[RFC PATCH 4/4] namespacefs: Couple namespacefs to the UTS namespace

["Yordan Karadzhov (VMware)" <y.karadz@gmail.com>]

When the UTS namespace gets initialized, a directory called 'uts'
is added to namespacesfs. This directory represents the main UTS
namespace and also serves as a trunk (parent) of all other UTS
namespaces. Every time when a new UTS namespace is created a
corresponding directory is added to 'namespacefs/uts/'. The 'inum'
of the new namespace gives the name of its directory. When a UTS
namespace is destroyed the corresponding directory is removed. Each
directory contains a file called 'uname' that can be used to get the
unique data fields of the uts namespaces(sysname, nodename, ...).

Signed-off-by: Yordan Karadzhov (VMware) <y.karadz@gmail.com>
---
 fs/namespacefs/inode.c      | 57 +++++++++++++++++++++++++++++++++++++
 include/linux/namespacefs.h | 13 +++++++++
 kernel/utsname.c            |  9 ++++++
 3 files changed, 79 insertions(+)

diff --git a/fs/namespacefs/inode.c b/fs/namespacefs/inode.c
index 55d71733164c..4b661bdd4d9c 100644
--- a/fs/namespacefs/inode.c
+++ b/fs/namespacefs/inode.c
@@ -14,6 +14,7 @@
 #include <linux/proc_ns.h>
 #include <linux/seq_file.h>
 #include <linux/pid_namespace.h>
+#include <linux/utsname.h>
 
 static struct vfsmount *namespacefs_mount;
 static int namespacefs_mount_count;
@@ -309,6 +310,58 @@ void namespacefs_remove_pid_ns_dir(struct pid_namespace *ns)
 	namespacefs_remove_dir(ns->ns.dentry);
 }
 
+#define _UNAME_N_FIELDS		6
+#define _UNAME_MAX_LEN		((__NEW_UTS_LEN + 1) * _UNAME_N_FIELDS)
+
+static ssize_t uts_ns_read(struct file *file, char __user *ubuf,
+			   size_t count, loff_t *pos)
+{
+	struct new_utsname *name = file->private_data;
+	char buff[_UNAME_MAX_LEN + 1];
+	int n;
+
+	n = snprintf(buff, _UNAME_MAX_LEN + 1,
+		     "%s %s %s %s %s %s\n",
+		     name->sysname,
+		     name->nodename,
+		     name->release,
+		     name->version,
+		     name->machine,
+		     name->domainname);
+
+	return simple_read_from_buffer(ubuf, count, pos, buff, n);
+}
+
+static const struct file_operations uts_fops = {
+	.open = simple_open,
+	.read = uts_ns_read,
+	.llseek = default_llseek,
+};
+
+int namespacefs_create_uts_ns_dir(struct uts_namespace *ns)
+{
+	struct dentry *dentry;
+	int err;
+
+	err = create_inode_dir(&ns->ns, init_uts_ns.ns.dentry, ns->user_ns);
+	if (err)
+		return err;
+
+	dentry = namespacefs_create_file("uname", ns->ns.dentry, ns->user_ns,
+					 &uts_fops, &ns->name);
+	if (IS_ERR(dentry)) {
+		dput(ns->ns.dentry);
+		return PTR_ERR(dentry);
+	}
+
+	return 0;
+}
+
+void namespacefs_remove_uts_ns_dir(struct uts_namespace *ns)
+{
+	namespacefs_remove_dir(ns->ns.dentry);
+}
+
 static int add_ns_dentry(struct ns_common *ns)
 {
 	struct dentry *dentry =
@@ -340,6 +393,10 @@ static int __init namespacefs_init(void)
 	if (err)
 		goto unreg;
 
+	err = add_ns_dentry(&init_uts_ns.ns);
+	if (err)
+		goto unreg;
+
 	return 0;
 
  unreg:
diff --git a/include/linux/namespacefs.h b/include/linux/namespacefs.h
index f41499a7635a..3815a7bbeb1c 100644
--- a/include/linux/namespacefs.h
+++ b/include/linux/namespacefs.h
@@ -21,6 +21,8 @@ namespacefs_create_dir(const char *name, struct dentry *parent,
 void namespacefs_remove_dir(struct dentry *dentry);
 int namespacefs_create_pid_ns_dir(struct pid_namespace *ns);
 void namespacefs_remove_pid_ns_dir(struct pid_namespace *ns);
+int namespacefs_create_uts_ns_dir(struct uts_namespace *ns);
+void namespacefs_remove_uts_ns_dir(struct uts_namespace *ns);
 
 #else
 
@@ -55,6 +57,17 @@ namespacefs_remove_pid_ns_dir(struct pid_namespace *ns)
 {
 }
 
+static inline int
+namespacefs_create_uts_ns_dir(struct uts_namespace *ns)
+{
+	return 0;
+}
+
+static inline void
+namespacefs_remove_uts_ns_dir(struct uts_namespace *ns)
+{
+}
+
 #endif /* CONFIG_NAMESPACE_FS */
 
 #endif
diff --git a/kernel/utsname.c b/kernel/utsname.c
index b1ac3ca870f2..d44b307cffdc 100644
--- a/kernel/utsname.c
+++ b/kernel/utsname.c
@@ -12,6 +12,7 @@
 #include <linux/slab.h>
 #include <linux/cred.h>
 #include <linux/user_namespace.h>
+#include <linux/namespacefs.h>
 #include <linux/proc_ns.h>
 #include <linux/sched/task.h>
 
@@ -70,8 +71,15 @@ static struct uts_namespace *clone_uts_ns(struct user_namespace *user_ns,
 	memcpy(&ns->name, &old_ns->name, sizeof(ns->name));
 	ns->user_ns = get_user_ns(user_ns);
 	up_read(&uts_sem);
+
+	err = namespacefs_create_uts_ns_dir(ns);
+	if (err)
+		goto fail_free_inum;
+
 	return ns;
 
+fail_free_inum:
+	ns_free_inum(&ns->ns);
 fail_free:
 	kmem_cache_free(uts_ns_cache, ns);
 fail_dec:
@@ -105,6 +113,7 @@ struct uts_namespace *copy_utsname(unsigned long flags,
 
 void free_uts_ns(struct uts_namespace *ns)
 {
+	namespacefs_remove_uts_ns_dir(ns);
 	dec_uts_namespaces(ns->ucounts);
 	put_user_ns(ns->user_ns);
 	ns_free_inum(&ns->ns);
-- 
2.33.1