Re: [PATCH v10 01/26] Documentation/x86: Add CET description

[Yu-cheng Yu]

On Wed, 2020-04-29 at 16:02 -0700, Yu-cheng Yu wrote:
> On Wed, 2020-04-29 at 15:53 -0700, Dave Hansen wrote:
> > On 4/29/20 3:07 PM, Yu-cheng Yu wrote:
> > > +Note:
> > > +  There is no CET-enabling arch_prctl function.  By design, CET is enabled
> > > +  automatically if the binary and the system can support it.
> > 
> > I think Andy and I danced around this last time.  Let me try to say it
> > more explicitly.
> > 
> > I want CET kernel enabling to able to be disconnected from the on-disk
> > binary.  I want a binary compiled with CET to be able to disable it, and
> > I want a binary not compiled with CET to be able to enable it.  I want
> > different threads in a process to be able to each have different CET status.
> 
> The kernel patches we have now can be modified to support this model.  If after
> discussion this is favorable, I will modify code accordingly.

To turn on/off and to lock CET are application-level decisions.  The kernel does
not prevent any of those.  Should there be a need to provide an arch_prctl() to
turn on CET, it can be added without any conflict to this series.

> > Which JITs was this tested with?  I think as a bare minimum we need to
> > know that this design can accommodate _a_ modern JIT.  It would be
> > horrible if the browser javascript engines couldn't use this design, for
> > instance.
> 
> JIT work is still in progress.  When that is available I will test it.

I found CET has been enabled in LLVM JIT, Mesa JIT as well as sljit which is
used by jit.  So the current model works with JIT.

Yu-cheng