[RFC PATCH v3 25/37] fuse-bpf: allow mounting with no userspace daemon

[Daniel Rosenberg <drosen@xxxxxxxxxx>]

When using fuse-bpf in pure passthrough mode, we don't explicitly need a
userspace daemon. This allows simple testing of the backing operations.

Signed-off-by: Daniel Rosenberg <drosen@xxxxxxxxxx>
Signed-off-by: Paul Lawrence <paullawrence@xxxxxxxxxx>
---
 fs/fuse/fuse_i.h |  4 ++++
 fs/fuse/inode.c  | 25 +++++++++++++++++++------
 2 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 121d31a04e79..2bd45c8658e8 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -566,6 +566,7 @@ struct fuse_fs_context {
 	bool no_control:1;
 	bool no_force_umount:1;
 	bool legacy_opts_show:1;
+	bool no_daemon:1;
 	enum fuse_dax_mode dax_mode;
 	unsigned int max_read;
 	unsigned int blksize;
@@ -847,6 +848,9 @@ struct fuse_conn {
 	/* Is tmpfile not implemented by fs? */
 	unsigned int no_tmpfile:1;
 
+	/** BPF Only, no Daemon running */
+	unsigned int no_daemon:1;
+
 	/** The number of requests waiting for completion */
 	atomic_t num_waiting;
 
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 3dfb9cfb6e73..31f34962bc9b 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -756,6 +756,7 @@ enum {
 	OPT_MAX_READ,
 	OPT_BLKSIZE,
 	OPT_ROOT_DIR,
+	OPT_NO_DAEMON,
 	OPT_ERR
 };
 
@@ -771,6 +772,7 @@ static const struct fs_parameter_spec fuse_fs_parameters[] = {
 	fsparam_u32	("blksize",		OPT_BLKSIZE),
 	fsparam_string	("subtype",		OPT_SUBTYPE),
 	fsparam_u32	("root_dir",		OPT_ROOT_DIR),
+	fsparam_flag	("no_daemon",		OPT_NO_DAEMON),
 	{}
 };
 
@@ -860,6 +862,11 @@ static int fuse_parse_param(struct fs_context *fsc, struct fs_parameter *param)
 			return invalfc(fsc, "Unable to open root directory");
 		break;
 
+	case OPT_NO_DAEMON:
+		ctx->no_daemon = true;
+		ctx->fd_present = true;
+		break;
+
 	default:
 		return -EINVAL;
 	}
@@ -1419,7 +1426,7 @@ void fuse_send_init(struct fuse_mount *fm)
 	ia->args.nocreds = true;
 	ia->args.end = process_init_reply;
 
-	if (fuse_simple_background(fm, &ia->args, GFP_KERNEL) != 0)
+	if (unlikely(fm->fc->no_daemon) || fuse_simple_background(fm, &ia->args, GFP_KERNEL) != 0)
 		process_init_reply(fm, &ia->args, -ENOTCONN);
 }
 EXPORT_SYMBOL_GPL(fuse_send_init);
@@ -1694,6 +1701,7 @@ int fuse_fill_super_common(struct super_block *sb, struct fuse_fs_context *ctx)
 	fc->destroy = ctx->destroy;
 	fc->no_control = ctx->no_control;
 	fc->no_force_umount = ctx->no_force_umount;
+	fc->no_daemon = ctx->no_daemon;
 
 	err = -ENOMEM;
 	root = fuse_get_root_inode(sb, ctx->rootmode, ctx->root_dir);
@@ -1740,7 +1748,7 @@ static int fuse_fill_super(struct super_block *sb, struct fs_context *fsc)
 	struct fuse_fs_context *ctx = fsc->fs_private;
 	int err;
 
-	if (!ctx->file || !ctx->rootmode_present ||
+	if (!!ctx->file == ctx->no_daemon || !ctx->rootmode_present ||
 	    !ctx->user_id_present || !ctx->group_id_present)
 		return -EINVAL;
 
@@ -1748,10 +1756,12 @@ static int fuse_fill_super(struct super_block *sb, struct fs_context *fsc)
 	 * Require mount to happen from the same user namespace which
 	 * opened /dev/fuse to prevent potential attacks.
 	 */
-	if ((ctx->file->f_op != &fuse_dev_operations) ||
-	    (ctx->file->f_cred->user_ns != sb->s_user_ns))
-		return -EINVAL;
-	ctx->fudptr = &ctx->file->private_data;
+	if (ctx->file) {
+		if ((ctx->file->f_op != &fuse_dev_operations) ||
+		    (ctx->file->f_cred->user_ns != sb->s_user_ns))
+			return -EINVAL;
+		ctx->fudptr = &ctx->file->private_data;
+	}
 
 	err = fuse_fill_super_common(sb, ctx);
 	if (err)
@@ -1801,6 +1811,9 @@ static int fuse_get_tree(struct fs_context *fsc)
 
 	fsc->s_fs_info = fm;
 
+	if (ctx->no_daemon)
+		return get_tree_nodev(fsc, fuse_fill_super);;
+
 	if (ctx->fd_present)
 		ctx->file = fget(ctx->fd);
 
-- 
2.40.0.634.g4ca3ef3211-goog