Michal Hocko <mhocko@xxxxxxxxxx> writes: > 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 Yes obviously. I didn't mean to imply we would change MAP_FIXED, rather we would add a new flag with the new semantics. >> 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 Yep. I thought that was implied :) >> 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. I'd have to look, but my memory of the arch code is that it doesn't deal with the vma so it wouldn't need any change. > 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. Sure. But in the scheme of things one more kernel release is not that big a deal to get it right. Given that the simple approach of dropping MAP_FIXED turns out to not be simple at all. cheers -- 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
