fuse: Add support for d_canonical_path

Allows FUSE to report to inotify that it is acting
as a layered filesystem. The userspace component
returns a string representing the location of the
underlying file. If the string cannot be resolved
into a path, the top level path is returned instead.

bug: 23904372
Change-Id: Iabdca0bbedfbff59e9c820c58636a68ef9683d9f
Signed-off-by: Daniel Rosenberg <drosen@google.com>

fs/fuse/dev.c

@@ -13,6 +13,7 @@
 #include <linux/poll.h>
 #include <linux/uio.h>
 #include <linux/miscdevice.h>
+#include <linux/namei.h>
 #include <linux/pagemap.h>
 #include <linux/file.h>
 #include <linux/slab.h>
@@ -1779,6 +1780,10 @@ static ssize_t fuse_dev_do_write(struct fuse_conn *fc,
 	spin_unlock(&fc->lock);
 
 	err = copy_out_args(cs, &req->out, nbytes);
+	if (req->in.h.opcode == FUSE_CANONICAL_PATH) {
+		req->out.h.error = kern_path((char *)req->out.args[0].value, 0,
+							req->canonical_path);
+	}
 	fuse_copy_finish(cs);
 
 	spin_lock(&fc->lock);

fs/fuse/dir.c

@@ -222,6 +222,50 @@ static int fuse_dentry_revalidate(struct dentry *entry, struct nameidata *nd)
 	return 1;
 }
 
+/*
+ * Get the canonical path. Since we must translate to a path, this must be done
+ * in the context of the userspace daemon, however, the userspace daemon cannot
+ * look up paths on its own. Instead, we handle the lookup as a special case
+ * inside of the write request.
+ */
+static void fuse_dentry_canonical_path(const struct path *path, struct path *canonical_path) {
+	struct inode *inode = path->dentry->d_inode;
+	struct fuse_conn *fc = get_fuse_conn(inode);
+	struct fuse_req *req;
+	int err;
+	char *path_name;
+
+	req = fuse_get_req(fc);
+	err = PTR_ERR(req);
+	if (IS_ERR(req))
+		goto default_path;
+
+	path_name = (char*)__get_free_page(GFP_KERNEL);
+	if (!path_name) {
+		fuse_put_request(fc, req);
+		goto default_path;
+	}
+
+	req->in.h.opcode = FUSE_CANONICAL_PATH;
+	req->in.h.nodeid = get_node_id(inode);
+	req->in.numargs = 0;
+	req->out.numargs = 1;
+	req->out.args[0].size = PATH_MAX;
+	req->out.args[0].value = path_name;
+	req->canonical_path = canonical_path;
+	req->out.argvar = 1;
+	fuse_request_send(fc, req);
+	err = req->out.h.error;
+	fuse_put_request(fc, req);
+	free_page((unsigned long)path_name);
+	if (!err)
+		return;
+default_path:
+	canonical_path->dentry = path->dentry;
+	canonical_path->mnt = path->mnt;
+	path_get(canonical_path);
+}
+
 static int invalid_nodeid(u64 nodeid)
 {
 	return !nodeid || nodeid == FUSE_ROOT_ID;
@@ -229,6 +273,7 @@ static int invalid_nodeid(u64 nodeid)
 
 const struct dentry_operations fuse_dentry_operations = {
 	.d_revalidate	= fuse_dentry_revalidate,
+	.d_canonical_path = fuse_dentry_canonical_path,
 };
 
 int fuse_valid_type(int m)

fs/fuse/fuse_i.h

@@ -302,6 +302,9 @@ struct fuse_req {
 	/** Inode used in the request or NULL */
 	struct inode *inode;
 
+	/** Path used for completing d_canonical_path */
+	struct path *canonical_path;
+
 	/** Link on fi->writepages */
 	struct list_head writepages_entry;
 

include/linux/fuse.h

@@ -278,6 +278,7 @@ enum fuse_opcode {
 	FUSE_POLL          = 40,
 	FUSE_NOTIFY_REPLY  = 41,
 	FUSE_BATCH_FORGET  = 42,
+	FUSE_CANONICAL_PATH= 2016,
 
 	/* CUSE specific operations */
 	CUSE_INIT          = 4096,