Messages in this thread | | | Date | Fri, 10 Aug 2007 20:45:46 +0400 | From | Oleg Nesterov <> | Subject | Re: [PATCH] Make access to task's nsproxy liter |
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On 08/10, Pavel Emelyanov wrote: > > Oleg Nesterov wrote: > >On 08/10, Serge E. Hallyn wrote: > >>Quoting Pavel Emelyanov (xemul@openvz.org): > >>>+/* > >>>+ * the namespaces access rules are: > >>>+ * > >>>+ * 1. only current task is allowed to change tsk->nsproxy pointer or > >>>+ * any pointer on the nsproxy itself > >>>+ * > >>>+ * 2. when accessing (i.e. reading) current task's namespaces - no > >>>+ * precautions should be taken - just dereference the pointers > >>>+ * > >>>+ * 3. the access to other task namespaces is performed like this > >>>+ * rcu_read_lock(); > >>>+ * nsproxy = task_nsproxy(tsk); > >>>+ * if (nsproxy != NULL) { > >>>+ * / * > >>>+ * * work with the namespaces here > >>>+ * * e.g. get the reference on one of them > >>>+ * * / > >>>+ * } / * > >>>+ * * NULL task_nsproxy() means that this task is > >>>+ * * almost dead (zombie) > >>>+ * * / > >>>+ * rcu_read_unlock(); > >>And lastly, I guess that the caller to switch_task_namespaces() has > >>to ensure that new_nsproxy either (1) is the init namespace, (2) is a > >>brand-new namespace to which noone else has a reference, or (3) the > >>caller has to hold a reference to the new_nsproxy across the call to > >>switch_task_namespaces(). > >> > >>As it happens the current calls fit (1) or (2). Again if we happen to > >>jump into the game of switching a task into another task's nsproxy, > >>we'll need to be mindful of (3) so that new_nsproxy can't be tossed into > >>the bin between > >> > >> if (new) > >> get_nsproxy(new); > > > >4) Unless tsk == current, get_task_namespaces(tsk) and get_nsproxy(tsk) > > are racy even if done under rcu_read_lock(). > > Yup :) > > It is already written in comment that only the current is allowed > to change its nsproxy. I.e. when switch_task_nsproxy() is called > for tsk other than current it's a BUG
Yes, but what I meant is that this code
rcu_read_lock(); nsproxy = task_nsproxy(tsk); if (nsproxy != NULL) get_nsproxy(nsproxy); rcu_read_unlock();
if (nsproxy) { use_it(nsproxy); put_nsproxy(nsproxy); }
is not safe despite the fact we are _not_ changing tsk->nsproxy.
The patch itself is correct because we don't do that, and the comment is right. Just it is not immediately obvious.
Oleg.
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