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Subject: [PATCH 11/23] fs: Add /proc/namespaces/ directory
From:   Kirill Tkhai <ktkhai@virtuozzo.com>
To:     viro@zeniv.linux.org.uk, adobriyan@gmail.com, davem@davemloft.net,
        ebiederm@xmission.com, akpm@linux-foundation.org,
        christian.brauner@ubuntu.com, areber@redhat.com, serge@hallyn.com,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        ktkhai@virtuozzo.com
Date:   Thu, 30 Jul 2020 15:00:19 +0300
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This is a new directory to show all namespaces, which can be
accessed from this /proc tasks credentials.

Every /proc is related to a pid_namespace, and the pid_namespace
is related to a user_namespace. The items, we show in this
/proc/namespaces/ directory, are the namespaces,
whose user_namespaces are the same as /proc's user_namespace,
or their descendants.

Say, /proc has pid_ns->user_ns, so in /proc/namespace we show
only a ns, which is in_userns(pid_ns->user_ns, ns->user_ns).

The final result is like below:

# ls /proc/namespaces/ -l
lrwxrwxrwx 1 root root 0 Jul 29 16:50 'cgroup:[4026531835]' -> 'cgroup:[4026531835]'
lrwxrwxrwx 1 root root 0 Jul 29 16:50 'ipc:[4026531839]' -> 'ipc:[4026531839]'
lrwxrwxrwx 1 root root 0 Jul 29 16:50 'mnt:[4026531840]' -> 'mnt:[4026531840]'
lrwxrwxrwx 1 root root 0 Jul 29 16:50 'mnt:[4026531861]' -> 'mnt:[4026531861]'
lrwxrwxrwx 1 root root 0 Jul 29 16:50 'mnt:[4026532133]' -> 'mnt:[4026532133]'
lrwxrwxrwx 1 root root 0 Jul 29 16:50 'mnt:[4026532134]' -> 'mnt:[4026532134]'
lrwxrwxrwx 1 root root 0 Jul 29 16:50 'mnt:[4026532135]' -> 'mnt:[4026532135]'
lrwxrwxrwx 1 root root 0 Jul 29 16:50 'mnt:[4026532136]' -> 'mnt:[4026532136]'
lrwxrwxrwx 1 root root 0 Jul 29 16:50 'net:[4026531993]' -> 'net:[4026531993]'
lrwxrwxrwx 1 root root 0 Jul 29 16:50 'pid:[4026531836]' -> 'pid:[4026531836]'
lrwxrwxrwx 1 root root 0 Jul 29 16:50 'time:[4026531834]' -> 'time:[4026531834]'
lrwxrwxrwx 1 root root 0 Jul 29 16:50 'user:[4026531837]' -> 'user:[4026531837]'
lrwxrwxrwx 1 root root 0 Jul 29 16:50 'uts:[4026531838]' -> 'uts:[4026531838]'

Every namespace may be open like ordinary file in /proc/[pid]/ns.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
---
 fs/nsfs.c               |    2 
 fs/proc/Makefile        |    1 
 fs/proc/internal.h      |   16 ++
 fs/proc/namespaces.c    |  314 +++++++++++++++++++++++++++++++++++++++++++++++
 fs/proc/root.c          |   17 ++-
 include/linux/proc_fs.h |    1 
 6 files changed, 345 insertions(+), 6 deletions(-)
 create mode 100644 fs/proc/namespaces.c

diff --git a/fs/nsfs.c b/fs/nsfs.c
index ee4be67d3a0b..61b789d2089c 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -58,7 +58,7 @@ static void nsfs_evict(struct inode *inode)
 	ns->ops->put(ns);
 }
 
-static int __ns_get_path(struct path *path, struct ns_common *ns)
+int __ns_get_path(struct path *path, struct ns_common *ns)
 {
 	struct vfsmount *mnt = nsfs_mnt;
 	struct dentry *dentry;
diff --git a/fs/proc/Makefile b/fs/proc/Makefile
index dc2d51f42905..34ff671c6d59 100644
--- a/fs/proc/Makefile
+++ b/fs/proc/Makefile
@@ -25,6 +25,7 @@ proc-y	+= util.o
 proc-y	+= version.o
 proc-y	+= softirqs.o
 proc-y	+= task_namespaces.o
+proc-y	+= namespaces.o
 proc-y	+= self.o
 proc-y	+= thread_self.o
 proc-$(CONFIG_PROC_SYSCTL)	+= proc_sysctl.o
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index 572757ff97be..d19fe5574799 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -134,10 +134,11 @@ void task_dump_owner(struct task_struct *task, umode_t mode,
 		     kuid_t *ruid, kgid_t *rgid);
 
 unsigned name_to_int(const struct qstr *qstr);
-/*
- * Offset of the first process in the /proc root directory..
- */
-#define FIRST_PROCESS_ENTRY 256
+
+/* Offset of "namespaces" entry in /proc root directory */
+#define NAMESPACES_ENTRY 256
+/* Offset of the first process in the /proc root directory */
+#define FIRST_PROCESS_ENTRY (NAMESPACES_ENTRY + 1)
 
 /* Worst case buffer size needed for holding an integer. */
 #define PROC_NUMBUF 13
@@ -168,6 +169,7 @@ extern void proc_pid_evict_inode(struct proc_inode *);
 extern struct inode *proc_pid_make_inode(struct super_block *, struct task_struct *, umode_t);
 extern void pid_update_inode(struct task_struct *, struct inode *);
 extern int pid_delete_dentry(const struct dentry *);
+extern int proc_emit_namespaces(struct file *, struct dir_context *);
 extern int proc_pid_readdir(struct file *, struct dir_context *);
 struct dentry *proc_pid_lookup(struct dentry *, unsigned int);
 extern loff_t mem_lseek(struct file *, loff_t, int);
@@ -222,6 +224,12 @@ void set_proc_pid_nlink(void);
 extern struct inode *proc_get_inode(struct super_block *, struct proc_dir_entry *);
 extern void proc_entry_rundown(struct proc_dir_entry *);
 
+/*
+ * namespaces.c
+ */
+extern int proc_setup_namespaces(struct super_block *);
+extern void proc_namespaces_init(void);
+
 /*
  * task_namespaces.c
  */
diff --git a/fs/proc/namespaces.c b/fs/proc/namespaces.c
new file mode 100644
index 000000000000..ab47e1555619
--- /dev/null
+++ b/fs/proc/namespaces.c
@@ -0,0 +1,314 @@
+#include <linux/pid_namespace.h>
+#include <linux/user_namespace.h>
+#include <linux/namei.h>
+#include "internal.h"
+
+static unsigned namespaces_inum __ro_after_init;
+
+int proc_emit_namespaces(struct file *file, struct dir_context *ctx)
+{
+	struct proc_fs_info *fs_info = proc_sb_info(file_inode(file)->i_sb);
+	struct inode *inode = d_inode(fs_info->proc_namespaces);
+
+	return dir_emit(ctx, "namespaces", 10, inode->i_ino, DT_DIR);
+}
+
+static int parse_namespace_dentry_name(const struct dentry *dentry,
+		const char **type, unsigned int *type_len, unsigned int *inum)
+{
+	const char *p, *name;
+	int count;
+
+	*type = name = dentry->d_name.name;
+	p = strchr(name, ':');
+	*type_len = p - name;
+	if (!p || p == name)
+		return -ENOENT;
+
+	p += 1;
+	if (sscanf(p, "[%u]%n", inum, &count) != 1 || *(p + count) != '\0' ||
+	    *inum < PROC_NS_MIN_INO)
+		return -ENOENT;
+
+	return 0;
+}
+
+static struct ns_common *get_namespace_by_dentry(struct pid_namespace *pid_ns,
+						 const struct dentry *dentry)
+{
+	unsigned int type_len, inum, p_inum;
+	struct user_namespace *user_ns;
+	struct ns_common *ns;
+	const char *type;
+
+	if (parse_namespace_dentry_name(dentry, &type, &type_len, &inum) < 0)
+		return NULL;
+
+	p_inum = inum - 1;
+	ns = ns_get_next(&p_inum);
+	if (!ns)
+		return NULL;
+
+	if (ns->inum != inum || strncmp(type, ns->ops->name, type_len) != 0 ||
+	    ns->ops->name[type_len] != '\0') {
+		ns->ops->put(ns);
+		return NULL;
+	}
+
+	if (ns->ops != &userns_operations)
+		user_ns = ns->ops->owner(ns);
+	else
+		user_ns = container_of(ns, struct user_namespace, ns);
+
+	if (!in_userns(pid_ns->user_ns, user_ns)) {
+		ns->ops->put(ns);
+		return NULL;
+	}
+
+	return ns;
+}
+
+static struct dentry *proc_namespace_instantiate(struct dentry *dentry,
+		struct task_struct *task, const void *ptr);
+
+static struct dentry *proc_namespaces_lookup(struct inode *dir, struct dentry *dentry,
+					     unsigned int flags)
+{
+	struct pid_namespace *pid_ns = proc_pid_ns(dir->i_sb);
+	struct task_struct *task;
+	struct ns_common *ns;
+
+	ns = get_namespace_by_dentry(pid_ns, dentry);
+	if (!ns)
+		return ERR_PTR(-ENOENT);
+
+	read_lock(&tasklist_lock);
+	task = get_task_struct(pid_ns->child_reaper);
+	read_unlock(&tasklist_lock);
+
+	dentry = proc_namespace_instantiate(dentry, task, ns);
+	put_task_struct(task);
+	ns->ops->put(ns);
+
+	return dentry;
+}
+
+static int proc_namespaces_permission(struct inode *inode, int mask)
+{
+	if ((mask & MAY_EXEC) && S_ISLNK(inode->i_mode))
+		return -EACCES;
+
+	return 0;
+}
+
+static int proc_namespaces_getattr(const struct path *path, struct kstat *stat,
+				   u32 request_mask, unsigned int query_flags)
+{
+	struct inode *inode = d_inode(path->dentry);
+
+	generic_fillattr(inode, stat);
+	return 0;
+}
+
+static const struct inode_operations proc_namespaces_inode_operations = {
+	.lookup		= proc_namespaces_lookup,
+	.permission	= proc_namespaces_permission,
+	.getattr	= proc_namespaces_getattr,
+};
+
+static int proc_namespaces_readlink(struct dentry *dentry, char __user *buffer, int buflen)
+{
+	struct inode *dir = dentry->d_parent->d_inode;
+	struct pid_namespace *pid_ns = proc_pid_ns(dir->i_sb);
+	struct ns_common *ns;
+
+	ns = get_namespace_by_dentry(pid_ns, dentry);
+	if (!ns)
+		return -ENOENT;
+	ns->ops->put(ns);
+
+	/* proc_namespaces_readdir() creates dentry names in namespace format */
+	return readlink_copy(buffer, buflen, dentry->d_iname);
+}
+
+int __ns_get_path(struct path *path, struct ns_common *ns);
+
+static const char *proc_namespaces_getlink(struct dentry *dentry,
+				struct inode *inode, struct delayed_call *done)
+{
+	struct pid_namespace *pid_ns = proc_pid_ns(inode->i_sb);
+	struct ns_common *ns;
+	struct path path;
+	int ret;
+
+	if (!dentry)
+		return ERR_PTR(-ECHILD);
+
+	while (1) {
+		ret = -ENOENT;
+		ns = get_namespace_by_dentry(pid_ns, dentry);
+		if (!ns)
+			goto out;
+
+		ret = __ns_get_path(&path, ns);
+		if (ret == -EAGAIN)
+			continue;
+		if (ret)
+			goto out;
+		break;
+	}
+
+	ret = nd_jump_link(&path);
+out:
+	return ERR_PTR(ret);
+}
+
+static const struct inode_operations proc_namespaces_link_inode_operations = {
+	.readlink	= proc_namespaces_readlink,
+	.get_link	= proc_namespaces_getlink,
+};
+
+static int namespace_delete_dentry(const struct dentry *dentry)
+{
+	struct inode *dir = dentry->d_parent->d_inode;
+	struct pid_namespace *pid_ns = proc_pid_ns(dir->i_sb);
+	struct ns_common *ns;
+
+	ns = get_namespace_by_dentry(pid_ns, dentry);
+	if (!ns)
+		return 1;
+
+	ns->ops->put(ns);
+	return 0;
+}
+
+const struct dentry_operations namespaces_dentry_operations = {
+	.d_delete	= namespace_delete_dentry,
+};
+
+static void namespace_update_inode(struct inode *inode)
+{
+	struct user_namespace *user_ns = proc_pid_ns(inode->i_sb)->user_ns;
+
+	inode->i_uid = make_kuid(user_ns, 0);
+	if (!uid_valid(inode->i_uid))
+		inode->i_uid = GLOBAL_ROOT_UID;
+
+	inode->i_gid = make_kgid(user_ns, 0);
+	if (!gid_valid(inode->i_gid))
+		inode->i_gid = GLOBAL_ROOT_GID;
+}
+
+static struct dentry *proc_namespace_instantiate(struct dentry *dentry,
+	struct task_struct *task, const void *ptr)
+{
+	const struct ns_common *ns = ptr;
+	struct inode *inode;
+	struct proc_inode *ei;
+
+	/*
+	 * Create inode with credentials of @task, and add it to @task's
+	 * quick removal list.
+	 */
+	inode = proc_pid_make_inode(dentry->d_sb, task, S_IFLNK | S_IRWXUGO);
+	if (!inode)
+		return ERR_PTR(-ENOENT);
+
+	ei = PROC_I(inode);
+	inode->i_op = &proc_namespaces_link_inode_operations;
+	ei->ns_ops = ns->ops;
+	namespace_update_inode(inode);
+
+	d_set_d_op(dentry, &namespaces_dentry_operations);
+	return d_splice_alias(inode, dentry);
+}
+
+static int proc_namespaces_readdir(struct file *file, struct dir_context *ctx)
+{
+	struct pid_namespace *pid_ns = proc_pid_ns(file_inode(file)->i_sb);
+	struct user_namespace *user_ns;
+	struct task_struct *task;
+	struct ns_common *ns;
+	unsigned int inum;
+
+	read_lock(&tasklist_lock);
+	task = get_task_struct(pid_ns->child_reaper);
+	read_unlock(&tasklist_lock);
+
+	if (!dir_emit_dots(file, ctx))
+		goto out;
+
+	inum = ctx->pos - 2;
+	while ((ns = ns_get_next(&inum)) != NULL) {
+		unsigned int len;
+		char name[32];
+
+		if (ns->ops != &userns_operations)
+			user_ns = ns->ops->owner(ns);
+		else
+			user_ns = container_of(ns, struct user_namespace, ns);
+
+		if (!in_userns(pid_ns->user_ns, user_ns))
+			goto next;
+
+		len = snprintf(name, sizeof(name), "%s:[%u]", ns->ops->name, inum);
+
+		if (!proc_fill_cache(file, ctx, name, len,
+			proc_namespace_instantiate, task, ns)) {
+			ns->ops->put(ns);
+			break;
+		}
+next:
+		ns->ops->put(ns);
+		ctx->pos = inum + 2;
+	}
+out:
+	put_task_struct(task);
+	return 0;
+}
+
+static const struct file_operations proc_namespaces_file_operations = {
+	.read		= generic_read_dir,
+	.iterate_shared	= proc_namespaces_readdir,
+	.llseek		= generic_file_llseek,
+};
+
+int proc_setup_namespaces(struct super_block *s)
+{
+	struct proc_fs_info *fs_info = proc_sb_info(s);
+	struct inode *root_inode = d_inode(s->s_root);
+	struct dentry *namespaces;
+	int ret = -ENOMEM;
+
+	inode_lock(root_inode);
+	namespaces = d_alloc_name(s->s_root, "namespaces");
+	if (namespaces) {
+		struct inode *inode = new_inode_pseudo(s);
+		if (inode) {
+			inode->i_ino = namespaces_inum;
+			inode->i_mtime = inode->i_atime = inode->i_ctime = current_time(inode);
+			inode->i_mode = S_IFDIR | S_IRUGO | S_IXUGO;
+			inode->i_uid = GLOBAL_ROOT_UID;
+			inode->i_gid = GLOBAL_ROOT_GID;
+			inode->i_op = &proc_namespaces_inode_operations;
+			inode->i_fop = &proc_namespaces_file_operations;
+			d_add(namespaces, inode);
+			ret = 0;
+		} else {
+			dput(namespaces);
+		}
+	}
+	inode_unlock(root_inode);
+
+	if (ret)
+		pr_err("proc_setup_namespaces: can't allocate /proc/namespaces\n");
+	else
+		fs_info->proc_namespaces = namespaces;
+
+	return ret;
+}
+
+void __init proc_namespaces_init(void)
+{
+	proc_alloc_inum(&namespaces_inum);
+}
diff --git a/fs/proc/root.c b/fs/proc/root.c
index 5e444d4f9717..e4e4f90fca3d 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -206,6 +206,10 @@ static int proc_fill_super(struct super_block *s, struct fs_context *fc)
 		return -ENOMEM;
 	}
 
+	ret = proc_setup_namespaces(s);
+	if (ret)
+		return ret;
+
 	ret = proc_setup_self(s);
 	if (ret) {
 		return ret;
@@ -272,6 +276,9 @@ static void proc_kill_sb(struct super_block *sb)
 	dput(fs_info->proc_self);
 	dput(fs_info->proc_thread_self);
 
+	if (fs_info->proc_namespaces)
+		dput(fs_info->proc_namespaces);
+
 	kill_anon_super(sb);
 	put_pid_ns(fs_info->pid_ns);
 	kfree(fs_info);
@@ -289,6 +296,7 @@ void __init proc_root_init(void)
 {
 	proc_init_kmemcache();
 	set_proc_pid_nlink();
+	proc_namespaces_init();
 	proc_self_init();
 	proc_thread_self_init();
 	proc_symlink("mounts", NULL, "self/mounts");
@@ -326,8 +334,15 @@ static struct dentry *proc_root_lookup(struct inode * dir, struct dentry * dentr
 
 static int proc_root_readdir(struct file *file, struct dir_context *ctx)
 {
-	if (ctx->pos < FIRST_PROCESS_ENTRY) {
+	if (ctx->pos < NAMESPACES_ENTRY) {
 		int error = proc_readdir(file, ctx);
+		if (unlikely(error <= 0))
+			return error;
+		ctx->pos = NAMESPACES_ENTRY;
+	}
+
+	if (ctx->pos == NAMESPACES_ENTRY) {
+		int error = proc_emit_namespaces(file, ctx);
 		if (unlikely(error <= 0))
 			return error;
 		ctx->pos = FIRST_PROCESS_ENTRY;
diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
index 97b3f5f06db9..8b0002a6cacf 100644
--- a/include/linux/proc_fs.h
+++ b/include/linux/proc_fs.h
@@ -61,6 +61,7 @@ struct proc_fs_info {
 	struct pid_namespace *pid_ns;
 	struct dentry *proc_self;        /* For /proc/self */
 	struct dentry *proc_thread_self; /* For /proc/thread-self */
+	struct dentry *proc_namespaces;	 /* For /proc/namespaces */
 	kgid_t pid_gid;
 	enum proc_hidepid hide_pid;
 	enum proc_pidonly pidonly;