ANDROID: 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
Bug: 171780975
Test: FileObserverTest and FileObserverTestLegacyPath on cuttlefish
Change-Id: Iabdca0bbedfbff59e9c820c58636a68ef9683d9f
Signed-off-by: Daniel Rosenberg <drosen@google.com>
Signed-off-by: Alessio Balsini <balsini@google.com>

fs/fuse/dev.c

@@ -14,6 +14,7 @@
 #include <linux/sched/signal.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>
@@ -1908,6 +1909,14 @@ static ssize_t fuse_dev_do_write(struct fuse_dev *fud,
 		err = copy_out_args(cs, req->args, nbytes);
 	fuse_copy_finish(cs);
 
+	if (!err && req->in.h.opcode == FUSE_CANONICAL_PATH) {
+		char *path = (char *)req->args->out_args[0].value;
+
+		path[req->args->out_args[0].size - 1] = 0;
+		req->out.h.error =
+			kern_path(path, 0, req->args->canonical_path);
+	}
+
 	spin_lock(&fpq->lock);
 	clear_bit(FR_LOCKED, &req->flags);
 	if (!fpq->connected)

fs/fuse/dir.c

@@ -374,6 +374,45 @@ out:
 	return ERR_PTR(err);
 }
 
+/*
+ * 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 = d_inode(path->dentry);
+	//struct fuse_conn *fc = get_fuse_conn(inode);
+	struct fuse_mount *fm = get_fuse_mount_super(path->mnt->mnt_sb);
+	FUSE_ARGS(args);
+	char *path_name;
+	int err;
+
+	path_name = (char *)__get_free_page(GFP_KERNEL);
+	if (!path_name)
+		goto default_path;
+
+	args.opcode = FUSE_CANONICAL_PATH;
+	args.nodeid = get_node_id(inode);
+	args.in_numargs = 0;
+	args.out_numargs = 1;
+	args.out_args[0].size = PATH_MAX;
+	args.out_args[0].value = path_name;
+	args.canonical_path = canonical_path;
+	args.out_argvar = 1;
+
+	err = fuse_simple_request(fm, &args);
+	free_page((unsigned long)path_name);
+	if (err > 0)
+		return;
+default_path:
+	canonical_path->dentry = path->dentry;
+	canonical_path->mnt = path->mnt;
+	path_get(canonical_path);
+}
+
 const struct dentry_operations fuse_dentry_operations = {
 	.d_revalidate	= fuse_dentry_revalidate,
 	.d_delete	= fuse_dentry_delete,
@@ -382,6 +421,7 @@ const struct dentry_operations fuse_dentry_operations = {
 	.d_release	= fuse_dentry_release,
 #endif
 	.d_automount	= fuse_dentry_automount,
+	.d_canonical_path = fuse_dentry_canonical_path,
 };
 
 const struct dentry_operations fuse_root_dentry_operations = {

fs/fuse/fuse_i.h

@@ -282,6 +282,9 @@ struct fuse_args {
 	struct fuse_in_arg in_args[3];
 	struct fuse_arg out_args[2];
 	void (*end)(struct fuse_mount *fm, struct fuse_args *args, int error);
+
+	/* Path used for completing d_canonical_path */
+	struct path *canonical_path;
 };
 
 struct fuse_args_pages {

include/uapi/linux/fuse.h

@@ -480,6 +480,7 @@ enum fuse_opcode {
 	FUSE_COPY_FILE_RANGE	= 47,
 	FUSE_SETUPMAPPING	= 48,
 	FUSE_REMOVEMAPPING	= 49,
+	FUSE_CANONICAL_PATH	= 2016,
 
 	/* CUSE specific operations */
 	CUSE_INIT		= 4096,