Re: [PATCH v6] Added PR_SET_PROCTITLE_AREA option for prctl()

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> 
> * KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:
> 
> > > The feature looks useful, but the choice of a prctl as an API is strange 
> > > - it limits us to the current task only - while the ability to set 
> > > arguments for another task looks a more generic (and potentially more 
> > > useful) solution.
> > 
> > No. It's impossible.
> > /proc/{pid}/cmdline read user process's memory. iow, this prctl() don't
> > receive string, it receive virtual address itself. [...]
> 
> it's not 'impossible' at all, you yourself mention ptrace:

Ah yes, 'impossible' was wrong word. but it doesn't works intentionally.

1. setproctitle() unaware application continue to see argv[0] directly.
  it makes some inconsistent behavior. 
2. proc title (i.e. string) injection need to map new page as process title area.
   implicit mapping increasing makes new trouble
   - mihgt cause to exceed max_map_count awhile after.
   - might cause leak proc title area (who know when it should be freed?)

I think reasonable way is 1. send signal (or use another inter process
communication way) to target process 2. target process change own proc title
themself.

Plus, I haven't seen the use-case of changin another task. iow I doubt
it's worth to change lots code.


> > [...] I don't want any task allow to change another task's memory 
> > except ptrace.
> 
> And i did not mean to allow 'any' task to be allowed to do this - 
> security checks apply, obviously.

Agreed. My 'any' didn't intent bypass security check ;-)