Re: [PATCH kcsan 3/3] kcsan: Fix encoding masks and regain address bit

[Boqun Feng]

On Fri, Nov 06, 2020 at 10:03:21AM +0100, Marco Elver wrote:
> On Fri, 6 Nov 2020 at 02:23, Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
> > Hi Marco,
> >
> > On Thu, Nov 05, 2020 at 02:03:24PM -0800, paulmck@xxxxxxxxxx wrote:
> > > From: Marco Elver <elver@xxxxxxxxxx>
> > >
> > > The watchpoint encoding masks for size and address were off-by-one bit
> > > each, with the size mask using 1 unnecessary bit and the address mask
> > > missing 1 bit. However, due to the way the size is shifted into the
> > > encoded watchpoint, we were effectively wasting and never using the
> > > extra bit.
> > >
> > > For example, on x86 with PAGE_SIZE==4K, we have 1 bit for the is-write
> > > bit, 14 bits for the size bits, and then 49 bits left for the address.
> > > Prior to this fix we would end up with this usage:
> > >
> > >       [ write<1> | size<14> | wasted<1> | address<48> ]
> > >
> > > Fix it by subtracting 1 bit from the GENMASK() end and start ranges of
> > > size and address respectively. The added static_assert()s verify that
> > > the masks are as expected. With the fixed version, we get the expected
> > > usage:
> > >
> > >       [ write<1> | size<14> |             address<49> ]
> > >
> > > Functionally no change is expected, since that extra address bit is
> > > insignificant for enabled architectures.
> > >
> > > Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > > ---
> > >  kernel/kcsan/encoding.h | 14 ++++++--------
> > >  1 file changed, 6 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/kernel/kcsan/encoding.h b/kernel/kcsan/encoding.h
> > > index 4f73db6..b50bda9 100644
> > > --- a/kernel/kcsan/encoding.h
> > > +++ b/kernel/kcsan/encoding.h
> > > @@ -37,14 +37,12 @@
> > >   */
> > >  #define WATCHPOINT_ADDR_BITS (BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
> > >
> > > -/*
> > > - * Masks to set/retrieve the encoded data.
> > > - */
> > > -#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG-1)
> > > -#define WATCHPOINT_SIZE_MASK                                                   \
> > > -     GENMASK(BITS_PER_LONG-2, BITS_PER_LONG-2 - WATCHPOINT_SIZE_BITS)
> > > -#define WATCHPOINT_ADDR_MASK                                                   \
> > > -     GENMASK(BITS_PER_LONG-3 - WATCHPOINT_SIZE_BITS, 0)
> > > +/* Bitmasks for the encoded watchpoint access information. */
> > > +#define WATCHPOINT_WRITE_MASK        BIT(BITS_PER_LONG-1)
> > > +#define WATCHPOINT_SIZE_MASK GENMASK(BITS_PER_LONG-2, BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
> > > +#define WATCHPOINT_ADDR_MASK GENMASK(BITS_PER_LONG-2 - WATCHPOINT_SIZE_BITS, 0)
> > > +static_assert(WATCHPOINT_ADDR_MASK == (1UL << WATCHPOINT_ADDR_BITS) - 1);
> >
> > Nit:
> >
> > Since you use the static_assert(), why not define WATCHPOINT_ADDR_MASK
> > as:
> >
> > #define WATCHPOINT_ADDR_MASK (BIT(WATCHPOINT_SIZE_BITS) - 1)
> 
> This is incorrect, as the static_assert()s would have indicated. It
> should probably be (BIT(WATCHPOINT_ADDR_BITS) - 1)?
> 
> As an aside, I explicitly did *not* want to use additional arithmetic
> to generate the masks but purely rely on BIT(), and GENMASK(), as it
> would be inconsistent otherwise. The static_assert()s then sanity
> check everything without BIT+GENMASK (because I've grown slightly
> paranoid about off-by-1s here). So I'd rather not start bikeshedding
> about which way around things should go.
> 
> In general, GENMASK() is safer, because subtracting 1 to get the mask
> doesn't always work, specifically e.g. (BIT(BITS_PER_LONG) - 1) does
> not work.
> 
> > Besides, WATCHPOINT_SIZE_MASK can also be defined as:
> 
> No, sorry it cannot.
> 
> > #define WATCHPOINT_SIZE_MASK GENMASK(BITS_PER_LONG - 2, WATCHPOINT_SIZE_BITS)
> 
>    GENMASK(BITS_PER_LONG - 2, WATCHPOINT_SIZE_BITS)
> 
> is not equivalent to the current
> 
>   GENMASK(BITS_PER_LONG-2, BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
> 
> Did you mean GENMASK(BITS_PER_LONG-2, WATCHPOINT_ADDR_BITS)? I can

You're right! Guess I should check first about what vim completes for me
;-) And I agree with you on the preference to GENMASK()

> send a v2 for this one.

Let me add an ack for that one, thanks!

Regards,
Boqun

> 
> Thanks,
> -- Marco