On Sat, Aug 15, 2020 at 10:09:24AM -0700, Kees Cook wrote: > Since the destination variable of the check_*_overflow() helpers will > contain a wrapped value on failure, it would be best to make sure callers > really did check the return result of the helper. Adjust the macros to use > a bool-wrapping static inline that is marked with __must_check. This means > the macros can continue to have their type-agnostic behavior while gaining > the function attribute (that cannot be applied directly to macros). > > Suggested-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> > --- > v2: > - de-generalized __must_check_overflow() from being named "bool" (willy) > - fix comment typos (rasmus) > v1: https://lore.kernel.org/lkml/202008121450.405E4A3@keescook > --- > include/linux/overflow.h | 39 ++++++++++++++++++++++++--------------- > 1 file changed, 24 insertions(+), 15 deletions(-) > > diff --git a/include/linux/overflow.h b/include/linux/overflow.h > index 93fcef105061..f1c4e7b56bd9 100644 > --- a/include/linux/overflow.h > +++ b/include/linux/overflow.h > @@ -43,6 +43,16 @@ > #define is_non_negative(a) ((a) > 0 || (a) == 0) > #define is_negative(a) (!(is_non_negative(a))) > > +/* > + * Allows for effectively applying __must_check to a macro so we can have > + * both the type-agnostic benefits of the macros while also being able to > + * enforce that the return value is, in fact, checked. > + */ > +static inline bool __must_check __must_check_overflow(bool overflow) > +{ > + return unlikely(overflow); How does the 'unlikely' hint propagate through return? It is in a static inline so compiler has complete information in order to use it, but I'm curious if it actually does. > +} > + > #ifdef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW > /* > * For simplicity and code hygiene, the fallback code below insists on > @@ -52,32 +62,32 @@ > * alias for __builtin_add_overflow, but add type checks similar to > * below. > */ > -#define check_add_overflow(a, b, d) ({ \ > +#define check_add_overflow(a, b, d) __must_check_overflow(({ \ > typeof(a) __a = (a); \ > typeof(b) __b = (b); \ > typeof(d) __d = (d); \ > (void) (&__a == &__b); \ > (void) (&__a == __d); \ > __builtin_add_overflow(__a, __b, __d); \ > -}) > +})) In case the hint gets dropped, the fix would probably be #define check_add_overflow(a, b, d) unlikely(__must_check_overflow(({ \ typeof(a) __a = (a); \ typeof(b) __b = (b); \ typeof(d) __d = (d); \ (void) (&__a == &__b); \ (void) (&__a == __d); \ __builtin_add_overflow(__a, __b, __d); \ })))
