[LLVMdev] MemorySanitizer, a tool that finds uninitialized reads and more

[Joerg Sonnenberger]

On Mon, Jun 18, 2012 at 06:44:57PM +0400, Kostya Serebryany wrote:
> On Mon, Jun 18, 2012 at 6:30 PM, Joerg Sonnenberger <joerg at britannica.bec.de
> > wrote:
> 
> > On Mon, Jun 18, 2012 at 05:52:49PM +0400, Kostya Serebryany wrote:
> > > On Mon, Jun 18, 2012 at 5:43 PM, Joerg Sonnenberger <
> > joerg at britannica.bec.de
> > > > wrote:
> > >
> > > > On Mon, Jun 18, 2012 at 05:19:11PM +0400, Kostya Serebryany wrote:
> > > > > On Mon, Jun 18, 2012 at 5:07 PM, Joerg Sonnenberger <
> > > > joerg at britannica.bec.de
> > > > > > wrote:
> > > > >
> > > > > > On Mon, Jun 18, 2012 at 02:39:34PM +0400, Kostya Serebryany wrote:
> > > > > > > Another difference from Memcheck is that we propose to use 8
> > shadow
> > > > bits
> > > > > > > per byte of application memory and use a
> > > > > > > direct shadow mapping (for 64-bit linux that is just clearing
> > 46-th
> > > > bit
> > > > > > of
> > > > > > > the application memory address).
> > > > > > > This greatly simplifies the instrumentation code and avoids
> > races on
> > > > > > shadow
> > > > > > > updates
> > > > > > > (Memcheck is single-threaded so races are not a concern there.
> > > > > > > Memcheck uses 2 shadow bits per byte with a slow path storage
> > that
> > > > uses 8
> > > > > > > bits per byte).
> > > > > >
> > > > > > Can you make it possible for ASAN to share the same layout?
> > > > >
> > > > >
> > > > > Not sure I understand you. What layout?
> > > >
> > > > Shadow memory.
> > > >
> > >
> > > yes, asan and msan shadow could co-exist, at least on 64-bit linux with
> > > disabled ASLR.
> > > But the problem is that the memory overheads will multiply -- the
> > combined
> > > tool will be more expensive to use
> > > than two separate tools together.
> >
> > Which is what I am asking about. I don't really have a problem with
> > using a one-to-one mapping, if it makes both ASAN and MSAN more
> > efficient in terms of runtime overhead.
> >
> 
> one-to-one mapping will make ASAN much less efficient.
> I meant that we may have both mappings (1:1 for MSAN and 8:1 for ASAN) in
> the same process, but it makes little sense to me.

Why can't ASAN use the same window as MSAN? That's the part I don't
understand.

Joerg