On Fri, 6 Nov 2020 at 02:23, Boqun Feng <boqun.f...@gmail.com> wrote: > Hi Marco, > > On Thu, Nov 05, 2020 at 02:03:24PM -0800, paul...@kernel.org wrote: > > From: Marco Elver <el...@google.com> > > > > 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 <el...@google.com> > > Signed-off-by: Paul E. McKenney <paul...@kernel.org> > > --- > > 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 send a v2 for this one. Thanks, -- Marco
