diff mbox series

[4/5] overflow: Introduce add_wrap(), sub_wrap(), and mul_wrap()

Message ID 20240129183411.3791340-4-keescook@chromium.org
State New
Headers
Series overflow: Introduce wrapping helpers |

Commit Message

Kees Cook Jan. 29, 2024, 6:34 p.m. UTC 
  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(+)

Comments

Rasmus Villemoes Jan. 29, 2024, 8:08 p.m. UTC | #1 
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.


> 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
Kees Cook Jan. 29, 2024, 8:26 p.m. UTC | #2 
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!
diff mbox series

Patch

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