On Mon 13-11-17 22:34:50, Michael Ellerman wrote: > Hi Michal, > > Michal Hocko <mhocko@xxxxxxxxxx> writes: > > On Mon 13-11-17 10:20:06, Michal Hocko wrote: > >> [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@xxxxxxxxxx > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-next" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
