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: 179164095
Link: https://lore.kernel.org/lkml/20210125153057.3623715-5-balsini@android.com/
Signed-off-by: Alessio Balsini <balsini@android.com>
Change-Id: I4d8290012302fb4547bce9bb261a03cc4f66b5aa
Signed-off-by: Alessio Balsini <balsini@google.com>

fs/fuse/inode.c

@@ -983,6 +983,11 @@ static void fuse_send_init(struct fuse_conn *fc, struct fuse_req *req)
 
 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

@@ -2,20 +2,104 @@
 
 #include "fuse_i.h"
 
+#include <linux/file.h>
 #include <linux/fuse.h>
+#include <linux/idr.h>
 
 int fuse_passthrough_open(struct fuse_dev *fud,
 			  struct fuse_passthrough_out *pto)
 {
-	return -EINVAL;
+	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)
 {
-	return -EINVAL;
+	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;
+	}
 }