linux-kernel - Re: [rfc 3/3] prctl: Add PR_SET_MM codes to tune up mm

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Message-ID: <20111208070724.GW21678@moon>
Date:	Thu, 8 Dec 2011 11:07:24 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org,
	Tejun Heo <tj@...nel.org>, Andrew Vagin <avagin@...nvz.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [rfc 3/3] prctl: Add PR_SET_MM codes to tune up mm_struct entires

On Wed, Dec 07, 2011 at 02:43:55PM -0800, Andrew Morton wrote:
> On Wed, 7 Dec 2011 16:27:18 +0400
> Cyrill Gorcunov <gorcunov@...il.com> wrote:
> 
> > At process of task restoration we need a way to tune up
> > a few members of mm_struct structure such as start_code,
> > end_code, start_data, end_data, start_stack, start_brk, brk.
> 
> I don't really know what "tune up" means in this context.  Can we
> please be more specific and detailed here?  It appears that the patch
> permits userspace to directly modify these fields.
> 

ok

> 
> The prctl(2) manpage will need to be updated.  Please Cc Michael on all
> such changes.
>

you mean -- Michael Kerrisk, mtk AT man7.org, right?

...
> > +
> > +	mm = get_task_mm(current);
> 
> Is it necessaary to run the expensive get_task_mm() for `current'? 
> `current' is known to be running and you have control of it here -
> nobody will be taking our mm away.  Simply use current->mm?  The
> function actually uses current->mm later on in several places.

hmm, indeed, i'll update, thanks!

> 
> > +	if (!mm)
> > +		return -ENOENT;
> > +
> > +	down_read(&mm->mmap_sem);
> > +	vma = find_vma(mm, addr);
> > +
> > +	if (opt != PR_SET_MM_START_BRK &&
> > +	    opt != PR_SET_MM_BRK) {
> 
> 80 columns, not 40 :)
> 
> > +		/* It must be existing VMA */
> > +		if (!vma || vma->vm_start > addr)
> > +			goto out;
> > +	}
> > +
> > +	error = -EINVAL;
> > +	switch (opt) {
> > +	case PR_SET_MM_START_CODE:
> > +	case PR_SET_MM_END_CODE:
> > +
> 
> You're adding unneeded and unconventional newlines after the `case'
> statements.
> 

no, I added them by a purpose -- it's a way easier to read these
assignments, but fine -- I'll drop this nits.

> 
> This is starting to add a non-trivial amount of code.  Perhaps we need
> to introduce a Kconfig variable to control such things as this, to
> prevent bloating up kernels which aren't require to support c/r?
> 

Dunno, Andrew. Actually I agreed that these snippets are mostly
needed for c/r only, but the initial idea over all changes was
to add levers into kernel which might be helpful not only
for c/r but for someone else as well.

	Cyrill
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