On Tue, Jan 3, 2017 at 2:07 PM, Arnd Bergmann [off-list ref] wrote:

On Tuesday, January 3, 2017 10:29:33 AM CET Andy Lutomirski wrote:

quoted

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.

quoted

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