[PATCH 2.6.24-rc8-mm1 14/15] (RFC) IPC/semaphores: prepare semundo code to work on another task than current

[pierre . peiffer]

From: Pierre Peiffer <pierre.peiffer@xxxxxxxx>

In order to modify the semundo-list of a task from procfs, we must be able to
work on any target task.
But all the existing code playing with the semundo-list, currently works
only on the 'current' task, and does not allow to specify any target task.

This patch changes all these routines to allow them to work on a specified
task, passed in parameter, instead of current.

This is mainly a preparation for the semundo_write() operation, on the
/proc/<pid>/semundo file, as provided in the next patch.

Signed-off-by: Pierre Peiffer <pierre.peiffer@xxxxxxxx>
---

 ipc/sem.c |   90 ++++++++++++++++++++++++++++++++++++++++++++++----------------
 1 file changed, 68 insertions(+), 22 deletions(-)

Index: b/ipc/sem.c
===================================================================
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -1017,8 +1017,9 @@ asmlinkage long sys_semctl (int semid, i
 }
 
 /* If the task doesn't already have a undo_list, then allocate one
- * here.  We guarantee there is only one thread using this undo list,
- * and current is THE ONE
+ * here.
+ * The target task (tsk) is current in the general case, except when
+ * accessed from the procfs (ie when writting to /proc/<pid>/semundo)
  *
  * If this allocation and assignment succeeds, but later
  * portions of this code fail, there is no need to free the sem_undo_list.
@@ -1026,22 +1027,60 @@ asmlinkage long sys_semctl (int semid, i
  * at exit time.
  *
  * This can block, so callers must hold no locks.
+ *
+ * Note: task_lock is used to synchronize 1. several possible concurrent
+ * creations and 2. the free of the undo_list (done when the task using it
+ * exits). In the second case, we check the PF_EXITING flag to not create
+ * an undo_list for a task which has exited.
+ * If there already is an undo_list for this task, there is no need
+ * to held the task-lock to retrieve it, as the pointer can not change
+ * afterwards.
  */
-static inline int get_undo_list(struct sem_undo_list **undo_listp)
+static inline int get_undo_list(struct task_struct *tsk,
+				struct sem_undo_list **ulp)
 {
-	struct sem_undo_list *undo_list;
+	if (tsk->sysvsem.undo_list == NULL) {
+		struct sem_undo_list *undo_list;
 
-	undo_list = current->sysvsem.undo_list;
-	if (!undo_list) {
-		undo_list = kzalloc(sizeof(*undo_list), GFP_KERNEL);
+		/* we must alloc a new one */
+		undo_list = kmalloc(sizeof(*undo_list), GFP_KERNEL);
 		if (undo_list == NULL)
 			return -ENOMEM;
+
+		task_lock(tsk);
+
+		/* check again if there is an undo_list for this task */
+		if (tsk->sysvsem.undo_list) {
+			if (tsk != current)
+				atomic_inc(&tsk->sysvsem.undo_list->refcnt);
+			task_unlock(tsk);
+			kfree(undo_list);
+			goto out;
+		}
+
 		spin_lock_init(&undo_list->lock);
-		atomic_set(&undo_list->refcnt, 1);
-		undo_list->ns = get_ipc_ns(current->nsproxy->ipc_ns);
-		current->sysvsem.undo_list = undo_list;
+		/*
+		 * If tsk is not current (meaning that current is creating
+		 * a semundo_list for a target task through procfs), and if
+		 * it's not being exited then refcnt must be 2: the target
+		 * task tsk + current.
+		 */
+		if (tsk == current)
+			atomic_set(&undo_list->refcnt, 1);
+		else if (!(tsk->flags & PF_EXITING))
+			atomic_set(&undo_list->refcnt, 2);
+		else {
+			task_unlock(tsk);
+			kfree(undo_list);
+			return -EINVAL;
+		}
+		undo_list->ns = get_ipc_ns(tsk->nsproxy->ipc_ns);
+		undo_list->proc_list = NULL;
+		tsk->sysvsem.undo_list = undo_list;
+		task_unlock(tsk);
 	}
-	*undo_listp = undo_list;
+out:
+	*ulp = tsk->sysvsem.undo_list;
 	return 0;
 }
 
@@ -1065,17 +1104,12 @@ static struct sem_undo *lookup_undo(stru
 	return un;
 }
 
-static struct sem_undo *find_undo(struct ipc_namespace *ns, int semid)
+static struct sem_undo *find_undo(struct sem_undo_list *ulp, int semid)
 {
 	struct sem_array *sma;
-	struct sem_undo_list *ulp;
 	struct sem_undo *un, *new;
+	struct ipc_namespace *ns;
 	int nsems;
-	int error;
-
-	error = get_undo_list(&ulp);
-	if (error)
-		return ERR_PTR(error);
 
 	spin_lock(&ulp->lock);
 	un = lookup_undo(ulp, semid);
@@ -1083,6 +1117,8 @@ static struct sem_undo *find_undo(struct
 	if (likely(un!=NULL))
 		goto out;
 
+	ns = ulp->ns;
+
 	/* no undo structure around - allocate one. */
 	sma = sem_lock_check(ns, semid);
 	if (IS_ERR(sma))
@@ -1133,6 +1169,7 @@ asmlinkage long sys_semtimedop(int semid
 	struct sem_array *sma;
 	struct sembuf fast_sops[SEMOPM_FAST];
 	struct sembuf* sops = fast_sops, *sop;
+	struct sem_undo_list *ulp;
 	struct sem_undo *un;
 	int undos = 0, alter = 0, max;
 	struct sem_queue queue;
@@ -1177,9 +1214,13 @@ asmlinkage long sys_semtimedop(int semid
 			alter = 1;
 	}
 
+	error = get_undo_list(current, &ulp);
+	if (error)
+		goto out_free;
+
 retry_undos:
 	if (undos) {
-		un = find_undo(ns, semid);
+		un = find_undo(ulp, semid);
 		if (IS_ERR(un)) {
 			error = PTR_ERR(un);
 			goto out_free;
@@ -1305,7 +1346,7 @@ int copy_semundo(unsigned long clone_fla
 	int error;
 
 	if (clone_flags & CLONE_SYSVSEM) {
-		error = get_undo_list(&undo_list);
+		error = get_undo_list(current, &undo_list);
 		if (error)
 			return error;
 		atomic_inc(&undo_list->refcnt);
@@ -1405,10 +1446,15 @@ next_entry:
 	kfree(undo_list);
 }
 
-/* called from do_exit() */
+/* exit_sem: called from do_exit()
+ * task_lock is used to synchronize with get_undo_list()
+ */
 void exit_sem(struct task_struct *tsk)
 {
-	struct sem_undo_list *ul = tsk->sysvsem.undo_list;
+	struct sem_undo_list *ul;
+	task_lock(tsk);
+	ul = tsk->sysvsem.undo_list;
+	task_unlock(tsk);
 	if (ul) {
 		rcu_assign_pointer(tsk->sysvsem.undo_list, NULL);
 		synchronize_rcu();

-- 
Pierre Peiffer
--
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