[PATCH v3 06/10] exportfs: allow VFS flags in struct file_handle

[Thomas Bertschinger <tahbertschinger@gmail.com>]

The handle_type field of struct file_handle is already being used to
pass "user" flags to open_by_handle_at() in the upper 16 bits.

Bits 8..15 are still unused, as FS implementations are expected to only
set the lower 8 bits.

This change prepares the VFS to pass flags to FS implementations of
fh_to_{dentry,parent}() using the previously unused bits 8..15 of
handle_type.

The user is prevented from setting VFS flags in a file handle--such a
handle will be rejected by open_by_handle_at(2). Only the VFS can set
those flags before passing the handle to the FS.

Suggested-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Thomas Bertschinger <tahbertschinger@gmail.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/exportfs/expfs.c      |  2 +-
 fs/fhandle.c             |  2 +-
 include/linux/exportfs.h | 29 ++++++++++++++++++++++++++---
 3 files changed, 28 insertions(+), 5 deletions(-)

diff --git a/fs/exportfs/expfs.c b/fs/exportfs/expfs.c
index d3e55de4a2a2..949ce6ef6c4e 100644
--- a/fs/exportfs/expfs.c
+++ b/fs/exportfs/expfs.c
@@ -391,7 +391,7 @@ int exportfs_encode_inode_fh(struct inode *inode, struct fid *fid,
 	else
 		type = nop->encode_fh(inode, fid->raw, max_len, parent);
 
-	if (type > 0 && FILEID_USER_FLAGS(type)) {
+	if (type > 0 && (type & ~FILEID_HANDLE_TYPE_MASK)) {
 		pr_warn_once("%s: unexpected fh type value 0x%x from fstype %s.\n",
 			     __func__, type, inode->i_sb->s_type->name);
 		return -EINVAL;
diff --git a/fs/fhandle.c b/fs/fhandle.c
index 7cc17e03e632..2dc669aeb520 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -342,7 +342,7 @@ struct file_handle *get_user_handle(struct file_handle __user *ufh)
 	    (f_handle.handle_bytes == 0))
 		return ERR_PTR(-EINVAL);
 
-	if (f_handle.handle_type < 0 ||
+	if (f_handle.handle_type < 0 || FILEID_FS_FLAGS(f_handle.handle_type) ||
 	    FILEID_USER_FLAGS(f_handle.handle_type) & ~FILEID_VALID_USER_FLAGS)
 		return ERR_PTR(-EINVAL);
 
diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
index cfb0dd1ea49c..30a9791d88e0 100644
--- a/include/linux/exportfs.h
+++ b/include/linux/exportfs.h
@@ -173,10 +173,33 @@ struct handle_to_path_ctx {
 #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().
+ * The 32 bits of the handle_type field of struct file_handle are used for a few
+ * different purposes:
+ *
+ *   Filesystems use only lower 8 bits of file_handle type for fid_type.
+ *
+ *   VFS uses bits 8..15 of the handle_type to pass flags to the FS
+ *   implementation of fh_to_{dentry,parent}().
+ *
+ *   name_to_handle_at() uses upper 16 bits of type as user flags to be
+ *   interpreted by open_by_handle_at().
+ *
+ *  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ *  |           user flags          |   VFS flags   |   fid_type    |
+ *  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ *  (MSB)                                                       (LSB)
+ *
+ * Filesystems are expected not to fill in any bits outside of fid_type in
+ * their encode_fh() implementation.
  */
+#define FILEID_HANDLE_TYPE_MASK	0xff
+#define FILEID_TYPE(type)	((type) & FILEID_HANDLE_TYPE_MASK)
+
+/* VFS flags: */
+#define FILEID_FS_FLAGS_MASK	0xff00
+#define FILEID_FS_FLAGS(flags)	((flags) & FILEID_FS_FLAGS_MASK)
+
+/* User flags: */
 #define FILEID_USER_FLAGS_MASK	0xffff0000
 #define FILEID_USER_FLAGS(type) ((type) & FILEID_USER_FLAGS_MASK)
 
-- 
2.51.0