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Message-Id: <20251202-configfs_netcon-v1-1-b4738ead8ee8@debian.org>
Date: Tue, 02 Dec 2025 07:29:01 -0800
From: Breno Leitao <leitao@...ian.org>
To: Andreas Hindborg <a.hindborg@...nel.org>,
Andrew Lunn <andrew+netdev@...n.ch>,
"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Cc: Breno Leitao <leitao@...ian.org>, linux-kernel@...r.kernel.org,
hch@...radead.org, jlbec@...lplan.org, linux-fsdevel@...r.kernel.org,
netdev@...r.kernel.org, gustavold@...il.com, asantostc@...il.com,
calvin@...nvd.org, kernel-team@...a.com
Subject: [PATCH RFC 1/2] configfs: add kernel-space item registration API
Add configfs_register_item() and configfs_unregister_item() functions
to allow kernel modules to register configfs items whose lifecycle is
controlled by kernel space rather than userspace.
This is useful for subsystems that need to expose configuration items
that are created based on kernel events (like boot parameters) rather
than explicit userspace mkdir operations. The items registered this
way are marked as default items (CONFIGFS_USET_DEFAULT) and cannot be
removed via rmdir.
The API follows the same pattern as configfs_register_group() but for
individual items:
- configfs_register_item() links the item into the parent group's
hierarchy and creates the filesystem representation
- configfs_unregister_item() reverses the registration, removing the
item from configfs
Signed-off-by: Breno Leitao <leitao@...ian.org>
---
fs/configfs/dir.c | 134 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
include/linux/configfs.h | 4 +++
2 files changed, 138 insertions(+)
diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
index 81f4f06bc87e..f7224bc51826 100644
--- a/fs/configfs/dir.c
+++ b/fs/configfs/dir.c
@@ -1866,6 +1866,140 @@ void configfs_unregister_default_group(struct config_group *group)
}
EXPORT_SYMBOL(configfs_unregister_default_group);
+/**
+ * configfs_register_item() - registers a kernel-created item with a parent group
+ * @parent_group: parent group for the new item
+ * @item: item to be registered
+ *
+ * This function allows kernel code to register configfs items whose lifecycle
+ * is controlled by kernel space rather than userspace (via mkdir/rmdir).
+ * The item must be already initialized with config_item_init_type_name().
+ *
+ * Return: 0 on success, negative errno on failure
+ */
+int configfs_register_item(struct config_group *parent_group,
+ struct config_item *item)
+{
+ struct configfs_subsystem *subsys = parent_group->cg_subsys;
+ struct configfs_fragment *frag;
+ struct dentry *parent, *child;
+ struct configfs_dirent *sd;
+ int ret;
+
+ if (!subsys || !item->ci_name)
+ return -EINVAL;
+
+ frag = new_fragment();
+ if (!frag)
+ return -ENOMEM;
+
+ parent = parent_group->cg_item.ci_dentry;
+ /* Allocate dentry for the item */
+ child = d_alloc_name(parent, item->ci_name);
+ if (!child) {
+ put_fragment(frag);
+ return -ENOMEM;
+ }
+
+ mutex_lock(&subsys->su_mutex);
+ link_obj(&parent_group->cg_item, item);
+ mutex_unlock(&subsys->su_mutex);
+
+ inode_lock_nested(d_inode(parent), I_MUTEX_PARENT);
+ d_add(child, NULL);
+
+ /* Attach the item to the filesystem */
+ ret = configfs_attach_item(&parent_group->cg_item, item, child, frag);
+ if (ret)
+ goto err_out;
+
+ /*
+ * CONFIGFS_USET_DEFAULT mark item as kernel-created (cannot be removed
+ * by userspace)
+ */
+ sd = child->d_fsdata;
+ sd->s_type |= CONFIGFS_USET_DEFAULT;
+
+ /* Mark directory as ready */
+ spin_lock(&configfs_dirent_lock);
+ configfs_dir_set_ready(child->d_fsdata);
+ spin_unlock(&configfs_dirent_lock);
+
+ inode_unlock(d_inode(parent));
+ put_fragment(frag);
+ return 0;
+
+err_out:
+ d_drop(child);
+ dput(child);
+ inode_unlock(d_inode(parent));
+ mutex_lock(&subsys->su_mutex);
+ unlink_obj(item);
+ mutex_unlock(&subsys->su_mutex);
+ put_fragment(frag);
+ return ret;
+}
+EXPORT_SYMBOL(configfs_register_item);
+
+/**
+ * configfs_unregister_item() - unregisters a kernel-created item
+ * @item: item to be unregistered
+ *
+ * This function reverses the effect of configfs_register_item(), removing
+ * the item from the configfs filesystem and releasing associated resources.
+ * The item must have been previously registered with configfs_register_item().
+ */
+void configfs_unregister_item(struct config_item *item)
+{
+ struct config_group *group = item->ci_group;
+ struct dentry *dentry = item->ci_dentry;
+ struct configfs_subsystem *subsys;
+ struct configfs_fragment *frag;
+ struct configfs_dirent *sd;
+ struct dentry *parent;
+
+ if (!group || !dentry)
+ return;
+
+ subsys = group->cg_subsys;
+ if (!subsys)
+ return;
+
+ parent = item->ci_parent->ci_dentry;
+ sd = dentry->d_fsdata;
+ frag = get_fragment(sd->s_frag);
+
+ if (WARN_ON(!(sd->s_type & CONFIGFS_USET_DEFAULT))) {
+ put_fragment(frag);
+ return;
+ }
+
+ /* Mark fragment as dead */
+ down_write(&frag->frag_sem);
+ frag->frag_dead = true;
+ up_write(&frag->frag_sem);
+
+ inode_lock_nested(d_inode(parent), I_MUTEX_PARENT);
+ spin_lock(&configfs_dirent_lock);
+ configfs_detach_prep(dentry, NULL);
+ spin_unlock(&configfs_dirent_lock);
+
+ configfs_detach_item(item);
+ d_inode(dentry)->i_flags |= S_DEAD;
+ dont_mount(dentry);
+ d_drop(dentry);
+ fsnotify_rmdir(d_inode(parent), dentry);
+ dput(dentry);
+ inode_unlock(d_inode(parent));
+
+ mutex_lock(&subsys->su_mutex);
+ unlink_obj(item);
+ mutex_unlock(&subsys->su_mutex);
+
+ put_fragment(frag);
+}
+EXPORT_SYMBOL(configfs_unregister_item);
+
int configfs_register_subsystem(struct configfs_subsystem *subsys)
{
int err;
diff --git a/include/linux/configfs.h b/include/linux/configfs.h
index 698520b1bfdb..70f2d113b4b3 100644
--- a/include/linux/configfs.h
+++ b/include/linux/configfs.h
@@ -244,6 +244,10 @@ int configfs_register_group(struct config_group *parent_group,
struct config_group *group);
void configfs_unregister_group(struct config_group *group);
+int configfs_register_item(struct config_group *parent_group,
+ struct config_item *item);
+void configfs_unregister_item(struct config_item *item);
+
void configfs_remove_default_groups(struct config_group *group);
struct config_group *
--
2.47.3
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