Re: [PATCH v7 3/3] overflow: Introduce wrapping_assign_add() and wrapping_assign_sub()

[Marco Elver]

On Wed, 14 Feb 2024 at 20:46, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> This allows replacements of the idioms "var += offset" and "var -=
> offset" with the wrapping_assign_add() and wrapping_assign_sub() helpers
> respectively. They will avoid wrap-around sanitizer instrumentation.
>
> Add to the selftests to validate behavior and lack of side-effects.
>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>

Reviewed-by: Marco Elver <elver@xxxxxxxxxx>

> ---
> Cc: Rasmus Villemoes <rasmus.villemoes@xxxxxxxxx>
> Cc: Marco Elver <elver@xxxxxxxxxx>
> Cc: Eric Biggers <ebiggers@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: "Gustavo A. R. Silva" <gustavoars@xxxxxxxxxx>
> Cc: linux-hardening@xxxxxxxxxxxxxxx
> ---
>  include/linux/overflow.h | 32 ++++++++++++++++++++++++++++++
>  lib/overflow_kunit.c     | 43 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 75 insertions(+)
>
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index d3ff8e2bec29..dede374832c9 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -81,6 +81,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
>                 __val;                                          \
>         })
>
> +/**
> + * wrapping_assign_add() - Intentionally perform a wrapping increment assignment
> + * @var: variable to be incremented
> + * @offset: amount to add
> + *
> + * Increments @var by @offset with wrap-around. Returns the resulting
> + * value of @var. Will not trip any wrap-around sanitizers.
> + *
> + * Returns the new value of @var.
> + */
> +#define wrapping_assign_add(var, offset)                               \
> +       ({                                                              \
> +               typeof(var) *__ptr = &(var);                            \
> +               *__ptr = wrapping_add(typeof(var), *__ptr, offset);     \
> +       })
> +
>  /**
>   * check_sub_overflow() - Calculate subtraction with overflow checking
>   * @a: minuend; value to subtract from
> @@ -111,6 +127,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
>                 __val;                                          \
>         })
>
> +/**
> + * wrapping_assign_sub() - Intentionally perform a wrapping decrement assign
> + * @var: variable to be decremented
> + * @offset: amount to subtract
> + *
> + * Decrements @var by @offset with wrap-around. Returns the resulting
> + * value of @var. Will not trip any wrap-around sanitizers.
> + *
> + * Returns the new value of @var.
> + */
> +#define wrapping_assign_sub(var, offset)                               \
> +       ({                                                              \
> +               typeof(var) *__ptr = &(var);                            \
> +               *__ptr = wrapping_sub(typeof(var), *__ptr, offset);     \
> +       })
> +
>  /**
>   * check_mul_overflow() - Calculate multiplication with overflow checking
>   * @a: first factor
> diff --git a/lib/overflow_kunit.c b/lib/overflow_kunit.c
> index d3fdb906d3fe..65e8a72a83bf 100644
> --- a/lib/overflow_kunit.c
> +++ b/lib/overflow_kunit.c
[...]
> +       /* wrapping_assign_{add,sub}() */                               \
> +       check_self_op(fmt, assign_add, +=, p->a, p->b);                 \
> +       check_self_op(fmt, assign_add, +=, p->b, p->a);                 \
> +       check_self_op(fmt, assign_sub, -=, p->a, p->b);                 \
>  }                                                                      \

Merely a curiosity, and am not suggesting this for this patch: I
wonder how much of this could be tested at compile-time. These are
very primitive operations, so I suspect the compiler could either
check these in a static_assert(), or if some of it isn't
constexpr-friendly, after optimizations with a BUILD_BUG.