usb: ch9: Replace 1-element array with flexible array

Since commit df8fc4e9 ("kbuild: Enable -fstrict-flex-arrays=3"),
UBSAN_BOUNDS no longer pretends 1-element arrays are unbounded. Walking
wData will trigger a warning, so make it a proper flexible array. Add a
union to keep the struct size identical for userspace in case anything
was depending on the old size.
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202306102333.8f5a7443-oliver.sang@intel.com
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: kernel test robot <lkp@intel.com>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: "Jó Ágila Bitsch" <jgilab@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Message-ID: <20230614181307.gonna.256-kees@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/uapi/linux/usb/ch9.h

@@ -376,7 +376,10 @@ struct usb_string_descriptor {
 	__u8  bLength;
 	__u8  bDescriptorType;
 
-	__le16 wData[1];		/* UTF-16LE encoded */
+	union {
+		__le16 legacy_padding;
+		__DECLARE_FLEX_ARRAY(__le16, wData);	/* UTF-16LE encoded */
+	};
 } __attribute__ ((packed));
 
 /* note that "string" zero is special, it holds language codes that