Re: [PATCH 1/2] x86/arch_prctl: add ARCH_SET_{COMPAT,NATIVE} to change compatible mode

[Dmitry Safonov]

On 04/07/2016 05:39 PM, Andy Lutomirski wrote:
> For 32-bit, the vdso *must* exist in memory at the address that the
> kernel thinks it's at.  Even if you had a pure 32-bit restore stub,
> you would still need vdso remap, because there's a chance the vdso
> could land at an unusable address, say one page off from where you
> want it.  You couldn't map a wrapper because there wouldn't be any
> space for it without moving the real vdso out of the way.
>
> Remember, you *cannot* mremap() the 32-bit vdso because you will
> crash.  It works by luck for 64-bit, but it's plausible that we'd want
> to change that some day.  (I have awful patches that speed a bunch of
> things up at the cost of a vdso trampoline for 64-bit code and a bunch
> of other hacks.  Those patches will never go in for real, but
> something else might want the ability to use 64-bit vdso trampolines.)
Hello again,
what do you think about attached patch?
I think it should fix landing problem for i386 vdso mremap.
It does not touch fast syscall path, so there should be no
speed regression.
> > I did remapping for vdso as blob for native x86_64 task differs
> > to compatible task. So it's just changing blobs, address value
> > is there for convenience - I may omit it and just remap
> > different vdso blob at the same place where was previous vdso.
> > I'm not sure, why do we need possibility to map 64-bit vdso blob
> > on native 32-bit builds?
> That would fail, but I think the API should exist.  But a native
> 32-bit program should be able to remap the 32-bit vdso.
>
> IOW, I think you should be able to do, roughly:
>
> map_new_vdso(VDSO_32BIT, addr);
>
> on any kernel.
>
> Am I making sense?
I will still work for this interface - just wanted that
usuall mremap to work on vdso mappings.

Thanks,
Dmitry.