Messages in this thread Patch in this message | | | From | paulmck@kernel ... | Subject | [PATCH kcsan 3/3] kcsan: Fix encoding masks and regain address bit | Date | Thu, 5 Nov 2020 14:03:24 -0800 |
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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); +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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