Date:   Wed, 5 May 2021 15:21:26 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Miklos Szeredi <miklos@...redi.hu>,
        Alessio Balsini <balsini@...roid.com>
Cc:     Akilesh Kailash <akailash@...gle.com>,
        Antonio SJ Musumeci <trapexit@...wn.link>,
        David Anderson <dvander@...gle.com>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Jann Horn <jannh@...gle.com>, Jens Axboe <axboe@...nel.dk>,
        Martijn Coenen <maco@...roid.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Lawrence <paullawrence@...gle.com>,
        Peng Tao <bergwolf@...il.com>,
        Stefano Duo <duostefano93@...il.com>,
        Zimuzo Ezeozue <zezeozue@...gle.com>, wuyan <wu-yan@....com>,
        fuse-devel <fuse-devel@...ts.sourceforge.net>,
        kernel-team <kernel-team@...roid.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND V12 4/8] fuse: Passthrough initialization and release

On Wed, Feb 17, 2021 at 3:52 PM Miklos Szeredi <miklos@...redi.hu> wrote:
>
> On Mon, Jan 25, 2021 at 4:31 PM Alessio Balsini <balsini@...roid.com> 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, user space 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 user space can be:
> > - > 0: success, the identifier can be used as part of an open/create
> > reply.
> > - <= 0: an error occurred.
> > The value 0 represents an error to preserve 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, and is
> > commonly zero-initialized (e.g., in the libfuse implementation).
> > Removing 0 from the correct values fixes the ambiguity between the case
> > in which 0 corresponds to a real passthrough_fh, a missing
> > implementation of FUSE passthrough or a request for a normal FUSE file,
> > simplifying the user space implementation.
> >
> > For the passthrough mode to be successfully activated, the lower file
> > system file must implement both read_iter 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@...roid.com>
> > ---
> >  fs/fuse/inode.c       |  5 +++
> >  fs/fuse/passthrough.c | 87 ++++++++++++++++++++++++++++++++++++++++++-
> >  2 files changed, 90 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> > index a1104d5abb70..7ebc398fbacb 100644
> > --- a/fs/fuse/inode.c
> > +++ b/fs/fuse/inode.c
> > @@ -1133,6 +1133,11 @@ 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..cf993e83803e 100644
> > --- a/fs/fuse/passthrough.c
> > +++ b/fs/fuse/passthrough.c
> > @@ -3,19 +3,102 @@
> >  #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 inode *passthrough_inode;
> > +       struct super_block *passthrough_sb;
> > +       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");
> > +               res = -EBADF;
> > +               goto err_free_file;
> > +       }
> > +
> > +       passthrough_inode = file_inode(passthrough_filp);
> > +       passthrough_sb = passthrough_inode->i_sb;
> > +       if (passthrough_sb->s_stack_depth >= FILESYSTEM_MAX_STACK_DEPTH) {
> > +               pr_err("FUSE: fs stacking depth exceeded for passthrough\n");
>
> No need to print an error to the logs, this can be a perfectly normal
> occurrence.  However I'd try to find a more unique error value than
> EINVAL so that the fuse server can interpret this as "not your fault,
> but can't support passthrough on this file".  E.g. EOPNOTSUPP.
>
>

Sorry for the fashionably late response...
Same comment for !{read,write}_iter case above.
EBAFD is really not appropriate there.
May I suggest ELOOP for s_stack_depth and EOPNOTSUPP
for no rw iter ops.

Are you planning to post another version of the patches soon?

Thanks,
Amir.

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