[PATCH] Lock session and group ID setting

 - session-IDs and group-IDs are set outside the tasklist lock. This
   causes breakage in the USB code. The correct fix is to do this:

I introduced the bug with the new pidhash.

fs/jffs/intrep.c

@@ -3344,8 +3344,7 @@ jffs_garbage_collect_thread(void *ptr)
 	lock_kernel();
 	exit_mm(c->gc_task);
 
-	current->session = 1;
-	current->pgrp = 1;
+	set_special_pids(1, 1);
 	init_completion(&c->gc_thread_comp); /* barrier */ 
 	spin_lock_irq(&current->sighand->siglock);
 	siginitsetinv (&current->blocked, sigmask(SIGHUP) | sigmask(SIGKILL) | sigmask(SIGSTOP) | sigmask(SIGCONT));

include/linux/sched.h

@@ -503,6 +503,8 @@ extern struct task_struct init_task;
 extern struct   mm_struct init_mm;
 
 extern struct task_struct *find_task_by_pid(int pid);
+extern void set_special_pids(pid_t session, pid_t pgrp);
+extern void __set_special_pids(pid_t session, pid_t pgrp);
 
 /* per-UID process charging. */
 extern struct user_struct * alloc_uid(uid_t);

kernel/exit.c

@@ -254,6 +254,29 @@ void reparent_to_init(void)
 	write_unlock_irq(&tasklist_lock);
 }
 
+void __set_special_pids(pid_t session, pid_t pgrp)
+{
+	struct task_struct *curr = current;
+
+	if (curr->session != session) {
+		detach_pid(curr, PIDTYPE_SID);
+		curr->session = session;
+		attach_pid(curr, PIDTYPE_SID, session);
+	}
+	if (curr->pgrp != pgrp) {
+		detach_pid(curr, PIDTYPE_PGID);
+		curr->pgrp = pgrp;
+		attach_pid(curr, PIDTYPE_PGID, pgrp);
+	}
+}
+
+void set_special_pids(pid_t session, pid_t pgrp)
+{
+	write_lock_irq(&tasklist_lock);
+	__set_special_pids(session, pgrp);
+	write_unlock_irq(&tasklist_lock);
+}
+
 /*
  *	Put all the gunge required to become a kernel thread without
  *	attached user resources in one place where it belongs.
@@ -271,8 +294,7 @@ void daemonize(void)
 	 */
 	exit_mm(current);
 
-	current->session = 1;
-	current->pgrp = 1;
+	set_special_pids(1, 1);
 	current->tty = NULL;
 
 	/* Become as one with the init task */

kernel/kmod.c

@@ -100,8 +100,7 @@ int exec_usermodehelper(char *program_path, char *argv[], char *envp[])
 	int i;
 	struct task_struct *curtask = current;
 
-	curtask->session = 1;
-	curtask->pgrp = 1;
+	set_special_pids(1, 1);
 
 	use_init_fs_context();
 

kernel/sys.c

@@ -1021,16 +1021,7 @@ asmlinkage long sys_setsid(void)
 		goto out;
 
 	current->leader = 1;
-	if (current->session != current->pid) {
-		detach_pid(current, PIDTYPE_SID);
-		current->session = current->pid;
-		attach_pid(current, PIDTYPE_SID, current->pid);
-	}
-	if (current->pgrp != current->pid) {
-		detach_pid(current, PIDTYPE_PGID);
-		current->pgrp = current->pid;
-		attach_pid(current, PIDTYPE_PGID, current->pid);
-	}
+	__set_special_pids(current->pid, current->pid);
 	current->tty = NULL;
 	current->tty_old_pgrp = 0;
 	err = current->pgrp;