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Re: [RFC] x86/vdso: Align vdso after searching for free area




On Tue, 2018-06-12 at 14:39 -0700, H. Peter Anvin wrote:
> On 06/12/18 14:24, Dmitry Safonov wrote:
> > > 
> > > Move align_vdso_addr() after get_unmapped_area() to make sure
> > > that
> > > errata for AMD 15h is always applied.
> > 
> > Alternative dirty-hacky idea:
> > specify some (struct file*) to get_unmapped_area() for vdso vma,
> > then
> > mapping would be automatically aligned. Dirty as hell as relies on
> > get_unmapped_area() realization details.
> > 
> 
> 
> I have mentioned several times that I would like to see the vdso
> actually be an actual file in a filesystem, that the kernel *or* user
> space can map (needs to be MAP_SHARED, of course.)

Yeah, I remember I did that previously:
https://lwn.net/Articles/698854/

But hadn't time to fix review replies.
Probably, worth to resurrect the patches and clean the dust out from
them.

> The vdso data page needs to be moved after the ELF object itself for
> this to work. Ideally it should be given an actual ELF segment (and
> ideally an ELF section as well.)  The easy way to do this is to give
> the
> linker a dummy vvar page as a properly aligned section at compile
> time,
> into which space the kernel can map the real vvar page. The only
> downside is that the linker likes to put section headings after the
> actual data, so it may end up taking up an extra page over the
> current
> arrangement. However, I think the gains outweigh the losses.

-- 
Thanks,
             Dmitry