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Date:   Sat, 8 Jun 2019 22:52:18 +0200
From:   Pavel Machek <pavel@ucw.cz>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Yu-cheng Yu <yu-cheng.yu@intel.com>, x86@kernel.org,
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        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
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        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
        Dave Martin <Dave.Martin@arm.com>
Subject: Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup
 function
Message-ID: <20190608205218.GA2359@xo-6d-61-c0.localdomain>
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Hi!

> > I've no idea what the kernel should do; since you failed to answer the
> > question what happens when you point this to garbage.
> > 
> > Does it then fault or what?
> 
> Yeah, I think you'll fault with a rather mysterious CR2 value since
> you'll go look at the instruction that faulted and not see any
> references to the CR2 value.
> 
> I think this new MSR probably needs to get included in oops output when
> CET is enabled.
> 
> Why don't we require that a VMA be in place for the entire bitmap?
> Don't we need a "get" prctl function too in case something like a JIT is
> running and needs to find the location of this bitmap to set bits itself?
> 
> Or, do we just go whole-hog and have the kernel manage the bitmap
> itself. Our interface here could be:
> 
> 	prctl(PR_MARK_CODE_AS_LEGACY, start, size);
> 
> and then have the kernel allocate and set the bitmap for those code
> locations.

For the record, that sounds like a better interface than userspace knowing
about the bitmap formats...
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html