fuse: Definitions and ioctl() for passthrough

Introduce the new FUSE passthrough ioctl(), which allows userspace to
specify a direct connection between a FUSE file and a lower file system
file.
Such ioctl() requires userspace to specify:
- the file descriptor of one of its opened files,
- the unique identifier of the FUSE request associated with a pending
  open/create operation,
both encapsulated into a fuse_passthrough_out data structure.
The ioctl() will search for the pending FUSE request matching the unique
identifier, and update the passthrough file pointer of the request with the
file pointer referenced by the passed file descriptor.
When that pending FUSE request is handled, the passthrough file pointer
is copied to the fuse_file data structure, so that the link between FUSE
and lower file system is consolidated.

In order for the passthrough mode to be successfully activated, the lower
file system file must implement both read_ and write_iter file operations.
This extra check avoids special pseudofiles to be targets for this feature.
An additional enforced limitation is that when FUSE passthrough is enabled,
no further file system stacking is allowed.

Signed-off-by: Alessio Balsini <balsini@android.com>

fs/fuse/Makefile

@@ -8,4 +8,5 @@ obj-$(CONFIG_CUSE) += cuse.o
 obj-$(CONFIG_VIRTIO_FS) += virtiofs.o
 
 fuse-objs := dev.o dir.o file.o inode.o control.o xattr.o acl.o readdir.o
+fuse-objs += passthrough.o
 virtiofs-y += virtio_fs.o

fs/fuse/dev.c

@@ -2219,21 +2219,53 @@ static int fuse_device_clone(struct fuse_conn *fc, struct file *new)
 	return 0;
 }
 
+int fuse_passthrough_open(struct fuse_dev *fud,
+			  struct fuse_passthrough_out *pto)
+{
+	int ret;
+	struct fuse_req *req;
+	struct fuse_pqueue *fpq = &fud->pq;
+	struct fuse_conn *fc = fud->fc;
+
+	if (!fc->passthrough)
+		return -EPERM;
+
+	/* This field is reserved for future use */
+	if (pto->len != 0)
+		return -EINVAL;
+
+	spin_lock(&fpq->lock);
+	req = request_find(fpq, pto->unique & ~FUSE_INT_REQ_BIT);
+	if (!req) {
+		spin_unlock(&fpq->lock);
+		return -ENOENT;
+	}
+	__fuse_get_request(req);
+	spin_unlock(&fpq->lock);
+
+	ret = fuse_passthrough_setup(req, pto->fd);
+
+	__fuse_put_request(req);
+	return ret;
+}
+
 static long fuse_dev_ioctl(struct file *file, unsigned int cmd,
 			   unsigned long arg)
 {
-	int err = -ENOTTY;
-
-	if (cmd == FUSE_DEV_IOC_CLONE) {
-		int oldfd;
+	int err;
+	int oldfd;
+	struct fuse_dev *fud;
+	struct fuse_passthrough_out pto;
 
+	switch (cmd) {
+	case FUSE_DEV_IOC_CLONE:
 		err = -EFAULT;
 		if (!get_user(oldfd, (__u32 __user *) arg)) {
 			struct file *old = fget(oldfd);
 
 			err = -EINVAL;
 			if (old) {
-				struct fuse_dev *fud = NULL;
+				fud = NULL;
 
 				/*
 				 * Check against file->f_op because CUSE
@@ -2251,6 +2283,21 @@ static long fuse_dev_ioctl(struct file *file, unsigned int cmd,
 				fput(old);
 			}
 		}
+		break;
+	case FUSE_DEV_IOC_PASSTHROUGH_OPEN:
+		err = -EFAULT;
+		if (!copy_from_user(&pto,
+				    (struct fuse_passthrough_out __user *)arg,
+				    sizeof(pto))) {
+			err = -EINVAL;
+			fud = fuse_get_dev(file);
+			if (fud)
+				err = fuse_passthrough_open(fud, &pto);
+		}
+		break;
+	default:
+		err = -ENOTTY;
+		break;
 	}
 	return err;
 }

fs/fuse/dir.c

@@ -477,6 +477,7 @@ static int fuse_create_open(struct inode *dir, struct dentry *entry,
 	args.out_args[0].value = &outentry;
 	args.out_args[1].size = sizeof(outopen);
 	args.out_args[1].value = &outopen;
+	args.passthrough_filp = NULL;
 	err = fuse_simple_request(fc, &args);
 	if (err)
 		goto out_free_ff;
@@ -489,6 +490,7 @@ static int fuse_create_open(struct inode *dir, struct dentry *entry,
 	ff->fh = outopen.fh;
 	ff->nodeid = outentry.nodeid;
 	ff->open_flags = outopen.open_flags;
+	ff->passthrough_filp = args.passthrough_filp;
 	inode = fuse_iget(dir->i_sb, outentry.nodeid, outentry.generation,
 			  &outentry.attr, entry_attr_timeout(&outentry), 0);
 	if (!inode) {

fs/fuse/file.c

@@ -33,10 +33,12 @@ static struct page **fuse_pages_alloc(unsigned int npages, gfp_t flags,
 }
 
 static int fuse_send_open(struct fuse_conn *fc, u64 nodeid, struct file *file,
-			  int opcode, struct fuse_open_out *outargp)
+			  int opcode, struct fuse_open_out *outargp,
+			  struct file **passthrough_filp)
 {
 	struct fuse_open_in inarg;
 	FUSE_ARGS(args);
+	int ret;
 
 	memset(&inarg, 0, sizeof(inarg));
 	inarg.flags = file->f_flags & ~(O_CREAT | O_EXCL | O_NOCTTY);
@@ -51,7 +53,10 @@ static int fuse_send_open(struct fuse_conn *fc, u64 nodeid, struct file *file,
 	args.out_args[0].size = sizeof(*outargp);
 	args.out_args[0].value = outargp;
 
-	return fuse_simple_request(fc, &args);
+	ret = fuse_simple_request(fc, &args);
+	*passthrough_filp = args.passthrough_filp;
+
+	return ret;
 }
 
 struct fuse_release_args {
@@ -144,14 +149,16 @@ int fuse_do_open(struct fuse_conn *fc, u64 nodeid, struct file *file,
 	/* Default for no-open */
 	ff->open_flags = FOPEN_KEEP_CACHE | (isdir ? FOPEN_CACHE_DIR : 0);
 	if (isdir ? !fc->no_opendir : !fc->no_open) {
+		struct file *passthrough_filp;
 		struct fuse_open_out outarg;
 		int err;
 
-		err = fuse_send_open(fc, nodeid, file, opcode, &outarg);
+		err = fuse_send_open(fc, nodeid, file, opcode, &outarg,
+				     &passthrough_filp);
 		if (!err) {
 			ff->fh = outarg.fh;
 			ff->open_flags = outarg.open_flags;
-
+			ff->passthrough_filp = passthrough_filp;
 		} else if (err != -ENOSYS) {
 			fuse_file_free(ff);
 			return err;
@@ -281,6 +288,8 @@ void fuse_release_common(struct file *file, bool isdir)
 	struct fuse_release_args *ra = ff->release_args;
 	int opcode = isdir ? FUSE_RELEASEDIR : FUSE_RELEASE;
 
+	fuse_passthrough_release(ff);
+
 	fuse_prepare_release(fi, ff, file->f_flags, opcode);
 
 	if (ff->flock) {

fs/fuse/fuse_i.h

@@ -208,6 +208,12 @@ struct fuse_file {
 
 	} readdir;
 
+	/**
+	 * Reference to lower filesystem file for read/write operations
+	 * handled in pass-through mode
+	 */
+	struct file *passthrough_filp;
+
 	/** RB node to be linked on fuse_conn->polled_files */
 	struct rb_node polled_node;
 
@@ -250,6 +256,8 @@ struct fuse_args {
 	bool page_zeroing:1;
 	bool page_replace:1;
 	bool may_block:1;
+	/** Lower filesystem file pointer used in pass-through mode */
+	struct file *passthrough_filp;
 	struct fuse_in_arg in_args[3];
 	struct fuse_arg out_args[2];
 	void (*end)(struct fuse_conn *fc, struct fuse_args *args, int error);
@@ -720,6 +728,9 @@ struct fuse_conn {
 	/* Do not show mount options */
 	unsigned int no_mount_options:1;
 
+	/** Pass-through mode for read/write IO */
+	unsigned int passthrough:1;
+
 	/** The number of requests waiting for completion */
 	atomic_t num_waiting;
 
@@ -1093,4 +1104,7 @@ unsigned int fuse_len_args(unsigned int numargs, struct fuse_arg *args);
 u64 fuse_get_unique(struct fuse_iqueue *fiq);
 void fuse_free_conn(struct fuse_conn *fc);
 
+int fuse_passthrough_setup(struct fuse_req *req, unsigned int fd);
+void fuse_passthrough_release(struct fuse_file *ff);
+
 #endif /* _FS_FUSE_I_H */

fs/fuse/inode.c

@@ -965,6 +965,12 @@ static void process_init_reply(struct fuse_conn *fc, struct fuse_args *args,
 					min_t(unsigned int, FUSE_MAX_MAX_PAGES,
 					max_t(unsigned int, arg->max_pages, 1));
 			}
+			if (arg->flags & FUSE_PASSTHROUGH) {
+				fc->passthrough = 1;
+				/* Prevent further stacking */
+				fc->sb->s_stack_depth =
+					FILESYSTEM_MAX_STACK_DEPTH;
+			}
 		} else {
 			ra_pages = fc->max_read / PAGE_SIZE;
 			fc->no_lock = 1;
@@ -1002,7 +1008,8 @@ void fuse_send_init(struct fuse_conn *fc)
 		FUSE_WRITEBACK_CACHE | FUSE_NO_OPEN_SUPPORT |
 		FUSE_PARALLEL_DIROPS | FUSE_HANDLE_KILLPRIV | FUSE_POSIX_ACL |
 		FUSE_ABORT_ERROR | FUSE_MAX_PAGES | FUSE_CACHE_SYMLINKS |
-		FUSE_NO_OPENDIR_SUPPORT | FUSE_EXPLICIT_INVAL_DATA;
+		FUSE_NO_OPENDIR_SUPPORT | FUSE_EXPLICIT_INVAL_DATA |
+		FUSE_PASSTHROUGH;
 	ia->args.opcode = FUSE_INIT;
 	ia->args.in_numargs = 1;
 	ia->args.in_args[0].size = sizeof(ia->in);

fs/fuse/passthrough.c

@@ -0,0 +1,55 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "fuse_i.h"
+
+int fuse_passthrough_setup(struct fuse_req *req, unsigned int fd)
+{
+	int ret;
+	int fs_stack_depth;
+	struct file *passthrough_filp;
+	struct inode *passthrough_inode;
+	struct super_block *passthrough_sb;
+
+	/* Passthrough mode can only be enabled at file open/create time */
+	if (req->in.h.opcode != FUSE_OPEN && req->in.h.opcode != FUSE_CREATE) {
+		pr_err("FUSE: invalid OPCODE for request.\n");
+		return -EINVAL;
+	}
+
+	passthrough_filp = fget(fd);
+	if (!passthrough_filp) {
+		pr_err("FUSE: invalid file descriptor for passthrough.\n");
+		return -EINVAL;
+	}
+
+	ret = -EINVAL;
+	if (!passthrough_filp->f_op->read_iter ||
+	    !passthrough_filp->f_op->write_iter) {
+		pr_err("FUSE: passthrough file misses file operations.\n");
+		goto out;
+	}
+
+	passthrough_inode = file_inode(passthrough_filp);
+	passthrough_sb = passthrough_inode->i_sb;
+	fs_stack_depth = passthrough_sb->s_stack_depth + 1;
+	ret = -EEXIST;
+	if (fs_stack_depth > FILESYSTEM_MAX_STACK_DEPTH) {
+		pr_err("FUSE: maximum fs stacking depth exceeded for passthrough\n");
+		goto out;
+	}
+
+	req->args->passthrough_filp = passthrough_filp;
+	return 0;
+out:
+	fput(passthrough_filp);
+	return ret;
+}
+
+void fuse_passthrough_release(struct fuse_file *ff)
+{
+	if (!ff->passthrough_filp)
+		return;
+
+	fput(ff->passthrough_filp);
+	ff->passthrough_filp = NULL;
+}

include/uapi/linux/fuse.h

@@ -342,6 +342,7 @@ struct fuse_file_lock {
 #define FUSE_NO_OPENDIR_SUPPORT (1 << 24)
 #define FUSE_EXPLICIT_INVAL_DATA (1 << 25)
 #define FUSE_MAP_ALIGNMENT	(1 << 26)
+#define FUSE_PASSTHROUGH	(1 << 27)
 
 /**
  * CUSE INIT request/reply flags
@@ -794,6 +795,14 @@ struct fuse_in_header {
 	uint32_t	padding;
 };
 
+struct fuse_passthrough_out {
+	uint64_t	unique;
+	uint32_t	fd;
+	/* For future implementation */
+	uint32_t	len;
+	void		*vec;
+};
+
 struct fuse_out_header {
 	uint32_t	len;
 	int32_t		error;
@@ -869,7 +878,8 @@ struct fuse_notify_retrieve_in {
 };
 
 /* Device ioctls: */
-#define FUSE_DEV_IOC_CLONE	_IOR(229, 0, uint32_t)
+#define FUSE_DEV_IOC_CLONE		_IOR(229, 0, uint32_t)
+#define FUSE_DEV_IOC_PASSTHROUGH_OPEN	_IOW(229, 1, struct fuse_passthrough_out)
 
 struct fuse_lseek_in {
 	uint64_t	fh;