hw/acpi: Use QEMU_NONSTRING for non NUL-terminated arrays

GCC 8 added a -Wstringop-truncation warning:

  The -Wstringop-truncation warning added in GCC 8.0 via r254630 for
  bug 81117 is specifically intended to highlight likely unintended
  uses of the strncpy function that truncate the terminating NUL
  character from the source string.

This new warning leads to compilation failures:

    CC      hw/acpi/core.o
  In function 'acpi_table_install', inlined from 'acpi_table_add' at qemu/hw/acpi/core.c:296:5:
  qemu/hw/acpi/core.c:184:9: error: 'strncpy' specified bound 4 equals destination size [-Werror=stringop-truncation]
           strncpy(ext_hdr->sig, hdrs->sig, sizeof ext_hdr->sig);
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  make: *** [qemu/rules.mak:69: hw/acpi/core.o] Error 1

Use the QEMU_NONSTRING attribute, since ACPI tables don't require the
strings to be NUL-terminated.

Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>

hw/acpi/core.c

@@ -35,14 +35,18 @@
 struct acpi_table_header {
     uint16_t _length;         /* our length, not actual part of the hdr */
                               /* allows easier parsing for fw_cfg clients */
-    char sig[4];              /* ACPI signature (4 ASCII characters) */
+    char sig[4]
+             QEMU_NONSTRING;  /* ACPI signature (4 ASCII characters) */
     uint32_t length;          /* Length of table, in bytes, including header */
     uint8_t revision;         /* ACPI Specification minor version # */
     uint8_t checksum;         /* To make sum of entire table == 0 */
-    char oem_id[6];           /* OEM identification */
-    char oem_table_id[8];     /* OEM table identification */
+    char oem_id[6]
+             QEMU_NONSTRING;  /* OEM identification */
+    char oem_table_id[8]
+             QEMU_NONSTRING;  /* OEM table identification */
     uint32_t oem_revision;    /* OEM revision number */
-    char asl_compiler_id[4];  /* ASL compiler vendor ID */
+    char asl_compiler_id[4]
+             QEMU_NONSTRING;  /* ASL compiler vendor ID */
     uint32_t asl_compiler_revision; /* ASL compiler revision number */
 } QEMU_PACKED;
 

include/hw/acpi/acpi-defs.h

@@ -41,8 +41,8 @@ enum {
 };
 
 typedef struct AcpiRsdpData {
-    uint8_t oem_id[6]; /* OEM identification */
-    uint8_t revision;  /* Must be 0 for 1.0, 2 for 2.0 */
+    uint8_t oem_id[6] QEMU_NONSTRING; /* OEM identification */
+    uint8_t revision;                 /* Must be 0 for 1.0, 2 for 2.0 */
 
     unsigned *rsdt_tbl_offset;
     unsigned *xsdt_tbl_offset;
@@ -57,10 +57,13 @@ typedef struct AcpiRsdpData {
     uint32_t length;                 /* Length of table, in bytes, including header */ \
     uint8_t  revision;               /* ACPI Specification minor version # */ \
     uint8_t  checksum;               /* To make sum of entire table == 0 */ \
-    uint8_t  oem_id [6];             /* OEM identification */ \
-    uint8_t  oem_table_id [8];       /* OEM table identification */ \
+    uint8_t  oem_id[6] \
+                 QEMU_NONSTRING;     /* OEM identification */ \
+    uint8_t  oem_table_id[8] \
+                 QEMU_NONSTRING;     /* OEM table identification */ \
     uint32_t oem_revision;           /* OEM revision number */ \
-    uint8_t  asl_compiler_id [4];    /* ASL compiler vendor ID */ \
+    uint8_t  asl_compiler_id[4] \
+                 QEMU_NONSTRING;     /* ASL compiler vendor ID */ \
     uint32_t asl_compiler_revision;  /* ASL compiler revision number */