Re: [PATCH v5] fuse: Add support for passthrough read/write

[Nikhilesh Reddy]

On 02/01/2016 11:45 AM, Jann Horn wrote:
> On Mon, Feb 01, 2016 at 11:28:51AM -0800, Nikhilesh Reddy wrote:
> > On Mon 01 Feb 2016 11:15:56 AM PST, Jann Horn wrote:
> > > On Mon, Feb 01, 2016 at 10:56:27AM -0800, Nikhilesh Reddy wrote:
> > > > diff --git a/fs/fuse/passthrough.c b/fs/fuse/passthrough.c
> > > [...]
> > > > +static ssize_t fuse_passthrough_read_write_iter(struct kiocb *iocb,
> > > > +					    struct iov_iter *iter, int do_write)
> > > > +{
> > > > +	ssize_t ret_val;
> > > > +	struct fuse_file *ff;
> > > > +	struct file *fuse_file, *passthrough_filp;
> > > > +	struct inode *fuse_inode, *passthrough_inode;
> > > > +
> > > > +	ff = iocb->ki_filp->private_data;
> > > > +	fuse_file = iocb->ki_filp;
> > > > +	passthrough_filp = ff->passthrough_filp;
> > > > +
> > > > +	/* lock passthrough file to prevent it from being released */
> > > > +	get_file(passthrough_filp);
> > > > +	iocb->ki_filp = passthrough_filp;
> > > > +	fuse_inode = fuse_file->f_path.dentry->d_inode;
> > > > +	passthrough_inode = file_inode(passthrough_filp);
> > > > +
> > > > +	if (do_write) {
> > > > +		if (!passthrough_filp->f_op->write_iter)
> > > > +			return -EIO;
> > > > +		ret_val = passthrough_filp->f_op->write_iter(iocb, iter);
> > > > +
> > > > +		if (ret_val >= 0 || ret_val == -EIOCBQUEUED) {
> > > > +			fsstack_copy_inode_size(fuse_inode, passthrough_inode);
> > > > +			fsstack_copy_attr_times(fuse_inode, passthrough_inode);
> > > > +		}
> > > > +	} else {
> > > > +		if (!passthrough_filp->f_op->read_iter)
> > > > +			return -EIO;
> > > > +		ret_val = passthrough_filp->f_op->read_iter(iocb, iter);
> > > > +		if (ret_val >= 0 || ret_val == -EIOCBQUEUED)
> > > > +			fsstack_copy_attr_atime(fuse_inode, passthrough_inode);
> > > > +	}
> > > > +
> > > > +	iocb->ki_filp = fuse_file;
> > > > +
> > > > +	/* unlock passthrough file */
> > > > +	fput(passthrough_filp);
> > >
> > > Why the get_file() and fput() in this method? This doesn't look right. There
> > > is no lock you're releasing between get_file() and fput(). What are they
> > > intended for?
> >
> > Hi
> >
> > Thanks for reviewing the code.
> >
> > The passthrough file could be released under our feet say  if the userspace
> > fuse daemon crashed or was killed  ( while we are processing the read or the
> > write) causing bad things to happen.
> > The calls here are to increase the count temporarily  and then decrease it
> > so that we dont release in the middle of a write and everything is
> > gracefully handled...
> >
> > I have a comment right before the get_file call above saying the same thing.
> > Please let me know if you have any more questions.
>
> If that is the case, why can't the passthrough file be released before the
> get_file() call, e.g. while the core processing the filesystem read request
> is entering fuse_passthrough_read_write_iter()?
>
> As far as I can tell, you can drop the get_file() and fput() calls.
> fuse_setup_passthrough() already took a reference to the file for you, that
> reference can only be dropped in fuse_passthrough_release(), and the VFS
> ensures that no release call happens while a read or write is pending.
I just feel uncomfortable with dropping them. I thought they could be 
released ( i/o ) takes longer than the actual execution... but if i can 
be sure of it then maybe..

--
Thanks
Nikhilesh Reddy

Qualcomm Innovation Center, Inc.
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