Re: [PATCH] sched/x86: Save [ER]FLAGS on context switch

[Peter Zijlstra]

On Tue, Feb 19, 2019 at 12:38:42PM +0100, Thomas Gleixner wrote:
> On Tue, 19 Feb 2019, Peter Zijlstra wrote:
> 
> > On Tue, Feb 19, 2019 at 10:04:09AM +0100, Peter Zijlstra wrote:
> > > > Does that make more sense?
> > > 
> > > It appears to me you're going about it backwards.
> > 
> > So how about you do a GCC plugin that verifies limits on code-gen
> > between user_access_begin/user_access_end() ?
> > 
> >  - No CALL/RET
> >    - implies user_access_end() happens
> >    - implies no fentry hooks
> >  - No __preempt_count frobbing
> >  - No tracepoints
> >  - ...
> > 
> > That way you put the burden on the special code, not on the rest of the
> > kernel.
> 
> And then you have kprobes ....

They prod the INT3 byte and then take an exception, and exceptions are
'fine'.