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overflow: Introduce wrapping_assign_add() and wrapping_assign_sub()

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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.

Reviewed-by: Marco Elver <elver@google.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
This commit is contained in:
Kees Cook
2024-01-26 22:09:50 -08:00
parent d70de8054c
commit 08d45ee84b
2 changed files with 75 additions and 0 deletions
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32
include/linux/overflow.h
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@@ -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