The check_*_overflow() helpers will return results with potentially
wrapped-around values. These values have always been checked by the
selftests, so avoid the confusing language in the kern-doc. The idea of
"safe for use" was relative to the expectation of whether or not the
caller wants a wrapped value -- the calculation itself will always follow
arithmetic wrapping rules.
Reviewed-by: Gustavo A. R. Silva <[email protected]>
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
---
include/linux/overflow.h | 18 ++++++------------
1 file changed, 6 insertions(+), 12 deletions(-)
diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 7b5cf4a5cd19..4e741ebb8005 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -57,11 +57,9 @@ static inline bool __must_check __must_check_overflow(bool overflow)
* @b: second addend
* @d: pointer to store sum
*
- * Returns 0 on success.
+ * Returns 0 on success, 1 on wrap-around.
*
- * *@d holds the results of the attempted addition, but is not considered
- * "safe for use" on a non-zero return value, which indicates that the
- * sum has overflowed or been truncated.
+ * *@d holds the results of the attempted addition, which may wrap-around.
*/
#define check_add_overflow(a, b, d) \
__must_check_overflow(__builtin_add_overflow(a, b, d))
@@ -72,11 +70,9 @@ static inline bool __must_check __must_check_overflow(bool overflow)
* @b: subtrahend; value to subtract from @a
* @d: pointer to store difference
*
- * Returns 0 on success.
+ * Returns 0 on success, 1 on wrap-around.
*
- * *@d holds the results of the attempted subtraction, but is not considered
- * "safe for use" on a non-zero return value, which indicates that the
- * difference has underflowed or been truncated.
+ * *@d holds the results of the attempted subtraction, which may wrap-around.
*/
#define check_sub_overflow(a, b, d) \
__must_check_overflow(__builtin_sub_overflow(a, b, d))
@@ -87,11 +83,9 @@ static inline bool __must_check __must_check_overflow(bool overflow)
* @b: second factor
* @d: pointer to store product
*
- * Returns 0 on success.
+ * Returns 0 on success, 1 on wrap-around.
*
- * *@d holds the results of the attempted multiplication, but is not
- * considered "safe for use" on a non-zero return value, which indicates
- * that the product has overflowed or been truncated.
+ * *@d holds the results of the attempted multiplication, which may wrap-around.
*/
#define check_mul_overflow(a, b, d) \
__must_check_overflow(__builtin_mul_overflow(a, b, d))
--
2.34.1
On Tue, Feb 13, 2024 at 02:10:57PM -0800, Kees Cook wrote:
> The check_*_overflow() helpers will return results with potentially
> wrapped-around values. These values have always been checked by the
> selftests, so avoid the confusing language in the kern-doc. The idea of
> "safe for use" was relative to the expectation of whether or not the
> caller wants a wrapped value -- the calculation itself will always follow
> arithmetic wrapping rules.
>
> Reviewed-by: Gustavo A. R. Silva <[email protected]>
> Cc: [email protected]
> Signed-off-by: Kees Cook <[email protected]>
> ---
> include/linux/overflow.h | 18 ++++++------------
> 1 file changed, 6 insertions(+), 12 deletions(-)
>
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index 7b5cf4a5cd19..4e741ebb8005 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -57,11 +57,9 @@ static inline bool __must_check __must_check_overflow(bool overflow)
> * @b: second addend
> * @d: pointer to store sum
> *
> - * Returns 0 on success.
> + * Returns 0 on success, 1 on wrap-around.
Sorry for the last minute bikeshedding, but could we clarify 'success' here?
e.g. I think it'd be clearer to say:
Returns true on wrap-around, false otherwise.
Note that also uses true/false since these all return bool (as do the
underlying __builtin_*_overflow() functions).
> *
> - * *@d holds the results of the attempted addition, but is not considered
> - * "safe for use" on a non-zero return value, which indicates that the
> - * sum has overflowed or been truncated.
> + * *@d holds the results of the attempted addition, which may wrap-around.
How about:
@d holds the results of the attempted addition, regardless of whether
wrap-around occurred.
.. and likewise for the others below?
Mark.
> */
> #define check_add_overflow(a, b, d) \
> __must_check_overflow(__builtin_add_overflow(a, b, d))
> @@ -72,11 +70,9 @@ static inline bool __must_check __must_check_overflow(bool overflow)
> * @b: subtrahend; value to subtract from @a
> * @d: pointer to store difference
> *
> - * Returns 0 on success.
> + * Returns 0 on success, 1 on wrap-around.
> *
> - * *@d holds the results of the attempted subtraction, but is not considered
> - * "safe for use" on a non-zero return value, which indicates that the
> - * difference has underflowed or been truncated.
> + * *@d holds the results of the attempted subtraction, which may wrap-around.
> */
> #define check_sub_overflow(a, b, d) \
> __must_check_overflow(__builtin_sub_overflow(a, b, d))
> @@ -87,11 +83,9 @@ static inline bool __must_check __must_check_overflow(bool overflow)
> * @b: second factor
> * @d: pointer to store product
> *
> - * Returns 0 on success.
> + * Returns 0 on success, 1 on wrap-around.
> *
> - * *@d holds the results of the attempted multiplication, but is not
> - * considered "safe for use" on a non-zero return value, which indicates
> - * that the product has overflowed or been truncated.
> + * *@d holds the results of the attempted multiplication, which may wrap-around.
> */
> #define check_mul_overflow(a, b, d) \
> __must_check_overflow(__builtin_mul_overflow(a, b, d))
> --
> 2.34.1
>
On Wed, Feb 14, 2024 at 11:57:28AM +0000, Mark Rutland wrote:
> On Tue, Feb 13, 2024 at 02:10:57PM -0800, Kees Cook wrote:
> > The check_*_overflow() helpers will return results with potentially
> > wrapped-around values. These values have always been checked by the
> > selftests, so avoid the confusing language in the kern-doc. The idea of
> > "safe for use" was relative to the expectation of whether or not the
> > caller wants a wrapped value -- the calculation itself will always follow
> > arithmetic wrapping rules.
> >
> > Reviewed-by: Gustavo A. R. Silva <[email protected]>
> > Cc: [email protected]
> > Signed-off-by: Kees Cook <[email protected]>
> > ---
> > include/linux/overflow.h | 18 ++++++------------
> > 1 file changed, 6 insertions(+), 12 deletions(-)
> >
> > diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> > index 7b5cf4a5cd19..4e741ebb8005 100644
> > --- a/include/linux/overflow.h
> > +++ b/include/linux/overflow.h
> > @@ -57,11 +57,9 @@ static inline bool __must_check __must_check_overflow(bool overflow)
> > * @b: second addend
> > * @d: pointer to store sum
> > *
> > - * Returns 0 on success.
> > + * Returns 0 on success, 1 on wrap-around.
>
> Sorry for the last minute bikeshedding, but could we clarify 'success' here?
> e.g. I think it'd be clearer to say:
>
> Returns true on wrap-around, false otherwise.
>
> Note that also uses true/false since these all return bool (as do the
> underlying __builtin_*_overflow() functions).
Yeah, that's a good point. I'll update this.
> > *
> > - * *@d holds the results of the attempted addition, but is not considered
> > - * "safe for use" on a non-zero return value, which indicates that the
> > - * sum has overflowed or been truncated.
> > + * *@d holds the results of the attempted addition, which may wrap-around.
>
> How about:
>
> @d holds the results of the attempted addition, regardless of whether
> wrap-around occurred.
>
> ... and likewise for the others below?
Yeah, that's more clear. Thanks!
-Kees
--
Kees Cook