Re: [PATCH V10 2/5] fuse: Passthrough initialization and release

[Peng Tao]

On Tue, Oct 27, 2020 at 12:19 AM Alessio Balsini <balsini@xxxxxxxxxxx> wrote:
>
> Implement the FUSE passthrough ioctl() that associates the lower
> (passthrough) file system file with the fuse_file.
>
> The file descriptor passed to the ioctl() by the FUSE daemon is used to
> access the relative file pointer, that will be copied to the fuse_file data
> structure to consolidate the link between the FUSE and lower file system.
>
> To enable the passthrough mode, userspace triggers the
> FUSE_DEV_IOC_PASSTHROUGH_OPEN ioctl() and, if the call succeeds,
> receives back an identifier that will be used at open/create response
> time in the fuse_open_out field to associate the FUSE file to the lower
> file system file.
> The value returned by the ioctl() to userspace can be:
> - > 0: success, the identifier can be used as part of an open/create
>   reply.
> - < 0: an error occurred.
> The value 0 has been left unused for backward compatibility: the
> fuse_open_out field that is used to pass the passthrough_fh back to the
> kernel uses the same bits that were previously as struct padding,
> zero-initialized in the common libfuse implementation. Removing the 0
> value fixes the ambiguity between the case in which 0 corresponds to a
> real passthrough_fh or a missing implementation, simplifying the
> userspace implementation.
>
> For the passthrough mode to be successfully activated, the lower file
> system file must implement both read_ and write_iter file operations.
> This extra check avoids special pseudo files to be targeted for this
> feature.
> Passthrough comes with another limitation: no further file system stacking
> is allowed for those FUSE file systems using passthrough.
>
> Signed-off-by: Alessio Balsini <balsini@xxxxxxxxxxx>
> ---
>  fs/fuse/inode.c       |  5 +++
>  fs/fuse/passthrough.c | 80 +++++++++++++++++++++++++++++++++++++++++--
>  2 files changed, 83 insertions(+), 2 deletions(-)
>
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index 6738dd5ff5d2..1e94c54d1455 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -1034,6 +1034,11 @@ EXPORT_SYMBOL_GPL(fuse_send_init);
>
>  static int free_fuse_passthrough(int id, void *p, void *data)
>  {
> +       struct fuse_passthrough *passthrough = (struct fuse_passthrough *)p;
> +
> +       fuse_passthrough_release(passthrough);
> +       kfree(p);
> +
>         return 0;
>  }
>
> diff --git a/fs/fuse/passthrough.c b/fs/fuse/passthrough.c
> index 594060c654f8..a135c955cc33 100644
> --- a/fs/fuse/passthrough.c
> +++ b/fs/fuse/passthrough.c
> @@ -3,19 +3,95 @@
>  #include "fuse_i.h"
>
>  #include <linux/fuse.h>
> +#include <linux/idr.h>
>
>  int fuse_passthrough_open(struct fuse_dev *fud,
>                           struct fuse_passthrough_out *pto)
>  {
> -       return -EINVAL;
> +       int res;
> +       struct file *passthrough_filp;
> +       struct fuse_conn *fc = fud->fc;
> +       struct fuse_passthrough *passthrough;
> +
> +       if (!fc->passthrough)
> +               return -EPERM;
> +
> +       /* This field is reserved for future implementation */
> +       if (pto->len != 0)
> +               return -EINVAL;
> +
> +       passthrough_filp = fget(pto->fd);
> +       if (!passthrough_filp) {
> +               pr_err("FUSE: invalid file descriptor for passthrough.\n");
> +               return -EBADF;
> +       }
> +
> +       if (!passthrough_filp->f_op->read_iter ||
> +           !passthrough_filp->f_op->write_iter) {
> +               pr_err("FUSE: passthrough file misses file operations.\n");
> +               return -EBADF;
> +       }
> +
> +       passthrough = kmalloc(sizeof(struct fuse_passthrough), GFP_KERNEL);
> +       if (!passthrough)
> +               return -ENOMEM;
> +
> +       passthrough->filp = passthrough_filp;
> +
> +       idr_preload(GFP_KERNEL);
> +       spin_lock(&fc->passthrough_req_lock);
> +       res = idr_alloc(&fc->passthrough_req, passthrough, 1, 0, GFP_ATOMIC);
> +       spin_unlock(&fc->passthrough_req_lock);
> +       idr_preload_end();
> +       if (res <= 0) {
> +               fuse_passthrough_release(passthrough);
> +               kfree(passthrough);
> +       }
> +
> +       return res;
>  }
>
>  int fuse_passthrough_setup(struct fuse_conn *fc, struct fuse_file *ff,
>                            struct fuse_open_out *openarg)
>  {
> -       return -EINVAL;
> +       struct inode *passthrough_inode;
> +       struct super_block *passthrough_sb;
> +       struct fuse_passthrough *passthrough;
> +       int passthrough_fh = openarg->passthrough_fh;
> +
> +       if (!fc->passthrough)
> +               return -EPERM;
> +
> +       /* Default case, passthrough is not requested */
> +       if (passthrough_fh <= 0)
> +               return -EINVAL;
> +
> +       spin_lock(&fc->passthrough_req_lock);
> +       passthrough = idr_remove(&fc->passthrough_req, passthrough_fh);
> +       spin_unlock(&fc->passthrough_req_lock);
> +
> +       if (!passthrough)
> +               return -EINVAL;
> +
> +       passthrough_inode = file_inode(passthrough->filp);
> +       passthrough_sb = passthrough_inode->i_sb;
> +       if (passthrough_sb->s_stack_depth >= FILESYSTEM_MAX_STACK_DEPTH) {
Hi Alessio,

passthrough_sb is the underlying filesystem superblock, right? It
seems to prevent fuse passthrough fs from stacking on another fully
stacked file system, instead of preventing other file systems from
stacking on this fuse passthrough file system. Am I misunderstanding
it?

Cheers,
Tao
--
Into Sth. Rich & Strange