Provide helpers that will perform wrapping addition, subtraction, or
multiplication without tripping the arithmetic wrap-around sanitizers. The
first argument is the type under which the wrap-around should happen
with. In other words, these two calls will get very different results:
add_wrap(int, 50, 50) == 2500
add_wrap(u8, 50, 50) == 196
Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
include/linux/overflow.h | 54 ++++++++++++++++++++++++++++++++++++++++
1 file changed, 54 insertions(+)
diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 3c46c648d2e8..4f945e9e7881 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -120,6 +120,24 @@ static inline bool __must_check __must_check_overflow(bool overflow)
check_add_overflow(var, offset, &__result); \
}))
+/**
+ * add_wrap() - Intentionally perform a wrapping addition
+ * @type: type to check overflow against
+ * @a: first addend
+ * @b: second addend
+ *
+ * Return the potentially wrapped-around addition without
+ * tripping any overflow sanitizers that may be enabled.
+ */
+#define add_wrap(type, a, b) \
+ ({ \
+ type __sum; \
+ if (check_add_overflow(a, b, &__sum)) { \
+ /* do nothing */ \
+ } \
+ __sum; \
+ })
+
/**
* check_sub_overflow() - Calculate subtraction with overflow checking
* @a: minuend; value to subtract from
@@ -133,6 +151,24 @@ static inline bool __must_check __must_check_overflow(bool overflow)
#define check_sub_overflow(a, b, d) \
__must_check_overflow(__builtin_sub_overflow(a, b, d))
+/**
+ * sub_wrap() - Intentionally perform a wrapping subtraction
+ * @type: type to check underflow against
+ * @a: minuend; value to subtract from
+ * @b: subtrahend; value to subtract from @a
+ *
+ * Return the potentially wrapped-around subtraction without
+ * tripping any overflow sanitizers that may be enabled.
+ */
+#define sub_wrap(type, a, b) \
+ ({ \
+ type __val; \
+ if (check_sub_overflow(a, b, &__val)) { \
+ /* do nothing */ \
+ } \
+ __val; \
+ })
+
/**
* check_mul_overflow() - Calculate multiplication with overflow checking
* @a: first factor
@@ -146,6 +182,24 @@ static inline bool __must_check __must_check_overflow(bool overflow)
#define check_mul_overflow(a, b, d) \
__must_check_overflow(__builtin_mul_overflow(a, b, d))
+/**
+ * mul_wrap() - Intentionally perform a wrapping multiplication
+ * @type: type to check underflow against
+ * @a: first factor
+ * @b: second factor
+ *
+ * Return the potentially wrapped-around multiplication without
+ * tripping any overflow sanitizers that may be enabled.
+ */
+#define mul_wrap(type, a, b) \
+ ({ \
+ type __val; \
+ if (check_mul_overflow(a, b, &__val)) { \
+ /* do nothing */ \
+ } \
+ __val; \
+ })
+
/**
* check_shl_overflow() - Calculate a left-shifted value and check overflow
* @a: Value to be shifted
--
2.34.1On 29/01/2024 19.34, Kees Cook wrote: > Provide helpers that will perform wrapping addition, subtraction, or > multiplication without tripping the arithmetic wrap-around sanitizers. The > first argument is the type under which the wrap-around should happen > with. In other words, these two calls will get very different results: > > add_wrap(int, 50, 50) == 2500 > add_wrap(u8, 50, 50) == 196 s/add/mul/g I suppose. > Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk> > Cc: Mark Rutland <mark.rutland@arm.com> > Cc: linux-hardening@vger.kernel.org > Signed-off-by: Kees Cook <keescook@chromium.org> > --- > include/linux/overflow.h | 54 ++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 54 insertions(+) > > diff --git a/include/linux/overflow.h b/include/linux/overflow.h > index 3c46c648d2e8..4f945e9e7881 100644 > --- a/include/linux/overflow.h > +++ b/include/linux/overflow.h > @@ -120,6 +120,24 @@ static inline bool __must_check __must_check_overflow(bool overflow) > check_add_overflow(var, offset, &__result); \ > })) > > +/** > + * add_wrap() - Intentionally perform a wrapping addition > + * @type: type to check overflow against Well, nothing is "checked", so why not just say "type of result"? > > +/** > + * sub_wrap() - Intentionally perform a wrapping subtraction > + * @type: type to check underflow against The terminology becomes muddy, is (INT_MAX) - (-1) an underflow or overflow? Anyway, see above. > > +/** > + * mul_wrap() - Intentionally perform a wrapping multiplication > + * @type: type to check underflow against And here there's definitely a copy-pasto. The code itself looks fine. Rasmus
On Mon, Jan 29, 2024 at 09:08:43PM +0100, Rasmus Villemoes wrote: > On 29/01/2024 19.34, Kees Cook wrote: > > Provide helpers that will perform wrapping addition, subtraction, or > > multiplication without tripping the arithmetic wrap-around sanitizers. The > > first argument is the type under which the wrap-around should happen > > with. In other words, these two calls will get very different results: > > > > add_wrap(int, 50, 50) == 2500 > > add_wrap(u8, 50, 50) == 196 > > s/add/mul/g I suppose. Oops, yes. > > Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk> > > Cc: Mark Rutland <mark.rutland@arm.com> > > Cc: linux-hardening@vger.kernel.org > > Signed-off-by: Kees Cook <keescook@chromium.org> > > --- > > include/linux/overflow.h | 54 ++++++++++++++++++++++++++++++++++++++++ > > 1 file changed, 54 insertions(+) > > > > diff --git a/include/linux/overflow.h b/include/linux/overflow.h > > index 3c46c648d2e8..4f945e9e7881 100644 > > --- a/include/linux/overflow.h > > +++ b/include/linux/overflow.h > > @@ -120,6 +120,24 @@ static inline bool __must_check __must_check_overflow(bool overflow) > > check_add_overflow(var, offset, &__result); \ > > })) > > > > +/** > > + * add_wrap() - Intentionally perform a wrapping addition > > + * @type: type to check overflow against > > Well, nothing is "checked", so why not just say "type of result"? Yeah, that's better. I was trying to describe that @type will affect the value of the result. > > +/** > > + * sub_wrap() - Intentionally perform a wrapping subtraction > > + * @type: type to check underflow against > > The terminology becomes muddy, is (INT_MAX) - (-1) an underflow or > overflow? Anyway, see above. Right, I should explicitly say "wrap-around". > > > > > +/** > > + * mul_wrap() - Intentionally perform a wrapping multiplication > > + * @type: type to check underflow against > > And here there's definitely a copy-pasto. Ek, yes. > The code itself looks fine. Thanks! -- Kees Cook
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