;
     uint64_t offset;
-    grant_ref_t grefs[1];  /* Variable length */
+    grant_ref_t grefs[XENPV_FLEX_ARRAY_DIM];
 };
 
 struct fsif_write_request {
@@ -48,7 +48,7 @@ struct fsif_write_request {
     int32_t pad;
     uint64_t len;
     uint64_t offset;
-    grant_ref_t grefs[1];  /* Variable length */
+    grant_ref_t grefs[XENPV_FLEX_ARRAY_DIM];
 };
 
 struct fsif_stat_request {

xen/include/public/io/pvcalls.h

@@ -30,7 +30,7 @@ struct pvcalls_data_intf {
     uint8_t pad2[52];
 
     RING_IDX ring_order;
-    grant_ref_t ref[];
+    grant_ref_t ref[XEN_FLEX_ARRAY_DIM];
 };
 DEFINE_XEN_FLEX_RING(pvcalls);
 

xen/include/public/io/ring.h

@@ -25,8 +25,16 @@
  * and grant_table.h from the Xen public headers.
  */
 
+#include "../xen.h"
 #include "../xen-compat.h"
 
+/* Some PV I/O interfaces need a compatibility variant. */
+#if __XEN_INTERFACE_VERSION__ < 0x00041300
+#define XENPV_FLEX_ARRAY_DIM  1 /* variable size */
+#else
+#define XENPV_FLEX_ARRAY_DIM  XEN_FLEX_ARRAY_DIM
+#endif
+
 #if __XEN_INTERFACE_VERSION__ < 0x00030208
 #define xen_mb()  mb()
 #define xen_rmb() rmb()
@@ -110,7 +118,7 @@ struct __name##_sring {                                                 \
         uint8_t pvt_pad[4];                                             \
     } pvt;                                                              \
     uint8_t __pad[44];                                                  \
-    union __name##_sring_entry ring[1]; /* variable-length */           \
+    union __name##_sring_entry ring[XENPV_FLEX_ARRAY_DIM];              \
 };                                                                      \
                                                                         \
 /* "Front" end's private variables */                                   \
@@ -479,7 +487,7 @@ struct name##_data_intf {                                                     \
     uint8_t pad2[56];                                                         \
                                                                               \
     RING_IDX ring_order;                                                      \
-    grant_ref_t ref[];                                                        \
+    grant_ref_t ref[XEN_FLEX_ARRAY_DIM];                                      \
 };                                                                            \
 DEFINE_XEN_FLEX_RING(name)
 

xen/include/public/io/sndif.h

@@ -659,7 +659,7 @@ struct xensnd_open_req {
 
 struct xensnd_page_directory {
     grant_ref_t gref_dir_next_page;
-    grant_ref_t gref[1]; /* Variable length */
+    grant_ref_t gref[XENPV_FLEX_ARRAY_DIM];
 };
 
 /*

xen/include/public/xen-compat.h

@@ -10,7 +10,7 @@
 #ifndef __XEN_PUBLIC_XEN_COMPAT_H__
 #define __XEN_PUBLIC_XEN_COMPAT_H__
 
-#define __XEN_LATEST_INTERFACE_VERSION__ 0x00040e00
+#define __XEN_LATEST_INTERFACE_VERSION__ 0x00041300
 
 #if defined(__XEN__) || defined(__XEN_TOOLS__)
 /* Xen is built with matching headers and implements the latest interface. */