pids: document task_pgrp/task_session is not safe without tasklist/rcu
Even if task == current, it is not safe to dereference the result of task_pgrp/task_session. We can race with another thread which changes the special pid via setpgid/setsid. Document this. The next 2 patches give an example of the unsafe usage, we have more bad users. [akpm@linux-foundation.org: coding-style fixes] Signed-off-by: Oleg Nesterov <oleg@redhat.com> Cc: Louis Rilling <Louis.Rilling@kerlabs.com> Cc: "Eric W. Biederman" <ebiederm@xmission.com> Cc: Pavel Emelyanov <xemul@openvz.org> Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com> Cc: Roland McGrath <roland@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -1489,6 +1489,11 @@ static inline struct pid *task_tgid(struct task_struct *task)
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return task->group_leader->pids[PIDTYPE_PID].pid;
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}
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/*
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* Without tasklist or rcu lock it is not safe to dereference
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* the result of task_pgrp/task_session even if task == current,
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* we can race with another thread doing sys_setsid/sys_setpgid.
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*/
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static inline struct pid *task_pgrp(struct task_struct *task)
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{
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return task->group_leader->pids[PIDTYPE_PGID].pid;
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