btrfs: send: prepare for v2 protocol

This is preparatory work for send protocol update to version 2 and
higher.

We have many pending protocol update requests but still don't have the
basic protocol rev in place, the first thing that must happen is to do
the actual versioning support.

The protocol version is u32 and is a new member in the send ioctl
struct. Validity of the version field is backed by a new flag bit. Old
kernels would fail when a higher version is requested. Version protocol
0 will pick the highest supported version, BTRFS_SEND_STREAM_VERSION,
that's also exported in sysfs.

The version is still unchanged and will be increased once we have new
incompatible commands or stream updates.

Reviewed-by: Nikolay Borisov <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/ioctl.c

···
 	compat_uptr_t clone_sources;	/* in */
 	__u64 parent_root;		/* in */
 	__u64 flags;			/* in */
-	__u64 reserved[4];		/* in */
+	__u32 version;			/* in */
+	__u8  reserved[28];		/* in */
 } __attribute__ ((__packed__));
 
 #define BTRFS_IOC_SEND_32 _IOW(BTRFS_IOCTL_MAGIC, 38, \

fs/btrfs/send.c

···
 	u64 total_send_size;
 	u64 cmd_send_size[BTRFS_SEND_C_MAX + 1];
 	u64 flags;	/* 'flags' member of btrfs_ioctl_send_args is u64 */
+	/* Protocol version compatibility requested */
+	u32 proto;
 
 	struct btrfs_root *send_root;
 	struct btrfs_root *parent_root;
···
 		  sctx->send_root->root_key.objectid,
 		  (sctx->parent_root ?
 		   sctx->parent_root->root_key.objectid : 0));
+}
+
+__maybe_unused
+static bool proto_cmd_ok(const struct send_ctx *sctx, int cmd)
+{
+	switch (sctx->proto) {
+	case 1:	 return cmd < __BTRFS_SEND_C_MAX_V1;
+	case 2:	 return cmd < __BTRFS_SEND_C_MAX_V2;
+	default: return false;
+	}
 }
 
 static int is_waiting_for_move(struct send_ctx *sctx, u64 ino);
···
 	INIT_LIST_HEAD(&sctx->name_cache_list);
 
 	sctx->flags = arg->flags;
+
+	if (arg->flags & BTRFS_SEND_FLAG_VERSION) {
+		if (arg->version > BTRFS_SEND_STREAM_VERSION) {
+			ret = -EPROTO;
+			goto out;
+		}
+		/* Zero means "use the highest version" */
+		sctx->proto = arg->version ?: BTRFS_SEND_STREAM_VERSION;
+	} else {
+		sctx->proto = 1;
+	}
 
 	sctx->send_filp = fget(arg->send_fd);
 	if (!sctx->send_filp) {

fs/btrfs/send.h

···
 enum btrfs_send_cmd {
 	BTRFS_SEND_C_UNSPEC,
 
+	/* Version 1 */
 	BTRFS_SEND_C_SUBVOL,
 	BTRFS_SEND_C_SNAPSHOT,
 
···
 
 	BTRFS_SEND_C_END,
 	BTRFS_SEND_C_UPDATE_EXTENT,
+	__BTRFS_SEND_C_MAX_V1,
+
+	/* Version 2 */
+	__BTRFS_SEND_C_MAX_V2,
+
+	/* End */
 	__BTRFS_SEND_C_MAX,
 };
 #define BTRFS_SEND_C_MAX (__BTRFS_SEND_C_MAX - 1)

include/uapi/linux/btrfs.h

···
  */
 #define BTRFS_SEND_FLAG_OMIT_END_CMD		0x4
 
+/*
+ * Read the protocol version in the structure
+ */
+#define BTRFS_SEND_FLAG_VERSION			0x8
+
 #define BTRFS_SEND_FLAG_MASK \
 	(BTRFS_SEND_FLAG_NO_FILE_DATA | \
 	 BTRFS_SEND_FLAG_OMIT_STREAM_HEADER | \
-	 BTRFS_SEND_FLAG_OMIT_END_CMD)
+	 BTRFS_SEND_FLAG_OMIT_END_CMD | \
+	 BTRFS_SEND_FLAG_VERSION)
 
 struct btrfs_ioctl_send_args {
 	__s64 send_fd;			/* in */
···
 	__u64 __user *clone_sources;	/* in */
 	__u64 parent_root;		/* in */
 	__u64 flags;			/* in */
-	__u64 reserved[4];		/* in */
+	__u32 version;			/* in */
+	__u8  reserved[28];		/* in */
 };
 
 /*