[PATCH v3 1/3] fs: prepare for "explicit connectable" file handles

[Amir Goldstein <amir73il@xxxxxxxxx>]

We would like to use the high 16bit of the handle_type field to encode
file handle traits, such as "connectable".

In preparation for this change, make sure that filesystems do not return
a handle_type value with upper bits set and that the open_by_handle_at(2)
syscall rejects these handle types.

Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
---
 fs/exportfs/expfs.c      | 14 ++++++++++++--
 fs/fhandle.c             |  6 ++++++
 include/linux/exportfs.h | 14 ++++++++++++++
 3 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/fs/exportfs/expfs.c b/fs/exportfs/expfs.c
index 4f2dd4ab4486..c8eb660fdde4 100644
--- a/fs/exportfs/expfs.c
+++ b/fs/exportfs/expfs.c
@@ -382,14 +382,21 @@ int exportfs_encode_inode_fh(struct inode *inode, struct fid *fid,
 			     int *max_len, struct inode *parent, int flags)
 {
 	const struct export_operations *nop = inode->i_sb->s_export_op;
+	enum fid_type type;
 
 	if (!exportfs_can_encode_fh(nop, flags))
 		return -EOPNOTSUPP;
 
 	if (!nop && (flags & EXPORT_FH_FID))
-		return exportfs_encode_ino64_fid(inode, fid, max_len);
+		type = exportfs_encode_ino64_fid(inode, fid, max_len);
+	else
+		type = nop->encode_fh(inode, fid->raw, max_len, parent);
+
+	if (WARN_ON_ONCE(FILEID_USER_FLAGS(type)))
+		return -EINVAL;
+
+	return type;
 
-	return nop->encode_fh(inode, fid->raw, max_len, parent);
 }
 EXPORT_SYMBOL_GPL(exportfs_encode_inode_fh);
 
@@ -436,6 +443,9 @@ exportfs_decode_fh_raw(struct vfsmount *mnt, struct fid *fid, int fh_len,
 	char nbuf[NAME_MAX+1];
 	int err;
 
+	if (WARN_ON_ONCE(FILEID_USER_FLAGS(fileid_type)))
+		return -EINVAL;
+
 	/*
 	 * Try to get any dentry for the given file handle from the filesystem.
 	 */
diff --git a/fs/fhandle.c b/fs/fhandle.c
index 82df28d45cd7..c5792cf3c6e9 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -307,6 +307,10 @@ static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
 		retval = -EINVAL;
 		goto out_path;
 	}
+	if (!FILEID_USER_TYPE_IS_VALID(f_handle.handle_type)) {
+		retval = -EINVAL;
+		goto out_path;
+	}
 	handle = kmalloc(struct_size(handle, f_handle, f_handle.handle_bytes),
 			 GFP_KERNEL);
 	if (!handle) {
@@ -322,6 +326,8 @@ static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
 		goto out_handle;
 	}
 
+	/* Filesystem code should not be exposed to user flags */
+	handle->handle_type &= ~FILEID_USER_FLAGS_MASK;
 	retval = do_handle_to_path(handle, path, &ctx);
 
 out_handle:
diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
index 893a1d21dc1c..76a3050b3593 100644
--- a/include/linux/exportfs.h
+++ b/include/linux/exportfs.h
@@ -160,6 +160,20 @@ struct fid {
 #define EXPORT_FH_FID		0x2 /* File handle may be non-decodeable */
 #define EXPORT_FH_DIR_ONLY	0x4 /* Only decode file handle for a directory */
 
+/*
+ * Filesystems use only lower 8 bits of file_handle type for fid_type.
+ * name_to_handle_at() uses upper 16 bits of type as user flags to be
+ * interpreted by open_by_handle_at().
+ */
+#define FILEID_USER_FLAGS_MASK	0xffff0000
+#define FILEID_USER_FLAGS(type) ((type) & FILEID_USER_FLAGS_MASK)
+
+/* Flags supported in encoded handle_type that is exported to user */
+#define FILEID_VALID_USER_FLAGS	(0)
+
+#define FILEID_USER_TYPE_IS_VALID(type) \
+	(!(FILEID_USER_FLAGS(type) & ~FILEID_VALID_USER_FLAGS))
+
 /**
  * struct export_operations - for nfsd to communicate with file systems
  * @encode_fh:      encode a file handle fragment from a dentry
-- 
2.34.1