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

[Amir Goldstein <amir73il@gmail.com>]

On Wed, Sep 10, 2025 at 11:47 PM Thomas Bertschinger
<tahbertschinger@gmail.com> wrote:
>
> 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 01fc209853d8..276c16454eb7 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
>