Re: Getting rid of dynamic TASK_SIZE (on x86, at least)

[Oleg Nesterov <oleg@redhat.com>]

On 05/10, Andy Lutomirski wrote:
>
> On May 10, 2016 11:21 AM, "Oleg Nesterov" <oleg@redhat.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.

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