[PATCH v4 1/3] fs: prepare for "explicit connectable" file handles

Amir Goldstein
2024-10-11T09:00:30Z

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@gmail.com>
---
fs/exportfs/expfs.c | 17 +++++++++++++++--
fs/fhandle.c | 7 +++++++
include/linux/exportfs.h | 11 +++++++++++
3 files changed, 33 insertions(+), 2 deletions(-)

diff --git a/fs/exportfs/expfs.c b/fs/exportfs/expfs.c
index 4f2dd4ab4486..0c899cfba578 100644
--- a/fs/exportfs/expfs.c
+++ b/fs/exportfs/expfs.c
@@ -382,14 +382,24 @@ 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 (type > 0 && FILEID_USER_FLAGS(type)) {
+ pr_warn_once("%s: unexpected fh type value 0x%x from fstype %s.\n",
+ __func__, type, inode->i_sb->s_type->name);
+ return -EINVAL;
+ }
+
+ return type;

- return nop->encode_fh(inode, fid->raw, max_len, parent);
}
EXPORT_SYMBOL_GPL(exportfs_encode_inode_fh);

@@ -436,6 +446,9 @@ exportfs_decode_fh_raw(struct vfsmount *mnt, struct fid *fid, int fh_len,
char nbuf[NAME_MAX+1];
int err;

+ if (fileid_type < 0 || FILEID_USER_FLAGS(fileid_type))
+ return ERR_PTR(-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..218511f38cbb 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -307,6 +307,11 @@ static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
retval = -EINVAL;
goto out_path;
}
+ if (f_handle.handle_type < 0 ||
+ FILEID_USER_FLAGS(f_handle.handle_type) & ~FILEID_VALID_USER_FLAGS) {
+ retval = -EINVAL;
+ goto out_path;
+ }
handle = kmalloc(struct_size(handle, f_handle, f_handle.handle_bytes),
GFP_KERNEL);
if (!handle) {
@@ -322,6 +327,8 @@ 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..5e14d4500a75 100644
--- a/include/linux/exportfs.h
+++ b/include/linux/exportfs.h
@@ -160,6 +160,17 @@ 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)
+
/**
* struct export_operations - for nfsd to communicate with file systems
* @encode_fh: encode a file handle fragment from a dentry
--
2.34.1