[RFC PATCH 5 1/7] kmod - add workqueue service thread store

[Ian Kent <raven@themaw.net>]

From: Ian Kent <ikent@redhat.com>

Persistent use of process information is needed for contained
execution in a namespace other than the root init namespace.

Use a simple random token as a key to create and store thread
information in a hashed list for use by the usermode helper
thread runner.

Signed-off-by: Ian Kent <ikent@redhat.com>
Cc: Benjamin Coddington <bcodding@redhat.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: J. Bruce Fields <bfields@fieldses.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: Oleg Nesterov <onestero@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Jeff Layton <jeff.layton@primarydata.com>
---
 include/linux/kmod.h |    3 +
 kernel/kmod.c        |  179 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 181 insertions(+), 1 deletion(-)

diff --git a/include/linux/kmod.h b/include/linux/kmod.h
index 0555cc6..fa46722 100644
--- a/include/linux/kmod.h
+++ b/include/linux/kmod.h
@@ -66,6 +66,9 @@ struct subprocess_info {
 	void *data;
 };
 
+extern int umh_wq_get_token(int token, const char *service);
+extern void umh_wq_put_token(int token);
+
 extern int
 call_usermodehelper(char *path, char **argv, char **envp, int wait);
 
diff --git a/kernel/kmod.c b/kernel/kmod.c
index 2777f40..55d20ce 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -40,13 +40,30 @@
 #include <linux/ptrace.h>
 #include <linux/async.h>
 #include <asm/uaccess.h>
+#include <linux/hash.h>
+#include <linux/list.h>
+#include <linux/random.h>
 
 #include <trace/events/module.h>
 
 extern int max_threads;
 
+#define KHELPER		"khelper"
 static struct workqueue_struct *khelper_wq;
 
+#define UMH_WQ_HASH_SHIFT  6
+#define UMH_WQ_HASH_SIZE   1 << UMH_WQ_HASH_SHIFT
+
+struct umh_wq_entry {
+	int token;
+	unsigned int count;
+	struct workqueue_struct *wq;
+	struct hlist_node umh_wq_hlist;
+};
+
+static DEFINE_SPINLOCK(umh_wq_hash_lock);
+static struct hlist_head umh_wq_hash[UMH_WQ_HASH_SIZE];
+
 #define CAP_BSET	(void *)1
 #define CAP_PI		(void *)2
 
@@ -475,6 +492,165 @@ static void helper_unlock(void)
 		wake_up(&running_helpers_waitq);
 }
 
+static void umh_wq_hash_init(void)
+{
+	int i;
+
+	for (i = 0; i < UMH_WQ_HASH_SIZE; i++)
+		INIT_HLIST_HEAD(&umh_wq_hash[i]);
+}
+
+static struct umh_wq_entry *umh_wq_find_entry(int token)
+{
+	struct umh_wq_entry *this, *entry;
+	struct hlist_head *bucket;
+	unsigned int hash;
+
+	hash = hash_32((unsigned long) token, UMH_WQ_HASH_SHIFT);
+	bucket = &umh_wq_hash[hash];
+
+	entry = ERR_PTR(-ENOENT);
+	if (hlist_empty(bucket))
+		goto out;
+
+	hlist_for_each_entry(this, bucket, umh_wq_hlist) {
+		if (this->token == token) {
+			entry = this;
+			break;
+		}
+	}
+out:
+	return entry;
+}
+
+static struct workqueue_struct *umh_find_wq(int token, unsigned int nowait)
+{
+	struct umh_wq_entry *entry;
+	unsigned long flags;
+
+	if (!token)
+		return khelper_wq;
+
+	if (nowait)
+		spin_lock_irqsave(&umh_wq_hash_lock, flags);
+	else
+		spin_lock(&umh_wq_hash_lock);
+	entry = umh_wq_find_entry(token);
+	if (nowait)
+		spin_unlock_irqrestore(&umh_wq_hash_lock, flags);
+	else
+		spin_unlock(&umh_wq_hash_lock);
+
+	return entry->wq;
+}
+
+/**
+ * umh_wq_get_token - create service thread and return an identifying token
+ * @token: token of an existing service thread or 0 to create a new
+ * 	   service thread.
+ * @name: service name to be appended to "khelper" for identification.
+ *
+ * Returns a token that used with calls to call_usermode_helper_service().
+ * If token corresponds to an existing service thread its reference count
+ * is increased and the token returned. On failure returns a negative errno.
+ */
+int umh_wq_get_token(int token, const char *service)
+{
+	struct workqueue_struct *wq;
+	char *wq_name;
+	int wq_name_len;
+	struct umh_wq_entry *entry;
+	struct hlist_head *bucket;
+	unsigned int hash;
+	unsigned int new_token;
+
+	if (token) {
+		spin_lock(&umh_wq_hash_lock);
+		entry = umh_wq_find_entry(token);
+		if (entry) {
+			entry->count++;
+			spin_unlock(&umh_wq_hash_lock);
+			return token;
+		}
+		spin_unlock(&umh_wq_hash_lock);
+	}
+
+	if (!service)
+		return -EINVAL;
+
+	wq_name_len = sizeof(KHELPER) + strlen(service) + 1;
+	wq_name = kmalloc(wq_name_len, GFP_KERNEL);
+	if (!wq_name)
+		return -ENOMEM;
+	strcpy(wq_name, "KHELPER-");
+	strcat(wq_name, service);
+
+	entry = kzalloc(sizeof(struct umh_wq_entry), GFP_KERNEL);
+	if (!entry) {
+		kfree(wq_name);
+		return -ENOMEM;
+	}
+
+	wq = create_singlethread_workqueue(wq_name);
+	if (IS_ERR(wq)) {
+		kfree(wq_name);
+		kfree(entry);
+		return PTR_ERR(wq);
+	}
+	kfree(wq_name);
+	entry->wq = wq;
+
+	do {
+		new_token = get_random_int();
+		if (!new_token)
+			continue;
+		spin_lock(&umh_wq_hash_lock);
+		entry = umh_wq_find_entry(new_token);
+		if (likely(IS_ERR(entry))) {
+			hash = hash_32(new_token, UMH_WQ_HASH_SHIFT);
+			bucket = &umh_wq_hash[hash];
+			hlist_add_head(&entry->umh_wq_hlist, bucket);
+			entry->token = (long) new_token;
+			spin_unlock(&umh_wq_hash_lock);
+			break;
+		}
+		spin_unlock(&umh_wq_hash_lock);
+	} while (1);
+
+	return (int) new_token;
+}
+EXPORT_SYMBOL(umh_wq_get_token);
+
+/**
+ * umh_ns_put_token - stop a service thread if it's not longer in use
+ * 		      and remove it from the serive thread list
+ * @token: token of a service thread.
+ */
+void umh_wq_put_token(int token)
+{
+	struct umh_wq_entry *entry;
+
+	if (!token)
+		return;
+
+	spin_lock(&umh_wq_hash_lock);
+	entry = umh_wq_find_entry(token);
+	if (unlikely(IS_ERR(entry)))
+		spin_unlock(&umh_wq_hash_lock);
+	else {
+		if (--entry->count)
+			spin_unlock(&umh_wq_hash_lock);
+		else {
+			hlist_del(&entry->umh_wq_hlist);
+			spin_unlock(&umh_wq_hash_lock);
+			destroy_workqueue(entry->wq);
+			kfree(entry);
+		}
+	}
+	return;
+}
+EXPORT_SYMBOL(umh_wq_put_token);
+
 /**
  * call_usermodehelper_setup - prepare to call a usermode helper
  * @path: path to usermode executable
@@ -689,6 +865,7 @@ struct ctl_table usermodehelper_table[] = {
 
 void __init usermodehelper_init(void)
 {
-	khelper_wq = create_singlethread_workqueue("khelper");
+	umh_wq_hash_init();
+	khelper_wq = create_singlethread_workqueue(KHELPER);
 	BUG_ON(!khelper_wq);
 }