Re: [git pull] changes for tip, and a nasty x86 page table bug

Steven Rostedt
2009-02-20T04:00:45Z

On Thu, 19 Feb 2009, Linus Torvalds wrote:

>
>
> On Thu, 19 Feb 2009, Steven Rostedt wrote:
> > >
> > > How do we ever have a PMD that is read-only? That sounds like a bug to
> > > begin with. There's no reason to ever do that.
> >
> > Patch 2/6 explains how this happened, and supplies the fix.
>
> I think your fix is for a real bug, but I think it's still bogus.
>
> That whole "ref_prot" code is SH*T. When we do a set_pmd(), the old
> huge-page protections do not matter AT ALL for the new pmd. It matters for
> the new _leaf_ entries (the "ref_prot" 20 lines higher up), but not for
> the upper level. That should have all bits set.
>
> So the whole
>
> ref_prot = pte_pgprot(pte_mkexec(pte_clrhuge(*kpte)));
> pgprot_val(ref_prot) |= _PAGE_PRESENT;
> __set_pmd_pte(kpte, address, mk_pte(base, ref_prot));
>
> sequence is utter crap, I think. The whole "ref_prot" there should be just
> _pgprot(_KERNPG_TABLE), I think. I don't think there is any other valid
> value.
>
> So I would argue that the comment above that piece of code is total and
> utter crap (all the protection info _and_ all the PAT bits are now in the
> pte, and trying to move them into the pmd is *buggy*), and the three lines
> should basically be
>
> __set_pmd_pte(kpte, address, mk_pte(base, _pgprot(_KERNPG_TABLE)));
>
> but let's see if somebody can tell me why I'm wrong.
>
> "git blame" attributes this all to Ying Huang and Thomas. And looking at
> the commit that introduced the pte_mkexec(), I really think the code was
> confused and people never thought about it deeply.
>
> Comments?

My original change was to just set the PMD to the KERNPG_TABLE entry like
you suggested, but I was thinking that there was a reason for the madness,
that I did not understand. So I just did the bare minimum to get my code
working.

Is this something worthy of 29? I could whip up a patch against your
latest tree.

-- Steve