On Tue, 16 Feb 2016 17:06:31 -0800 John Stultz <john.stu...@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;
> +}
> +
