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Unverified Commit 27233d34 authored by Alessio Balsini's avatar Alessio Balsini Committed by Michael Bestas
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FROMLIST: fuse: Introduce synchronous read and write for passthrough 

All the read and write operations performed on fuse_files which have the
passthrough feature enabled are forwarded to the associated lower file
system file via VFS.

Sending the request directly to the lower file system avoids the
userspace round-trip that, because of possible context switches and
additional operations might reduce the overall performance, especially
in those cases where caching doesn't help, for example in reads at
random offsets.

Verifying if a fuse_file has a lower file system file associated with
can be done by checking the validity of its passthrough_filp pointer.
This pointer is not NULL only if passthrough has been successfully
enabled via the appropriate ioctl().
When a read/write operation is requested for a FUSE file with
passthrough enabled, a new equivalent VFS request is generated, which
instead targets the lower file system file.
The VFS layer performs additional checks that allow for safer operations
but may cause the operation to fail if the process accessing the FUSE
file system does not have access to the lower file system.

This change only implements synchronous requests in passthrough,
returning an error in the case of asynchronous operations, yet covering
the majority of the use cases.

Bug: 168023149
Link: https://lore.kernel.org/lkml/20210125153057.3623715-6-balsini@android.com/


Signed-off-by: default avatarAlessio Balsini <balsini@android.com>
Change-Id: Ifbe6a247fe7338f87d078fde923f0252eeaeb668
Signed-off-by: default avatarAlessio Balsini <balsini@google.com>
parent 95b3206d

fs/fuse/file.c

+6 −2
Original line number Diff line number Diff line
@@ -1598,7 +1598,9 @@ static ssize_t fuse_file_read_iter(struct kiocb *iocb, struct iov_iter *to) 
	if (fuse_is_bad(file_inode(file)))

		return -EIO;



	if (!(ff->open_flags & FOPEN_DIRECT_IO))

	if (ff->passthrough.filp)

		return fuse_passthrough_read_iter(iocb, to);

	else if (!(ff->open_flags & FOPEN_DIRECT_IO))

		return fuse_cache_read_iter(iocb, to);

	else

		return fuse_direct_read_iter(iocb, to);
@@ -1612,7 +1614,9 @@ static ssize_t fuse_file_write_iter(struct kiocb *iocb, struct iov_iter *from) 
	if (fuse_is_bad(file_inode(file)))

		return -EIO;



	if (!(ff->open_flags & FOPEN_DIRECT_IO))

	if (ff->passthrough.filp)

		return fuse_passthrough_write_iter(iocb, from);

	else if (!(ff->open_flags & FOPEN_DIRECT_IO))

		return fuse_cache_write_iter(iocb, from);

	else

		return fuse_direct_write_iter(iocb, from);

fs/fuse/fuse_i.h

+2 −0
Original line number Diff line number Diff line
@@ -1137,5 +1137,7 @@ int fuse_passthrough_open(struct fuse_dev *fud, 
int fuse_passthrough_setup(struct fuse_conn *fc, struct fuse_file *ff,

			   struct fuse_open_out *openarg);

void fuse_passthrough_release(struct fuse_passthrough *passthrough);

ssize_t fuse_passthrough_read_iter(struct kiocb *iocb, struct iov_iter *to);

ssize_t fuse_passthrough_write_iter(struct kiocb *iocb, struct iov_iter *from);



#endif /* _FS_FUSE_I_H */

fs/fuse/passthrough.c

+57 −0
Original line number Diff line number Diff line
@@ -4,6 +4,63 @@ 


#include <linux/fuse.h>

#include <linux/idr.h>

#include <linux/uio.h>



#define PASSTHROUGH_IOCB_MASK                                                  \

	(IOCB_APPEND | IOCB_DSYNC | IOCB_HIPRI | IOCB_NOWAIT | IOCB_SYNC)



static void fuse_copyattr(struct file *dst_file, struct file *src_file)

{

	struct inode *dst = file_inode(dst_file);

	struct inode *src = file_inode(src_file);



	i_size_write(dst, i_size_read(src));

}



ssize_t fuse_passthrough_read_iter(struct kiocb *iocb_fuse,

				   struct iov_iter *iter)

{

	ssize_t ret;

	struct file *fuse_filp = iocb_fuse->ki_filp;

	struct fuse_file *ff = fuse_filp->private_data;

	struct file *passthrough_filp = ff->passthrough.filp;



	if (!iov_iter_count(iter))

		return 0;



	ret = vfs_iter_read(passthrough_filp, iter, &iocb_fuse->ki_pos,

			    iocb_to_rw_flags(iocb_fuse->ki_flags,

					     PASSTHROUGH_IOCB_MASK));



	return ret;

}



ssize_t fuse_passthrough_write_iter(struct kiocb *iocb_fuse,

				    struct iov_iter *iter)

{

	ssize_t ret;

	struct file *fuse_filp = iocb_fuse->ki_filp;

	struct fuse_file *ff = fuse_filp->private_data;

	struct inode *fuse_inode = file_inode(fuse_filp);

	struct file *passthrough_filp = ff->passthrough.filp;



	if (!iov_iter_count(iter))

		return 0;



	inode_lock(fuse_inode);



	file_start_write(passthrough_filp);

	ret = vfs_iter_write(passthrough_filp, iter, &iocb_fuse->ki_pos,

			     iocb_to_rw_flags(iocb_fuse->ki_flags,

					      PASSTHROUGH_IOCB_MASK));

	file_end_write(passthrough_filp);

	if (ret > 0)

		fuse_copyattr(fuse_filp, passthrough_filp);



	inode_unlock(fuse_inode);



	return ret;

}



int fuse_passthrough_open(struct fuse_dev *fud,

			  struct fuse_passthrough_out *pto)