namespacefs: Add methods to create/remove PID namespace directories

Each existing namespace on the system will be represented by a
corresponding directory in namespacesfs. When a namespace is created
a new directory will be added. When a namespace is destroyed, its
corresponding directory will be removed. The hierarchy of the
directories will follow the hierarchy of the namespaces. As a first
step towards this, here we add methods for creating/removing PID
namespace directories. For the moment the PID namespace directory
contains only one file called 'tasks'. This is a read only pseudo
file that provides the list of PIDs of all tasks enclosed into the
namespace.

Signed-off-by: Yordan Karadzhov (VMware) <y.karadz@gmail.com>

3 files changed, 108 insertions, 7 deletions

diff --git a/fs/namespacefs/inode.c b/fs/namespacefs/inode.c
index 338694f3e5d643..2f897daf30d7e8 100644
--- a/fs/namespacefs/inode.c
+++ b/fs/namespacefs/inode.c
@@ -150,16 +150,16 @@ static struct dentry *_create(const char *name, struct dentry *parent,
 	return ERR_PTR(-ESTALE);
 }
 
-struct dentry *namespacefs_create_file(const char *name,
-				       struct dentry *parent,
-				       const struct file_operations *fops,
-				       void *data)
+static struct dentry * namespacefs_create_file(const char *name,
+					       struct dentry *parent,
+					       const struct file_operations *fops,
+					       void *data)
 {
 	return _create(name, parent, fops, data);
 }
 
-struct dentry *namespacefs_create_dir(const char *name,
-				      struct dentry *parent)
+static struct dentry *namespacefs_create_dir(const char *name,
+					     struct dentry *parent)
 {
 	return _create(name, parent, NULL, NULL);
 }
@@ -169,7 +169,7 @@ static void _remove_one(struct dentry *d)
 	_release_namespacefs();
 }
 
-void namespacefs_remove_dir(struct dentry *dentry)
+static void namespacefs_remove_dir(struct dentry *dentry)
 {
 	if (IS_ERR_OR_NULL(dentry))
 		return;
@@ -249,6 +249,71 @@ static int idr_seq_open(struct file *file, struct idr *idr,
 	return 0;
 }
 
+static inline int pid_seq_show(struct seq_file *m, void *v)
+{
+	struct pid *pid = v;
+	seq_printf(m, "%d\n", pid_nr(pid));
+	return 0;
+}
+
+static const struct seq_operations pid_seq_ops = {
+	.start		= idr_seq_start,
+	.next		= idr_seq_next,
+	.stop		= idr_seq_stop,
+	.show		= pid_seq_show,
+};
+
+static int pid_seq_open(struct inode *inode, struct file *file)
+{
+	struct idr *idr = inode->i_private;
+	return idr_seq_open(file, idr, &pid_seq_ops);
+}
+
+static const struct file_operations tasks_fops = {
+	.open		= pid_seq_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= idr_seq_release,
+};
+
+static int _create_inod_dir(struct ns_common *ns, struct dentry *parent_dentry)
+{
+	char *dir = kasprintf(GFP_KERNEL, "%u", ns->inum);
+
+	if (!dir)
+		return -ENOMEM;
+
+	ns->dentry = namespacefs_create_dir(dir, parent_dentry);
+	kfree(dir);
+	if (IS_ERR(ns->dentry))
+		return PTR_ERR(ns->dentry);
+
+	return 0;
+}
+
+int namespacefs_create_pid_ns_dir(struct pid_namespace *ns)
+{
+	int err = _create_inod_dir(&ns->ns, ns->parent->ns.dentry);
+	struct dentry *dentry;
+
+	if (err)
+		return err;
+
+	dentry = namespacefs_create_file("tasks", ns->ns.dentry,
+					 &tasks_fops, &ns->idr);
+	if (IS_ERR(dentry)) {
+		dput(ns->ns.dentry);
+		return PTR_ERR(dentry);
+	}
+
+	return 0;
+}
+
+void namespacefs_remove_pid_ns_dir(struct pid_namespace *ns)
+{
+	namespacefs_remove_dir(ns->ns.dentry);
+}
+
 static int __init namespacefs_init(void)
 {
 	int err;
diff --git a/include/linux/namespacefs.h b/include/linux/namespacefs.h
new file mode 100644
index 00000000000000..020bf5bbb3b534
--- /dev/null
+++ b/include/linux/namespacefs.h
@@ -0,0 +1,32 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * namespacefs.h - a pseudo file system for examining namespaces.
+ */
+
+#ifndef _NAMESPACEFS_H_
+#define _NAMESPACEFS_H_
+
+#ifdef CONFIG_NAMESPACE_FS
+
+#include <linux/fs.h>
+
+int namespacefs_create_pid_ns_dir(struct pid_namespace *ns);
+
+void namespacefs_remove_pid_ns_dir(struct pid_namespace *ns);
+
+#else
+
+static inline int
+namespacefs_create_pid_ns_dir(struct pid_namespace *ns)
+{
+	return 0;
+}
+
+static inline void
+namespacefs_remove_pid_ns_dir(struct pid_namespace *ns)
+{
+}
+
+#endif /* CONFIG_NAMESPACE_FS */
+
+#endif
diff --git a/include/linux/ns_common.h b/include/linux/ns_common.h
index 0f1d024bd95826..1dec75c51b2c52 100644
--- a/include/linux/ns_common.h
+++ b/include/linux/ns_common.h
@@ -11,6 +11,10 @@ struct ns_common {
 	const struct proc_ns_operations *ops;
 	unsigned int inum;
 	refcount_t count;
+
+#ifdef CONFIG_NAMESPACE_FS
+	struct dentry *dentry;
+#endif /* CONFIG_NAMESPACE_FS */
 };
 
 #endif