Messages in this thread | | | Date | Mon, 26 Oct 2020 16:33:54 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] kcsan: Fix encoding masks and regain address bit |
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On Fri, Oct 23, 2020 at 02:12:24PM +0200, Marco Elver wrote: > 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>
Queued and pushed, thank you!!!
Thanx, Paul
> --- > 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 64b3c0f2a685..fc5154dd2475 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); > +static_assert((WATCHPOINT_WRITE_MASK ^ WATCHPOINT_SIZE_MASK ^ WATCHPOINT_ADDR_MASK) == ~0UL); > > static inline bool check_encodable(unsigned long addr, size_t size) > { > -- > 2.29.0.rc1.297.gfa9743e501-goog >
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