On 2018-03-16 00:46, Linus Torvalds wrote: > On Thu, Mar 15, 2018 at 4:41 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: >> >> I much prefer explicit typing, but both you and Rasmus mentioned >> wanting the int/sizeof_t mixing. > > Well, the explicit typing allows that mixing, in that you can just > have "const_max_t(5,sizeof(x))" > > So I'm ok with that. > > 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.. I don't like const_max_t, at least not as the "primary" interface - forcing the user to pass in a type, or equivalently passing in cast expressions to a const_max(), can hide errors, e.g. if the -1 is really SOME_MACRO or some complicated expression that is usually positive, but that expression always gets cast to size_t because the user was forced to do const_max_t(size_t, SOME_MACRO, sizeof(foo)) to make the code compile. Not to mention that it's both easier to read and write if one could just do const_max(SOME_MACRO, sizeof(foo)) Can we instead do one of the following: (1) Effectively do the comparison in an infinitely wide signed integer, i.e. implement x < 0 && y >= 0 --> y x >= 0 && y < 0 --> x otherwise, if both have the same sign (but not necessarily the same signedness of their types), the type promotions do not alter either's value, so __builtin_choose_expr(x > y, x, y) will do the right thing with the resulting thing having the same type as the chosen one of x and y. [Or having type typeof(x+y), which would just be a cast in the macro.] This would allow const_max(-1, sizeof(foo)) and give sizeof(foo), but perhaps that's too magic. (2) Allow mixed types, but ensure the build fails if one of the values is not representable in typeof(x+y) (i.e., one value is negative but the common type is unsigned). That allows the const_max(SOME_MACRO, sizeof()), but prevents silent failure in case some weird combination of CONFIG options make SOME_MACRO evaluate to something negative. The user can always pass in (size_t)-1 explicitly if needed, or cast the sizeof() to int if that's what makes sense, but that's a case-by-case thing. I'd really like that the simple case const_max(16, sizeof(foo)) Just Works. Then if a lot users turn up that do need some casting, const_max_t can be implemented as a trivial const_max wrapper. Rasmus (1) something like __builtin_choose_expr((x >= 0 && y < 0) || \ (x >= 0 && y >= 0 && x > y) || \ (x < 0 && y < 0 && x > y), x, y) (2) something like // 1 or build error #define __check_promotion(t, x) ( 1/(((t)(x) < 0) == ((x) < 0)) ) __builtin_choose_expr(__check_promotion(typeof((x)+(y)), x) && \ __check_promotion(typeof((x)+(y)), y) && \ (x) > (y), x, y) Not sure how to get a more sensible error message, I'd like this to also work outside functions. -- 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
