linux-kernel - Re: [RFCv2 03/10] x86: Introduce userspace API to handle per-thread features

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Date:   Thu, 12 May 2022 14:02:50 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFCv2 03/10] x86: Introduce userspace API to handle per-thread
 features

On Wed, May 11 2022 at 05:27, Kirill A. Shutemov wrote:
> Add three new arch_prctl() handles:
>  
> +static long thread_feature_prctl(struct task_struct *task, int option,
> +				 unsigned long features)

Bah. I really hate the task pointer on all these x86 prctls. @task must
always be current, so this @task argument is just confusion.

> +{
> +	const unsigned long known_features = 0;
> +
> +	if (features & ~known_features)
> +		return -EINVAL;

This implementation allows to task->read features_[locked] with
@features == 0. That should be documented somewhere.

> +	if (option == ARCH_THREAD_FEATURE_LOCK) {
> +		task->thread.features_locked |= features;
> +		return task->thread.features_locked;
> +	}


> +	/* Do not allow to change locked features */
> +	if (features & task->thread.features_locked)
> +		return -EPERM;
> +
> +	if (option == ARCH_THREAD_FEATURE_DISABLE) {
> +		task->thread.features &= ~features;
> +		goto out;
> +	}
> +
> +	/* Handle ARCH_THREAD_FEATURE_ENABLE */
> +
> +	task->thread.features |= features;
> +out:
> +	return task->thread.features;

Eyes bleed.

	if (option == ARCH_THREAD_FEATURE_ENABLE)
        	task->thread.features |= features;
        else
        	task->thread.features &= ~features;

	return task->thread.features;

It's bloody obvious from the code that the counterpart of enable is
disable, no? So neither the goto nor the 'comment the obvious' is useful
in any way.

Thanks,

        tglx