linux-kernel - Re: [PATCH] x86/boot/64/clang: Use fixup_pointer() to access '__supported_pte

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Date:   Tue, 8 May 2018 09:25:01 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        Michael Davidson <md@...gle.com>
Subject: Re: [PATCH] x86/boot/64/clang: Use fixup_pointer() to access
 '__supported_pte_mask'

On 05/08/2018 07:50 AM, Alexander Potapenko wrote:
>>> Similarly to commit 187e91fe5e91
>>> ("x86/boot/64/clang: Use fixup_pointer() to access 'next_early_pgt'"),
>>> '__supported_pte_mask' must be also accessed using fixup_pointer() to
>>> avoid position-dependent relocations.
>>>
>>> Signed-off-by: Alexander Potapenko <glider@...gle.com>
>>> Fixes: fb43d6cb91ef ("x86/mm: Do not auto-massage page protections")
> 
>> In the interests of standalone changelogs, I'd really appreciate an
>> actual explanation of what's going on here.  Your patch makes the code
>> uglier and doesn't fix anything functional from what I can see.
> You're right, sure. I'll send a patch with an updated description.

Great, thanks!

>> Do we have anything we can do to keep us from recreating these kinds of
>> regressions all the time?
> I'm not really aware of the possible options in the kernel land. Looks like
> a task for some objtool-like utility?
> As long as these regressions are caught with Clang, setting up a 0day Clang
> builder might help.

I've asked the 0day folks if this is doable.  It would be great to see
it added.