Messages in this thread | | | Date | Fri, 6 Nov 2020 09:23:35 +0800 | From | Boqun Feng <> | Subject | Re: [PATCH kcsan 3/3] kcsan: Fix encoding masks and regain address bit |
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Hi Marco,
On Thu, Nov 05, 2020 at 02:03:24PM -0800, paulmck@kernel.org wrote: > From: Marco Elver <elver@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 <elver@google.com> > Signed-off-by: Paul E. McKenney <paulmck@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)
Besides, WATCHPOINT_SIZE_MASK can also be defined as:
#define WATCHPOINT_SIZE_MASK GENMASK(BITS_PER_LONG - 2, WATCHPOINT_SIZE_BITS)
Regards, Boqun
> +static_assert((WATCHPOINT_WRITE_MASK ^ WATCHPOINT_SIZE_MASK ^ WATCHPOINT_ADDR_MASK) == ~0UL); > > static inline bool check_encodable(unsigned long addr, size_t size) > { > -- > 2.9.5 >
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