Commit 94e4fe2c authored Jan 12, 2015 by Tom Van Braeckel Committed by Miklos Szeredi Mar 19, 2015

fuse: explicitly set /dev/fuse file's private_data

The misc subsystem (which is used for /dev/fuse) initializes private_data to
point to the misc device when a driver has registered a custom open file
operation, and initializes it to NULL when a custom open file operation has
*not* been provided.

This subtle quirk is confusing, to the point where kernel code registers
*empty* file open operations to have private_data point to the misc device
structure. And it leads to bugs, where the addition or removal of a custom open
file operation surprisingly changes the initial contents of a file's
private_data structure.

So to simplify things in the misc subsystem, a patch [1] has been proposed to
*always* set the private_data to point to the misc device, instead of only
doing this when a custom open file operation has been registered.

But before this patch can be applied we need to modify drivers that make the
assumption that a misc device file's private_data is initialized to NULL
because they didn't register a custom open file operation, so they don't rely
on this assumption anymore. FUSE uses private_data to store the fuse_conn and
errors out if this is not initialized to NULL at mount time.

Hence, we now set a file's private_data to NULL explicitly, to be independent
of whatever value the misc subsystem initializes it to by default.

[1] https://lkml.org/lkml/2014/12/4/939



Reported-by: Giedrius Statkevicius <giedriuswork@gmail.com>
Reported-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Tom Van Braeckel <tomvanbraeckel@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

parent aa991b3b

fs/fuse/dev.c

+12 −0

Original line number	Diff line number	Diff line
		@@ -1353,6 +1353,17 @@ static ssize_t fuse_dev_do_read(struct fuse_conn fc, struct file file,
		return err;
		}

		static int fuse_dev_open(struct inode inode, struct file file)
		{
		/*
		* The fuse device's file's private_data is used to hold
		* the fuse_conn(ection) when it is mounted, and is used to
		* keep track of whether the file has been mounted already.
		*/
		file->private_data = NULL;
		return 0;
		}

		static ssize_t fuse_dev_read(struct kiocb iocb, const struct iovec iov,
		unsigned long nr_segs, loff_t pos)
		{
		@@ -2220,6 +2231,7 @@ static int fuse_dev_fasync(int fd, struct file *file, int on)

		const struct file_operations fuse_dev_operations = {
		.owner = THIS_MODULE,
		.open = fuse_dev_open,
		.llseek = no_llseek,
		.read = do_sync_read,
		.aio_read = fuse_dev_read,