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Date: Wed, 18 Aug 2004 01:58:50 -0700
From: William Lee Irwin III <wli@holomorphy.com>
To: DervishD <disposable1@telefonica.net>
Cc: Linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: setproctitle
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	DervishD <disposable1@telefonica.net>,
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On Wed, Aug 18, 2004 at 10:28:51AM +0200, DervishD wrote:
>     Is there any special reason not to implement setproctitle in the
> kernel? In user space is a bit difficult to implement since 'argv[0]'
> cannot grow beyond the initially allocated space, better said, it can
> grow but only changing the pointer to another place or eating the
> space occupied by the other arguments.
>     proftpd has a not-very-polite set_proc_title that misses the
> final NULL, and a couple of other programs out there uses it, too.
> Applications should be free to change theirs proc titles to some
> pretty if they want, shouldn't they?
>     In proc/base.c you can read about 'setproctitle(3)', that is, in
> library space (user space), not kernel space, but AFAIK only FreeBSD
> has setproctitle :?

Observe the following, from fs/proc/base.c:

static int proc_pid_cmdline(struct task_struct *task, char * buffer)
{
	int res = 0;
	unsigned int len;
	struct mm_struct *mm = get_task_mm(task);
	if (!mm)
		goto out;
	if (!mm->arg_end)
		goto out;	/* Shh! No looking before we're done */

 	len = mm->arg_end - mm->arg_start;
 
	if (len > PAGE_SIZE)
		len = PAGE_SIZE;
 
	res = access_process_vm(task, mm->arg_start, buffer, len, 0);

	// If the nul at the end of args has been overwritten, then
	// assume application is using setproctitle(3).
	if (res > 0 && buffer[res-1] != '\0') {
		len = strnlen(buffer, res);
		if (len < res) {
		    res = len;
		} else {
			len = mm->env_end - mm->env_start;
			if (len > PAGE_SIZE - res)
				len = PAGE_SIZE - res;
			res += access_process_vm(task, mm->env_start, buffer+res, len, 0);
			res = strnlen(buffer, res);
		}
	}
	mmput(mm);

out:
	return res;
}

The command-line arguments are being fetched from the process address
space, i.e. simply editing argv[] in userspace will have the desired
effect. Though this code is butt ugly.


-- wli