Re: [PATCH v2] kcsan: Fix encoding masks and regain address bit

[Boqun Feng]

On Fri, Nov 06, 2020 at 10:34:56AM +0100, 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@xxxxxxxxxx>

Acked-by: Boqun Feng <boqun.feng@xxxxxxxxx>

Regards,
Boqun

> ---
> v2:
> * Use WATCHPOINT_ADDR_BITS to avoid duplicating "BITS_PER_LONG-1 -
>   WATCHPOINT_SIZE_BITS" per Boqun's suggestion.
> ---
>  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 4f73db6d1407..7ee405524904 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, WATCHPOINT_ADDR_BITS)
> +#define WATCHPOINT_ADDR_MASK	GENMASK(WATCHPOINT_ADDR_BITS-1, 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)
>  {
> -- 
> 2.29.2.222.g5d2a92d10f8-goog
>