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

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Message-ID: <20160201194526.GA11837@pc.thejh.net>
Date:	Mon, 1 Feb 2016 20:45:26 +0100
From:	Jann Horn <jann@...jh.net>
To:	Nikhilesh Reddy <reddyn@...eaurora.org>
Cc:	torvalds@...ux-foundation.org, Miklos Szeredi <miklos@...redi.hu>,
	fuse-devel <fuse-devel@...ts.sourceforge.net>,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	gregkh@...uxfoundation.org, linux-fsdevel@...r.kernel.org,
	viro@...iv.linux.org.uk, Richard Weinberger <richard@....at>,
	Theodore Ts'o <tytso@....edu>, jack@...e.cz,
	Antonio SJ Musumeci <trapexit@...wn.link>, sven.utcke@....de,
	Nikolaus Rath <nikolaus@...h.org>,
	Jann Horn <jannhorn@...glemail.com>,
	Mike Shal <marfey@...il.com>
Subject: Re: [PATCH v5] fuse: Add support for passthrough read/write

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.

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