Re: [RFC 00/12] xfs: more and better verifiers

[Dave Chinner <david@fromorbit.com>]

On Fri, Aug 18, 2017 at 05:58:16PM -0700, Darrick J. Wong wrote:
> On Sat, Aug 19, 2017 at 10:33:00AM +1000, Dave Chinner wrote:
> > On Fri, Aug 18, 2017 at 11:45:11AM -0700, Darrick J. Wong wrote:
> > > On Fri, Aug 18, 2017 at 10:06:07AM -0700, Darrick J. Wong wrote:
> > > > ...which is totally worthless, unless we want to compile all the verifier
> > > > functions with __attribute__((optimize("O0"))), which is bogus.
> > > > 
> > > > <sigh> Back to the drawing board on that one.
> > > 
> > > Ok, there's /slightly/ less awful way to prevent gcc from optimizing the
> > > verifier function to the point of imprecise pointer value, but it involves
> > > writing to a volatile int:
> > > 
> > > /* stupidly prevent gcc from over-optimizing getting the instruction ptr */
> > > extern volatile int xfs_lineno;
> > > #define __this_address ({ __label__ __here; __here: xfs_lineno = __LINE__; &&__here; })
> > > 
> > > <grumble> Yucky, but it more or less works.
> > 
> > Can you declare the label as volatile, like you can an asm
> > statement to prevent the compiler from optimising out asm
> > statements?
> > 
> > Even so, given the yuckiness is very isolated and should only affect
> > the slow path code, I can live with this.
> 
> Hmmm.  I can't declare the label as volatile, but I /can/ inject
> asm volatile("") and that seems to prevent gcc from moving code hunks
> around:
> 
> #define __this_address	({ __label__ __here; __here: asm volatile(""); &&__here; })

That seems cleaner to me, and I /think/ the gcc manual says it won't
remove such statements, but it also says:

	Under certain circumstances, GCC may duplicate (or remove duplicates
	of) your assembly code when optimizing.

So I have no real idea whether this is going to be robust or not.
I'm not a gcc/asm expert at all (that stuff is mostly black magic
to me).

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com