[RFC PATCH 3/5] use next syscall data to predefine process ids

[Nadia.Derbey-6ktuUTfB/bM@public.gmane.org]

[-- Attachment #1: proccreate_use_next_syscall_data.patch --]
[-- Type: text/plain, Size: 6648 bytes --]

[PATCH 03/05]

This patch uses the value written into the next_syscall_data proc file
as a target upid nr for the next process to be created.
The following syscalls have a new behavior if next_syscall_data is set:
. fork()
. vfork()
. clone()

In the current version, if the process belongs to nested namespaces, only
the upper namespace level upid nr is allowed to be predefined, since there
is not yet a way to take a snapshot of upid nrs at all namespaces levels.

But this can easily be extended in the future.

Signed-off-by: Nadia Derbey <Nadia.Derbey-6ktuUTfB/bM@public.gmane.org>

---
 include/linux/next_syscall_data.h |    2 
 include/linux/pid.h               |    2 
 kernel/fork.c                     |    3 -
 kernel/pid.c                      |  111 ++++++++++++++++++++++++++++++++------
 4 files changed, 98 insertions(+), 20 deletions(-)

Index: linux-2.6.26-rc5-mm3/kernel/pid.c
===================================================================
--- linux-2.6.26-rc5-mm3.orig/kernel/pid.c	2008-07-01 10:25:46.000000000 +0200
+++ linux-2.6.26-rc5-mm3/kernel/pid.c	2008-07-01 11:25:38.000000000 +0200
@@ -122,6 +122,26 @@ static void free_pidmap(struct upid *upi
 	atomic_inc(&map->nr_free);
 }
 
+static inline int alloc_pidmap_page(struct pidmap *map)
+{
+	if (unlikely(!map->page)) {
+		void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
+		/*
+		 * Free the page if someone raced with us
+		 * installing it:
+		 */
+		spin_lock_irq(&pidmap_lock);
+		if (map->page)
+			kfree(page);
+		else
+			map->page = page;
+		spin_unlock_irq(&pidmap_lock);
+		if (unlikely(!map->page))
+			return -1;
+	}
+	return 0;
+}
+
 static int alloc_pidmap(struct pid_namespace *pid_ns)
 {
 	int i, offset, max_scan, pid, last = pid_ns->last_pid;
@@ -134,21 +154,8 @@ static int alloc_pidmap(struct pid_names
 	map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
 	max_scan = (pid_max + BITS_PER_PAGE - 1)/BITS_PER_PAGE - !offset;
 	for (i = 0; i <= max_scan; ++i) {
-		if (unlikely(!map->page)) {
-			void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
-			/*
-			 * Free the page if someone raced with us
-			 * installing it:
-			 */
-			spin_lock_irq(&pidmap_lock);
-			if (map->page)
-				kfree(page);
-			else
-				map->page = page;
-			spin_unlock_irq(&pidmap_lock);
-			if (unlikely(!map->page))
-				break;
-		}
+		if (unlikely(alloc_pidmap_page(map)))
+			break;
 		if (likely(atomic_read(&map->nr_free))) {
 			do {
 				if (!test_and_set_bit(offset, map->page)) {
@@ -182,6 +189,33 @@ static int alloc_pidmap(struct pid_names
 	return -1;
 }
 
+/*
+ * Return 0 if successful (i.e. next_nr could be assigned as a upid nr).
+ * -errno else
+ */
+static int alloc_fixed_pidmap(struct pid_namespace *pid_ns, int next_nr)
+{
+	int offset;
+	struct pidmap *map;
+
+	if (next_nr < RESERVED_PIDS || next_nr >= pid_max)
+		return -EINVAL;
+
+	map = &pid_ns->pidmap[next_nr / BITS_PER_PAGE];
+
+	if (unlikely(alloc_pidmap_page(map)))
+		return -ENOMEM;
+
+	offset = next_nr & BITS_PER_PAGE_MASK;
+	if (test_and_set_bit(offset, map->page))
+		return -EBUSY;
+
+	atomic_dec(&map->nr_free);
+	pid_ns->last_pid = max(pid_ns->last_pid, next_nr);
+
+	return 0;
+}
+
 int next_pidmap(struct pid_namespace *pid_ns, int last)
 {
 	int offset;
@@ -239,7 +273,25 @@ void free_pid(struct pid *pid)
 	call_rcu(&pid->rcu, delayed_put_pid);
 }
 
-struct pid *alloc_pid(struct pid_namespace *ns)
+/*
+ * Sets a predefined upid nr for the process' upper namespace level
+ */
+static int set_predefined_pid(struct pid_namespace *ns, struct pid *pid,
+				int next_nr)
+{
+	int i = ns->level;
+	int rc;
+
+	rc = alloc_fixed_pidmap(ns, next_nr);
+	if (rc < 0)
+		return rc;
+
+	pid->numbers[i].nr = next_nr;
+	pid->numbers[i].ns = ns;
+	return 0;
+}
+
+struct pid *alloc_pid(struct pid_namespace *ns, int *retval)
 {
 	struct pid *pid;
 	enum pid_type type;
@@ -247,12 +299,37 @@ struct pid *alloc_pid(struct pid_namespa
 	struct pid_namespace *tmp;
 	struct upid *upid;
 
+	*retval = -ENOMEM;
 	pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
 	if (!pid)
 		goto out;
 
 	tmp = ns;
-	for (i = ns->level; i >= 0; i--) {
+	i = ns->level;
+	if (next_data_set(current)) {
+		/*
+		 * There is a upid nr specified, use it instead of letting
+		 * the kernel chose it for us.
+		 */
+		int next_nr = get_next_data(current);
+		int rc;
+
+		rc = set_predefined_pid(tmp, pid, next_nr);
+		if (rc < 0) {
+			*retval = rc;
+			goto out_free;
+		}
+		/* Go up one level */
+		tmp = tmp->parent;
+		i--;
+		reset_next_syscall_data(current);
+	}
+
+	/*
+	 * Let the lower levels upid nrs be automatically allocated
+	 */
+	*retval = -ENOMEM;
+	for ( ; i >= 0; i--) {
 		nr = alloc_pidmap(tmp);
 		if (nr < 0)
 			goto out_free;
Index: linux-2.6.26-rc5-mm3/include/linux/pid.h
===================================================================
--- linux-2.6.26-rc5-mm3.orig/include/linux/pid.h	2008-07-01 10:25:46.000000000 +0200
+++ linux-2.6.26-rc5-mm3/include/linux/pid.h	2008-07-01 10:49:07.000000000 +0200
@@ -121,7 +121,7 @@ extern struct pid *find_get_pid(int nr);
 extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
 int next_pidmap(struct pid_namespace *pid_ns, int last);
 
-extern struct pid *alloc_pid(struct pid_namespace *ns);
+extern struct pid *alloc_pid(struct pid_namespace *, int *);
 extern void free_pid(struct pid *pid);
 
 /*
Index: linux-2.6.26-rc5-mm3/kernel/fork.c
===================================================================
--- linux-2.6.26-rc5-mm3.orig/kernel/fork.c	2008-07-01 10:25:46.000000000 +0200
+++ linux-2.6.26-rc5-mm3/kernel/fork.c	2008-07-01 10:49:07.000000000 +0200
@@ -1110,8 +1110,7 @@ static struct task_struct *copy_process(
 		goto bad_fork_cleanup_io;
 
 	if (pid != &init_struct_pid) {
-		retval = -ENOMEM;
-		pid = alloc_pid(task_active_pid_ns(p));
+		pid = alloc_pid(task_active_pid_ns(p), &retval);
 		if (!pid)
 			goto bad_fork_cleanup_io;
 
Index: linux-2.6.26-rc5-mm3/include/linux/next_syscall_data.h
===================================================================
--- linux-2.6.26-rc5-mm3.orig/include/linux/next_syscall_data.h	2008-07-01 10:41:36.000000000 +0200
+++ linux-2.6.26-rc5-mm3/include/linux/next_syscall_data.h	2008-07-01 11:09:35.000000000 +0200
@@ -5,6 +5,7 @@
  * following is supported today:
  *    . object creation with a predefined id
  *         . for a sysv ipc object
+ *         . for a process
  *
  */
 
@@ -19,6 +20,7 @@
  * For example, it can be used to pre-set the id of the object to be created
  * by next syscall. The following syscalls support this feature:
  *    . msgget(), semget(), shmget()
+ *    . fork(), vfork(), clone()
  */
 struct next_syscall_data {
 	int ndata;

--