Re: [RFC, PATCHv2 29/29] mm, x86: introduce RLIMIT_VADDR

[Andy Lutomirski]

On Tue, Jan 3, 2017 at 2:07 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Tuesday, January 3, 2017 10:29:33 AM CET Andy Lutomirski wrote:
>>
>> Hmm.  What if we approached this a bit differently?  We could add a
>> single new personality bit ADDR_LIMIT_EXPLICIT.  Setting this bit
>> cause PER_LINUX32_3GB etc to be automatically cleared.
>
> Both the ADDR_LIMIT_32BIT and ADDR_LIMIT_3GB flags I guess?

Yes.

>
>> When
>> ADDR_LIMIT_EXPLICIT is in effect, prctl can set a 64-bit numeric
>> limit.  If ADDR_LIMIT_EXPLICIT is cleared, the prctl value stops being
>> settable and reading it via prctl returns whatever is implied by the
>> other personality bits.
>
> I don't see anything wrong with it, but I'm a bit confused now
> what this would be good for, compared to using just prctl.
>
> Is this about setuid clearing the personality but not the prctl,
> or something else?

It's to avid ambiguity as to what happens if you set ADDR_LIMIT_32BIT
and use the prctl.  ISTM it would be nice for the semantics to be
fully defined in all cases.

--Andy