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SubjectRe: [RFC PATCH tip/master 2/3] kprobes: Allocate kretprobe instance if its free list is empty

* Masami Hiramatsu <mhiramat@kernel.org> wrote:

> @@ -1824,6 +1823,30 @@ void unregister_jprobes(struct jprobe **jps, int num)
> EXPORT_SYMBOL_GPL(unregister_jprobes);
>
> #ifdef CONFIG_KRETPROBES
> +
> +/* Try to use free instance first, if failed, try to allocate new instance */
> +struct kretprobe_instance *kretprobe_alloc_instance(struct kretprobe *rp)
> +{
> + struct kretprobe_instance *ri = NULL;
> + unsigned long flags = 0;
> +
> + raw_spin_lock_irqsave(&rp->lock, flags);
> + if (!hlist_empty(&rp->free_instances)) {
> + ri = hlist_entry(rp->free_instances.first,
> + struct kretprobe_instance, hlist);
> + hlist_del(&ri->hlist);
> + }
> + raw_spin_unlock_irqrestore(&rp->lock, flags);
> +
> + /* Populate max active instance if possible */
> + if (!ri && rp->maxactive < KRETPROBE_MAXACTIVE_ALLOC) {
> + ri = kmalloc(sizeof(*ri) + rp->data_size, GFP_ATOMIC);
> + if (ri)
> + rp->maxactive++;
> + }
> +
> + return ri;
> +}
> /*
> * This kprobe pre_handler is registered with every kretprobe. When probe
> * hits it will set up the return probe.
> @@ -1846,14 +1869,8 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
> }
>
> /* TODO: consider to only swap the RA after the last pre_handler fired */
> - hash = hash_ptr(current, KPROBE_HASH_BITS);
> - raw_spin_lock_irqsave(&rp->lock, flags);
> - if (!hlist_empty(&rp->free_instances)) {
> - ri = hlist_entry(rp->free_instances.first,
> - struct kretprobe_instance, hlist);
> - hlist_del(&ri->hlist);
> - raw_spin_unlock_irqrestore(&rp->lock, flags);
> -
> + ri = kretprobe_alloc_instance(rp);
> + if (ri) {
> ri->rp = rp;
> ri->task = current;
>
> @@ -1868,13 +1885,13 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
>
> /* XXX(hch): why is there no hlist_move_head? */
> INIT_HLIST_NODE(&ri->hlist);
> + hash = hash_ptr(current, KPROBE_HASH_BITS);
> kretprobe_table_lock(hash, &flags);
> hlist_add_head(&ri->hlist, &kretprobe_inst_table[hash]);
> kretprobe_table_unlock(hash, &flags);
> - } else {
> + } else
> rp->nmissed++;
> - raw_spin_unlock_irqrestore(&rp->lock, flags);
> - }
> +
> return 0;
> }
> NOKPROBE_SYMBOL(pre_handler_kretprobe);

So this is something I missed while the original code was merged, but the concept
looks a bit weird: why do we do any "allocation" while a handler is executing?

That's fundamentally fragile. What's the maximum number of parallel
'kretprobe_instance' required per kretprobe - one per CPU?

If so then we should preallocate all of them when they are installed and not do
any alloc/free dance when executing them.

This will also speed them up, and increase robustness all around.

Thanks,

Ingo

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