linux-kernel - Re: Getting rid of dynamic TASK

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Message-ID: <20160511180847.GA27195@redhat.com>
Date:	Wed, 11 May 2016 20:08:47 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Dmitry Safonov <0x7f454c46@...il.com>,
	Borislav Petkov <bp@...en8.de>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, X86 ML <x86@...nel.org>,
	Ruslan Kabatsayev <b7.10110111@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Getting rid of dynamic TASK_SIZE (on x86, at least)

On 05/10, Andy Lutomirski wrote:
>
> On May 10, 2016 11:21 AM, "Oleg Nesterov" <oleg@...hat.com> wrote:
> >
> > On 05/10, Andy Lutomirski wrote:
> > >
> > >  - xol_add_vma: This one is weird: uprobes really is doing something
> > > behind the task's back, and the addresses need to be consistent with
> > > the address width.  I'm not quite sure what to do here.
> >
> > It can use mm->task_size instead, plus this is just a hint. And perhaps
> > mm->task_size should have more users, say get_unmapped_area...
>
> Ick.  I hadn't noticed mm->task_size.  We have a *lot* of different
> indicators of task size.  mm->task_size appears to have basically no
> useful uses except maybe for ppc.
>
> On x86, bitness can change without telling the kernel, and tasks
> running in 64-bit mode can do 32-bit syscalls.

Sure, but imo this doesn't mean that mm->task_size or (say) is_64bit_mm()
make no sense.

> So maybe I should add mm->task_size to my list of things that would be
> nice to remove.  Or maybe I'm just tilting at windmills.

I dunno. But afaics there is no other way to look at foreign mm and find
out its limit. Say, the usage of mm->task_size in validate_range() looks
valid even if (afaics) nothing bad can happen if start/end >= task_size,
so validate_range() could just check that len+start doesn't overflow.

Oleg.