Re: [PATCH 2/2] proc: fix PAGE_SIZE limit of /proc/$PID/cmdline

[Cyrill Gorcunov <gorcunov@gmail.com>]

On Thu, May 28, 2015 at 12:49:53AM +0300, Alexey Dobriyan wrote:
...
> -static int proc_pid_cmdline(struct seq_file *m, struct pid_namespace *ns,
> -			    struct pid *pid, struct task_struct *task)
> +static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
> +				     size_t _count, loff_t *pos)
>  {
> +	struct task_struct *tsk;
> +	struct mm_struct *mm;
> +	char *page;
> +	unsigned long count = _count;
> +	unsigned long arg_start, arg_end, env_start, env_end;
> +	unsigned long len1, len2, len;
> +	unsigned long p;
> +	char c;
> +	ssize_t rv;
> +
> +	BUG_ON(*pos < 0);
> +
> +	tsk = get_proc_task(file_inode(file));
> +	if (!tsk)
> +		return -ESRCH;
> +	mm = get_task_mm(tsk);
> +	put_task_struct(tsk);
> +	if (!mm)
> +		return 0;
> +	/* Check if process spawned far enough to have cmdline. */
> +	if (!mm->env_end) {
> +		rv = 0;
> +		goto out_mmput;
> +	}
> +
> +	page = (char *)__get_free_page(GFP_TEMPORARY);
> +	if (!page) {
> +		rv = -ENOMEM;
> +		goto out_mmput;
> +	}
> +
> +	down_read(&mm->mmap_sem);
> +	arg_start = mm->arg_start;
> +	arg_end = mm->arg_end;
> +	env_start = mm->env_start;
> +	env_end = mm->env_end;
> +	up_read(&mm->mmap_sem);

Could you please explain why this down/up is needed?
(the rest is hard to review at night :)