fs-verity: add UAPI header

Add the UAPI header for fs-verity, including two ioctls:

- FS_IOC_ENABLE_VERITY
- FS_IOC_MEASURE_VERITY

These ioctls are documented in the "User API" section of
Documentation/filesystems/fsverity.rst.

Examples of using these ioctls can be found in fsverity-utils
(https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/fsverity-utils.git).

I've also written xfstests that test these ioctls
(https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/xfstests-dev.git/log/?h=fsverity).
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>

Documentation/ioctl/ioctl-number.rst

@@ -233,6 +233,7 @@ Code  Seq#    Include File                                           Comments
 'f'   00-0F  fs/ext4/ext4.h                                          conflict!
 'f'   00-0F  linux/fs.h                                              conflict!
 'f'   00-0F  fs/ocfs2/ocfs2_fs.h                                     conflict!
+'f'   81-8F  linux/fsverity.h
 'g'   00-0F  linux/usb/gadgetfs.h
 'g'   20-2F  linux/usb/g_printer.h
 'h'   00-7F                                                          conflict! Charon filesystem

include/uapi/linux/fsverity.h

+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+/*
+ * fs-verity user API
+ *
+ * These ioctls can be used on filesystems that support fs-verity.  See the
+ * "User API" section of Documentation/filesystems/fsverity.rst.
+ *
+ * Copyright 2019 Google LLC
+ */
+#ifndef _UAPI_LINUX_FSVERITY_H
+#define _UAPI_LINUX_FSVERITY_H
+
+#include <linux/ioctl.h>
+#include <linux/types.h>
+
+#define FS_VERITY_HASH_ALG_SHA256	1
+
+struct fsverity_enable_arg {
+	__u32 version;
+	__u32 hash_algorithm;
+	__u32 block_size;
+	__u32 salt_size;
+	__u64 salt_ptr;
+	__u32 sig_size;
+	__u32 __reserved1;
+	__u64 sig_ptr;
+	__u64 __reserved2[11];
+};
+
+struct fsverity_digest {
+	__u16 digest_algorithm;
+	__u16 digest_size; /* input/output */
+	__u8 digest[];
+};
+
+#define FS_IOC_ENABLE_VERITY	_IOW('f', 133, struct fsverity_enable_arg)
+#define FS_IOC_MEASURE_VERITY	_IOWR('f', 134, struct fsverity_digest)
+
+#endif /* _UAPI_LINUX_FSVERITY_H */