Re: +c-r-prctl-add-ability-to-set-new-mm_struct-exe_file-update-after-mm-num_exe_file_vmas-removal.patch added to -mm tree

From: Cyrill Gorcunov
Date: Thu Apr 19 2012 - 18:02:06 EST

On Thu, Apr 19, 2012 at 11:51:09PM +0200, Oleg Nesterov wrote:
> On 04/20, Konstantin Khlebnikov wrote:
> >
> > Oleg Nesterov wrote:
> >>
> >> You misunderstood. I am not arguing with "one-shot", I do not really
> >> care.
> >>
> >> My question is: unless I missed something "it can't be changed again"
> >> is not actually true. A task does PR_SET_MM_EXE_FILE, then it forks
> >> the new child. The child can do PR_SET_MM_EXE_FILE again. Is this
> >> by design?
> >>
> >> Oleg.
> >>
> >
> > I found more weird case: child thread (with CLONE_THREAD and without CLONE_VM)
> > changes its exe_file...
> No. copy_process() does:
> if ((clone_flags & CLONE_THREAD) && !(clone_flags & CLONE_SIGHAND))
> return ERR_PTR(-EINVAL);
> if ((clone_flags & CLONE_SIGHAND) && !(clone_flags & CLONE_VM))
> return ERR_PTR(-EINVAL);

Guys, while I more-less agree with Matt about single-shot behaviour

[ let me copy my and his email

>> With mm->exe_file this prctl option become a one-shot
>> only, and while at moment our user-space tool can perfectly
>> live with that I thought that there is no strict need to
>> limit the option this way from the very beginning.
> As far as backward compatibility, isn't it better to lift that restriction
> later rather than add it? I think the latter would very likely "break"
> things whereas the former would not.
> I also prefer that restriction because it establishes a bound on how
> frequently the symlink can change. Keeping it a one-shot deal makes the
> values that show up in tools like top more reliable for admins.

I guess maybe it's time to drop one-shot requirement and as result
we can drop MMF_EXE_FILE_CHANGED bit completely, making overall code

Our tool can live with any behaviour (and one shot and multishot,
doesn't matter).

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