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Unverified Commit 95b3206d authored by Alessio Balsini's avatar Alessio Balsini Committed by Michael Bestas
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FROMLIST: fuse: Passthrough initialization and release 

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.

Bug: 168023149
Link: https://lore.kernel.org/lkml/20210125153057.3623715-5-balsini@android.com/


Signed-off-by: default avatarAlessio Balsini <balsini@android.com>
Change-Id: I4d8290012302fb4547bce9bb261a03cc4f66b5aa
Signed-off-by: default avatarAlessio Balsini <balsini@google.com>
parent 2fa6e18b

fs/fuse/inode.c

+5 −0
Original line number Diff line number Diff line
@@ -1041,6 +1041,11 @@ 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;

}

fs/fuse/passthrough.c

+85 −2
Original line number Diff line number Diff line
@@ -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)

{

	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");

		res = -EINVAL;

		goto err_free_file;

	}



	passthrough = kmalloc(sizeof(struct fuse_passthrough), GFP_KERNEL);

	if (!passthrough) {

		res = -ENOMEM;

		goto err_free_file;

	}



	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)

		return res;



	fuse_passthrough_release(passthrough);

	kfree(passthrough);



err_free_file:

	fput(passthrough_filp);



	return res;

}



int fuse_passthrough_setup(struct fuse_conn *fc, struct fuse_file *ff,

			   struct fuse_open_out *openarg)

{

	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;



	ff->passthrough = *passthrough;

	kfree(passthrough);



	return 0;

}



void fuse_passthrough_release(struct fuse_passthrough *passthrough)

{

	if (passthrough->filp) {

		fput(passthrough->filp);

		passthrough->filp = NULL;

	}

}