vfs: plumb i_version handling into struct kstat

The NFS server has a lot of special handling for different types of
change attribute access, depending on the underlying filesystem. In
most cases, it's doing a getattr anyway and then fetching that value
after the fact.

Rather that do that, add a new STATX_CHANGE_COOKIE flag that is a
kernel-only symbol (for now). If requested and getattr can implement it,
it can fill out this field. For IS_I_VERSION inodes, add a generic
implementation in vfs_getattr_nosec. Take care to mask
STATX_CHANGE_COOKIE off in requests from userland and in the result
mask.

Since not all filesystems can give the same guarantees of monotonicity,
claim a STATX_ATTR_CHANGE_MONOTONIC flag that filesystems can set to
indicate that they offer an i_version value that can never go backward.

Eventually if we decide to make the i_version available to userland, we
can just designate a field for it in struct statx, and move the
STATX_CHANGE_COOKIE definition to the uapi header.
Reviewed-by: NeilBrown <neilb@suse.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jeff Layton <jlayton@kernel.org>

fs/stat.c

@@ -18,6 +18,7 @@
 #include <linux/syscalls.h>
 #include <linux/pagemap.h>
 #include <linux/compat.h>
+#include <linux/iversion.h>
 
 #include <linux/uaccess.h>
 #include <asm/unistd.h>
@@ -122,6 +123,11 @@ int vfs_getattr_nosec(const struct path *path, struct kstat *stat,
 	stat->attributes_mask |= (STATX_ATTR_AUTOMOUNT |
 				  STATX_ATTR_DAX);
 
+	if ((request_mask & STATX_CHANGE_COOKIE) && IS_I_VERSION(inode)) {
+		stat->result_mask |= STATX_CHANGE_COOKIE;
+		stat->change_cookie = inode_query_iversion(inode);
+	}
+
 	mnt_userns = mnt_user_ns(path->mnt);
 	if (inode->i_op->getattr)
 		return inode->i_op->getattr(mnt_userns, path, stat,
@@ -602,9 +608,11 @@ cp_statx(const struct kstat *stat, struct statx __user *buffer)
 
 	memset(&tmp, 0, sizeof(tmp));
 
-	tmp.stx_mask = stat->result_mask;
+	/* STATX_CHANGE_COOKIE is kernel-only for now */
+	tmp.stx_mask = stat->result_mask & ~STATX_CHANGE_COOKIE;
 	tmp.stx_blksize = stat->blksize;
-	tmp.stx_attributes = stat->attributes;
+	/* STATX_ATTR_CHANGE_MONOTONIC is kernel-only for now */
+	tmp.stx_attributes = stat->attributes & ~STATX_ATTR_CHANGE_MONOTONIC;
 	tmp.stx_nlink = stat->nlink;
 	tmp.stx_uid = from_kuid_munged(current_user_ns(), stat->uid);
 	tmp.stx_gid = from_kgid_munged(current_user_ns(), stat->gid);
@@ -643,6 +651,11 @@ int do_statx(int dfd, struct filename *filename, unsigned int flags,
 	if ((flags & AT_STATX_SYNC_TYPE) == AT_STATX_SYNC_TYPE)
 		return -EINVAL;
 
+	/* STATX_CHANGE_COOKIE is kernel-only for now. Ignore requests
+	 * from userland.
+	 */
+	mask &= ~STATX_CHANGE_COOKIE;
+
 	error = vfs_statx(dfd, filename, flags, &stat, mask);
 	if (error)
 		return error;

include/linux/stat.h

@@ -52,6 +52,15 @@ struct kstat {
 	u64		mnt_id;
 	u32		dio_mem_align;
 	u32		dio_offset_align;
+	u64		change_cookie;
 };
 
+/* These definitions are internal to the kernel for now. Mainly used by nfsd. */
+
+/* mask values */
+#define STATX_CHANGE_COOKIE		0x40000000U	/* Want/got stx_change_attr */
+
+/* file attribute values */
+#define STATX_ATTR_CHANGE_MONOTONIC	0x8000000000000000ULL /* version monotonically increases */
+
 #endif