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Date: Wed, 17 Feb 2016 11:35:47 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        Arjan van de Ven <arjan@linux.intel.com>,
        lkml <linux-kernel@vger.kernel.org>, Oren Laadan <orenl@cellrox.com>,
        Ruchi Kandoi <kandoiruchi@google.com>,
        Rom Lemarchand <romlem@android.com>, Kees Cook <keescook@chromium.org>,
        Android Kernel Team <kernel-team@android.com>
Subject: Re: [PATCH 2/2] proc: Add /proc/<pid>/timerslack_ns interface
Message-Id: <20160217113547.6487174b9c6d365927095080@linux-foundation.org>
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On Tue, 16 Feb 2016 17:06:31 -0800 John Stultz <john.stultz@linaro.org> wrote:

> This patch provides a proc/PID/timerslack_ns interface which
> exposes a task's timerslack value in nanoseconds and allows it
> to be changed.
> 
> This allows power/performance management software to set timer
> slack for other threads according to its policy for the thread
> (such as when the thread is designated foreground vs. background
> activity)
> 
> If the value written is non-zero, slack is set to that value.
> Otherwise sets it to the default for the thread.
> 
> This interface checks that the calling task has permissions to
> to use PTRACE_MODE_ATTACH_FSCREDS on the target task, so that we
> can ensure arbitrary apps do not change the timer slack for other
> apps.

hm.  What the heck is PTRACE_MODE_ATTACH_FSCREDS and why was it chosen?

The procfs file's permissions are 0644, yes?  So a process's
timer_slack is world-readable?  hm.

> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -2257,6 +2257,74 @@ static const struct file_operations proc_timers_operations = {
>  	.release	= seq_release_private,
>  };
>  
> +static ssize_t timerslack_ns_write(struct file *file, const char __user *buf,
> +					size_t count, loff_t *offset)
> +{
> +	struct inode *inode = file_inode(file);
> +	struct task_struct *p;
> +	char buffer[PROC_NUMBUF];
> +	u64 slack_ns;
> +	int err;
> +
> +	memset(buffer, 0, sizeof(buffer));
> +	if (count > sizeof(buffer) - 1)
> +		count = sizeof(buffer) - 1;
> +
> +	if (copy_from_user(buffer, buf, count))
> +		return -EFAULT;
> +
> +	err = kstrtoull(strstrip(buffer), 10, &slack_ns);
> +	if (err < 0)
> +		return err;

Use kstrtoull_from_user()?

> +	p = get_proc_task(inode);
> +	if (!p)
> +		return -ESRCH;
> +
> +	if (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS)) {
> +		if (slack_ns == 0)
> +			p->timer_slack_ns = p->default_timer_slack_ns;
> +		else
> +			p->timer_slack_ns = slack_ns;
> +	} else
> +		count = -EINVAL;
> +
> +	put_task_struct(p);
> +
> +	return count;
> +}
> +