[Pkg-shadow-devel] why the nulls in cmdline?

[Nicolas François]

Hello,

On Wed, Jan 16, 2008 at 07:07:11PM +0800, jidanni at jidanni.org wrote:
> Why the nulls?
> $ pstree -a|grep \ $|cat -e
> init  $
>   |-login --        $
> $ od -c /proc/`pidof login`/cmdline
> 0000000   /   b   i   n   /   l   o   g   i   n  \0   -   -  \0  \0  \0
> 0000020  \0  \0  \0  \0  \0  \0
> 0000026
> $ od -c /proc/1/cmdline
> 0000000   i   n   i   t       [   2   ]  \0  \0  \0
> 0000013

On the consoles, init runs some getty. When a username is entered to
the getty's prompt, getty executes login with:

	login -- <username>

That is, in you case:
	/proc/`pidof login`/cmdline:
0000000   /   b   i   n   /   l   o   g   i   n  \0   -   -  \0  j   i
0000020   d   a   n   n   i   \0

When login parses it arguments, it zeros the username
	(strzero (argv[optind]); in login.c)

This changes /proc/`pidof login`/cmdline to the one you reported:
	/proc/`pidof login`/cmdline:
0000000   /   b   i   n   /   l   o   g   i   n  \0   -   -  \0  \0  \0
0000020  \0  \0  \0  \0  \0  \0

In my opinion, this is a feature in case an user entered a password
instead of a username. Without this feature, some passwords could be
gathered by running a ps regularly.

Best Regards,
-- 
Nekral