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

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Message-ID: <20190219115840.GV32477@hirez.programming.kicks-ass.net>
Date:   Tue, 19 Feb 2019 12:58:40 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...capital.net>,
        Julien Thierry <julien.thierry@....com>,
        Will Deacon <will.deacon@....com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, mingo@...hat.com,
        catalin.marinas@....com, james.morse@....com,
        valentin.schneider@....com, brgerst@...il.com, jpoimboe@...hat.com,
        luto@...nel.org, bp@...en8.de, dvlasenk@...hat.com,
        torvalds@...ux-foundation.org
Subject: Re: [PATCH] sched/x86: Save [ER]FLAGS on context switch

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'.