Messages in this thread | | | Date | Wed, 28 Oct 2009 13:05:08 +1000 | Subject | Re: is avoiding compat ioctls possible? | From | Dave Airlie <> |
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On Wed, Oct 28, 2009 at 12:53 PM, Andi Kleen <andi@firstfloor.org> wrote: > Dave Airlie <airlied@gmail.com> writes: > >> They used uint64_t to represent userspace pointers and userspace >> casted into those and the kernel casts back out and passes it to copy_*_user > > uint64_t is actually dangerous due to different alignment on x86-32 vs 64, > better use compat_u64/s64
We've designed that into a/c also, we pad all 64-bit values to 64-bit alignment on all the ioctls we've added to the drm in the past couple of years. Just because of this particular insanity.
> >> Now I thought cool I don't need to worry about compat ioctl hackery I can >> run 32 on 64 bit apps fine and it'll all just work. >> >> Now Dave Miller points out that I'm obivously deluded and we really need >> to add compat ioctls so that the kernel can truncate correctly 32-bit address >> in case userspace shoves garbage into the top 32bits of the u64. > > When the user space sees a u64 field it should never shove garbage here. > You just have to cast on 32bit for this, which is a bit ugly. > > However some architectures need special operations on compat pointers > (s390 iirc), but if you don't support those it might be reasonable > to not support that. > >> Is there really no way to avoid compat ioctls? was I delusional in >> thinking there was? > > Experience shows that people make mistakes and you sooner or > later need them anyways to work around them. >
Assume no mistakes are made, new ioctls designed from scratch and reviewed to do 32/64-bit properly. The s390 was something I didn't know about but KMS on s390 is probably never going to be something that sees the light of day.
I'm just amazed that compat_ioctl should be required for all new code.
DrNick on irc suggested just doing: if (is_compat_task()) ptr &= 0x00000000FFFFFFFF;
Is there a one liner I can just do in the actual ioctls instead of adding 20 compat ones?
Dave.
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