io_uring: add support for probing opcodes

The application currently has no way of knowing if a given opcode is
supported or not without having to try and issue one and see if we get
-EINVAL or not. And even this approach is fraught with peril, as maybe
we're getting -EINVAL due to some fields being missing, or maybe it's
just not that easy to issue that particular command without doing some
other leg work in terms of setup first.

This adds IORING_REGISTER_PROBE, which fills in a structure with info
on what it supported or not. This will work even with sparse opcode
fields, which may happen in the future or even today if someone
backports specific features to older kernels.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/uapi/linux/io_uring.h

@@ -194,6 +194,7 @@ struct io_uring_params {
 #define IORING_UNREGISTER_EVENTFD	5
 #define IORING_REGISTER_FILES_UPDATE	6
 #define IORING_REGISTER_EVENTFD_ASYNC	7
+#define IORING_REGISTER_PROBE		8
 
 struct io_uring_files_update {
 	__u32 offset;
@@ -201,4 +202,21 @@ struct io_uring_files_update {
 	__aligned_u64 /* __s32 * */ fds;
 };
 
+#define IO_URING_OP_SUPPORTED	(1U << 0)
+
+struct io_uring_probe_op {
+	__u8 op;
+	__u8 resv;
+	__u16 flags;	/* IO_URING_OP_* flags */
+	__u32 resv2;
+};
+
+struct io_uring_probe {
+	__u8 last_op;	/* last opcode supported */
+	__u8 ops_len;	/* length of ops[] array below */
+	__u16 resv;
+	__u32 resv2[3];
+	struct io_uring_probe_op ops[0];
+};
+
 #endif