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SubjectRe: introduce task_rcu_dereference?
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В Ср, 22/10/2014 в 23:30 +0200, Oleg Nesterov пишет:
> On 10/22, Kirill Tkhai wrote:
> >
> > Unlocked access to dst_rq->curr in task_numa_compare() is racy.
> > If curr task is exiting this may be a reason of use-after-free:
>
> Thanks.
>
> And as you pointed out, there are other examples of unlocked
> foreign_rq->curr usage.
>
> So, Kirill, Peter, do you think that the patch below can help? Can
> we change task_numa_group() and ->select_task_rq() to do nothing if
> rq_curr_rcu_safe() returns NULL? It seems we can...
>
> task_numa_compare() can use it too, we can make another patch on
> top of this one.
>
> - Obviously just for the early review. Lacks the changelog
> and the comments (at least).
>
> - Once again, I won't insist on probe_slab_address(). We can
> add SDBR and change task_rcu_dereference() to simply read
> ->sighand.
>
> - Also, I won't argue if you think that we do not need a
> generic helper. In this case we can move this logic into
> rq_curr_rcu_safe() and it will be a bit simpler.
>
> - OTOH, I am not sure we need rq_curr_rcu_safe(). The callers
> can just use task_rcu_dereference() and check IS_ERR_OR_NULL,
> I guess retry doesn't buy too much in this case.
>
> Or do you think we need something else?
>
> Oleg.
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 857ba40..0ba420e 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2300,6 +2300,7 @@ extern void block_all_signals(int (*notifier)(void *priv), void *priv,
> sigset_t *mask);
> extern void unblock_all_signals(void);
> extern void release_task(struct task_struct * p);
> +extern struct task_struct *task_rcu_dereference(struct task_struct **ptask);
> extern int send_sig_info(int, struct siginfo *, struct task_struct *);
> extern int force_sigsegv(int, struct task_struct *);
> extern int force_sig_info(int, struct siginfo *, struct task_struct *);
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 32c58f7..4aa00c7 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -213,6 +213,37 @@ repeat:
> goto repeat;
> }
>
> +struct task_struct *task_rcu_dereference(struct task_struct **ptask)
> +{
> + struct task_struct *task;
> + struct sighand_struct *sighand;
> +
> + task = rcu_dereference(*ptask);
> + if (!task)
> + return NULL;
> +
> + /* If it fails the check below must fail too */
> + probe_slab_address(&task->sighand, sighand);
> + /*
> + * Pairs with atomic_dec_and_test() in put_task_struct(task).
> + * If we have read the freed/reused memory, we must see that
> + * the pointer was updated. The caller might want to retry in
> + * this case.
> + */
> + smp_rmb();
> + if (unlikely(task != ACCESS_ONCE(*ptask)))
> + return ERR_PTR(-EAGAIN);
> +
> + /*
> + * release_task(task) was already called; potentially before
> + * the caller took rcu_read_lock() and in this case it can be
> + * freed before rcu_read_unlock().
> + */
> + if (!sighand)
> + return ERR_PTR(-EINVAL);
> + return task;
> +}
> +
> /*
> * This checks not only the pgrp, but falls back on the pid if no
> * satisfactory pgrp is found. I dunno - gdb doesn't work correctly
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 579712f..249c0c1 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -655,6 +655,18 @@ DECLARE_PER_CPU(struct rq, runqueues);
> #define cpu_curr(cpu) (cpu_rq(cpu)->curr)
> #define raw_rq() (&__raw_get_cpu_var(runqueues))
>
> +static inline struct task_struct *rq_curr_rcu_safe(struct rq *rq)
> +{
> + for (;;) {
> + struct task_struct *curr = task_rcu_dereference(&rq->curr);
> + /* NULL is not possible */
> + if (likely(!IS_ERR(curr)))
> + return curr;
> + if (PTR_ERR(curr) != -EAGAIN)
> + return NULL;
> + }
> +}
> +
> static inline u64 rq_clock(struct rq *rq)
> {
> return rq->clock;
>

I'm agree generic helper is better. But probe_slab_address() has a sence
if we know that SDBR is worse in our subject area. Less of code is
easier to support :) probe_slab_address() it's not a trivial logic.
Also, if we use mm primitives this increases kernel modularity.

Kirill

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