Re: linux-next: Tree for Nov 7

On Mon 13-11-17 22:34:50, Michael Ellerman wrote:

Hi Michal,

Michal Hocko [off-list ref] writes:

quoted

On Mon 13-11-17 10:20:06, Michal Hocko wrote:

quoted

[Cc arm and ppc maintainers]

Hmm, it turned out to be a problem on other architectures as well.
CCing more maintainers. For your reference, we are talking about
http://lkml.kernel.org/r/20171023082608.6167-1-mhocko@kernel.org
which has broken architectures which do apply aligning on the mmap
address hint without MAP_FIXED applied. See below my proposed way
around this issue because I belive that the above patch is quite
valuable on its own to be dropped for all archs.

I don't really like your solution sorry :)  The fact that you've had to
patch seven arches seems like a red flag.

I think this is a generic problem with MAP_FIXED, which I've heard
userspace folks complain about in the past.

The thing is that we canno  change MAP_FIXED behavior as it is carved in
stone

Currently MAP_FIXED does two things:
  1. makes addr not a hint but the required address
  2. blasts any existing mapping

You want 1) but not 2).

+ fail if there is a clashing range

So the right solution IMHO would be to add a new mmap flag to request
that behaviour, ie. a fixed address but iff there is nothing already
mapped there.

I don't know the mm code well enough to know if that's hard for some
reason, but it *seems* like it should be doable.

Yes, I have mentioned that in the previous email but the amount of code
would be even larger. Basically every arch which reimplements
arch_get_unmapped_area would have to special case new MAP_FIXED flag to
do vma lookup. So this was the most simple solution I could come up
with. If there was a general interest for MAP_FIXED_SAFE then we can
introduce it later of course. I would just like the hardening merged
sooner rather than later.
-- 
Michal Hocko
SUSE Labs