Re: [patch 050/160] prctl: add PR_SET_PROCTITLE_AREA option for prctl()

[Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On 03/06, Linus Torvalds wrote:
>
> On Fri, 5 Mar 2010, akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org wrote:
> >
> > This patch makes it possible for userspace to implement setproctitle()
> > cleanly.  It adds a new PR_SET_PROCTITLE_AREA option for prctl(), which
> > updates task's mm_struct->arg_start and arg_end to the given area.
>
> This looks overly complicated. Why do you change the whole locking rules,
> instead of protecting _only_ the "arg_start/arg_end" case?
>
> The thing is, there's no reason to hold the mmap_sem over the whole thing,
> and I don't think this is important enough to be a valid reason for
> exposing a whole new "locked" access variant, when a simple "protect just
> the arg_start/end" would handle it.

It was me who suggested to re-use mm->mmap_sem instead of adding the new
lock, but looking at this patch again I do not understand the reason this
lock should be held throughout in proc_pid_cmdline(). If there is no such
a reason, then the new access_process_vm_locked (cough, also suggested by
me ;) is not needed.

Oleg.