On Fri, Mar 16, 2018 at 12:49 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > On Thu, Mar 15, 2018 at 4:46 PM, Linus Torvalds > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: >> What I'm *not* so much ok with is "const_max(5,sizeof(x))" erroring >> out, or silently causing insane behavior due to hidden subtle type >> casts.. > > Yup! I like it as an explicit argument. Thanks! > What about something like this? #define INTMAXT_MAX LLONG_MAX typedef int64_t intmax_t; #define const_max(x, y) \ __builtin_choose_expr( \ !__builtin_constant_p(x) || !__builtin_constant_p(y), \ __error_not_const_arg(), \ __builtin_choose_expr( \ (x) > INTMAXT_MAX || (y) > INTMAXT_MAX, \ __error_too_big(), \ __builtin_choose_expr( \ (intmax_t)(x) >= (intmax_t)(y), \ (x), \ (y) \ ) \ ) \ ) Works for different types, allows to mix negatives and positives and returns the original type, e.g.: const_max(-1, sizeof(char)); is of type 'long unsigned int', but: const_max(2, sizeof(char)); is of type 'int'. While I am not a fan that the return type depends on the arguments, it is useful if you are going to use the expression in something that needs expects a precise (a printk() for instance?). The check against the INTMAXT_MAX is there to avoid complexity (if we do not handle those cases, it is safe to use intmax_t for the comparison; otherwise you have to have another compile time branch for the case positive-positive using uintmax_t) and also avoids odd warnings for some cases above LLONG_MAX about comparisons with 0 for unsigned expressions being always true. On the positive side, it prevents using the macro for thing like "(size_t)-1". Cheers, Miguel -- To unsubscribe from this list: send the line "unsubscribe linux-input" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html