On 2019-09-07, Jeff Layton [off-list ref] wrote:

On Thu, 2019-09-05 at 06:19 +1000, Aleksa Sarai wrote:

quoted

+ * @flags: O_* flags.
+ * @mode: O_CREAT/O_TMPFILE file mode.
+ * @upgrade_mask: UPGRADE_* flags (to restrict O_PATH re-opening).
+ * @resolve: RESOLVE_* flags.
+ */
+struct open_how {
+	__u32 flags;
+	union {
+		__u16 mode;
+		__u16 upgrade_mask;
+	};
+	__u16 resolve;
+};
+
+#define OPEN_HOW_SIZE_VER0	8 /* sizeof first published struct */
+

Hmm, there is no version field. When you want to expand this in the
future, what is the plan? Add a new flag to indicate that it's some
length?

The "version number" is the size of the struct. Any extensions we make
are appended to the struct (openat2 now takes a size_t argument), and
the new copy_struct_{to,from}_user() helpers handle all of the
permutations of {old,new} kernel and {old,new} user space.

This is how clone3(), sched_[gs]etattr() and perf_event_open() all
operate (all of the sigset syscalls operate similarly but don't
gracefully handle different kernel vintages -- you just get -EINVAL).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>