On 08/16, Eric W. Biederman wrote: > Oleg Nesterov <oleg at tv-sign.ru> writes: > > > On 08/15, Eric W. Biederman wrote: > >> > >> +static inline pid_t pid_nr(struct pid *pid) > >> +{ > >> + pid_t nr = 0; > >> + if (pid) > >> + nr = pid->nr; > >> + return nr; > >> +} > > > > I think this is not safe, you need rcu locks here or the caller should > > do some locking. > > > > Let's look at f_getown() (PATCH 7/7). What if original task which was > > pointed by ->f_owner.pid has gone, another thread does fcntl(F_SETOWN), > > and pid_nr() takes a preemtion after 'if (pid)'? In this case 'pid->nr' > > may follow a freed memory. > > This isn't an rcu reference. I hold a hard reference count on > the pid entry. So this should be safe. -static void f_modown(struct file *filp, unsigned long pid, +static void f_modown(struct file *filp, struct pid *pid, enum pid_type type, uid_t uid, uid_t euid, int force) { write_lock_irq(&filp->f_owner.lock); if (force || !filp->f_owner.pid) { - filp->f_owner.pid = pid; + put_pid(filp->f_owner.pid); This 'put_pid()' can actually free 'struct pid' if the task/group has already gone away. Another thread doing f_getown() can access a freed memory, no? > What is an rcu reference is going from struct pid to the task > it points to. Yes, you are right... But I'd say it is going form task to pid :) Oleg.