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: Pixel 4.19
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>
@@ -573,6 +574,7 @@ ssize_t fuse_simple_request(struct fuse_conn *fc, struct fuse_args *args)
 	req->out.numargs = args->out.numargs;
 	memcpy(req->out.args, args->out.args,
 	       args->out.numargs * sizeof(struct fuse_arg));
+	req->out.canonical_path = args->out.canonical_path;
 	fuse_request_send(fc, req);
 	ret = req->out.h.error;
 	if (!ret && args->out.argvar) {
@@ -1932,6 +1934,13 @@ static ssize_t fuse_dev_do_write(struct fuse_dev *fud,
 	err = copy_out_args(cs, &req->out, nbytes);
 	fuse_copy_finish(cs);
 
+	if (!err && req->in.h.opcode == FUSE_CANONICAL_PATH) {
+		char *path = (char *)req->out.args[0].value;
+
+		path[req->out.args[0].size - 1] = 0;
+		req->out.h.error = kern_path(path, 0, req->out.canonical_path);
+	}
+
 	spin_lock(&fpq->lock);
 	clear_bit(FR_LOCKED, &req->flags);
 	if (!fpq->connected)

fs/fuse/dir.c

@@ -281,10 +281,49 @@ static void fuse_dentry_release(struct dentry *dentry)
 	kfree_rcu(fd, rcu);
 }
 
+/*
+ * 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);
+	FUSE_ARGS(args);
+	char *path_name;
+	int err;
+
+	path_name = (char *)__get_free_page(GFP_KERNEL);
+	if (!path_name)
+		goto default_path;
+
+	args.in.h.opcode = FUSE_CANONICAL_PATH;
+	args.in.h.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.out.argvar = 1;
+	args.out.canonical_path = canonical_path;
+
+	err = fuse_simple_request(fc, &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_init		= fuse_dentry_init,
 	.d_release	= fuse_dentry_release,
+	.d_canonical_path = fuse_dentry_canonical_path,
 };
 
 const struct dentry_operations fuse_root_dentry_operations = {

fs/fuse/fuse_i.h

@@ -213,6 +213,9 @@ struct fuse_out {
 
 	/** Array of arguments */
 	struct fuse_arg args[2];
+
+	/* Path used for completing d_canonical_path */
+	struct path *canonical_path;
 };
 
 /** FUSE page descriptor */
@@ -235,6 +238,9 @@ struct fuse_args {
 		unsigned argvar:1;
 		unsigned numargs;
 		struct fuse_arg args[2];
+
+		/* Path used for completing d_canonical_path */
+		struct path *canonical_path;
 	} out;
 };
 

include/uapi/linux/fuse.h

@@ -383,6 +383,7 @@ enum fuse_opcode {
 	FUSE_READDIRPLUS   = 44,
 	FUSE_RENAME2       = 45,
 	FUSE_LSEEK         = 46,
+	FUSE_CANONICAL_PATH= 2016,
 
 	/* CUSE specific operations */
 	CUSE_INIT          = 4096,