[PATCH V10 3/5] fuse: Introduce synchronous read and write for passthrough

[Alessio Balsini]

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.

Signed-off-by: Alessio Balsini <balsini@xxxxxxxxxxx>
---
 fs/fuse/file.c        |  8 +++--
 fs/fuse/fuse_i.h      |  2 ++
 fs/fuse/passthrough.c | 70 +++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 78 insertions(+), 2 deletions(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 84daaf084197..f7a12489c0ef 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1545,7 +1545,9 @@ static ssize_t fuse_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 	if (is_bad_inode(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);
@@ -1559,7 +1561,9 @@ static ssize_t fuse_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	if (is_bad_inode(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);
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 32da45ce86e0..a888d3df5877 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -1118,5 +1118,7 @@ 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 */
diff --git a/fs/fuse/passthrough.c b/fs/fuse/passthrough.c
index a135c955cc33..5a78cb336db4 100644
--- a/fs/fuse/passthrough.c
+++ b/fs/fuse/passthrough.c
@@ -4,6 +4,76 @@
 
 #include <linux/fuse.h>
 #include <linux/idr.h>
+#include <linux/uio.h>
+
+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));
+}
+
+static inline rwf_t iocb_to_rw_flags(int ifl)
+{
+	rwf_t flags = 0;
+
+	if (ifl & IOCB_APPEND)
+		flags |= RWF_APPEND;
+	if (ifl & IOCB_DSYNC)
+		flags |= RWF_DSYNC;
+	if (ifl & IOCB_HIPRI)
+		flags |= RWF_HIPRI;
+	if (ifl & IOCB_NOWAIT)
+		flags |= RWF_NOWAIT;
+	if (ifl & IOCB_SYNC)
+		flags |= RWF_SYNC;
+
+	return flags;
+}
+
+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));
+
+	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));
+	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)
-- 
2.29.0.rc1.297.gfa9743e501-goog