BTW my main email addess is now: uecker@xxxxxxxxx My suggestion would also to limit explanation. Nobody should write such code and if you need to, you can find explanations all over the internet. Finally, I still think the motivation for this macro (removing VLAs) is misguided if security is the goal because VLAs provide precise bounds and larger worst-case fixed-size arrays do not. It would be better to use the compiler options that detect possibly use of VLAs of unbounded size and if there a problems with this, improve this on the compiler side. Martin Am Freitag, dem 01.03.2024 um 09:32 +0000 schrieb David Laight: > From: Kees Cook > > Sent: 01 March 2024 04:45 > > To: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> > > > > The __is_constexpr() macro is dark magic. Shed some light on it with > > a comment to explain how and why it works. > > All the 8s don't help... > > I don't think you need that much explanation. > > Perhaps just saying that the type of ?: depends on the types > of the values and is independent of the condition. > The type of (0 ? (void *)p : (foo *)q) is normally 'void *' > (so that both values can be assigned to it). > But if 'p' is 'an integer constant expression with value 0' > then (void *)p is NULL and the type is 'foo *'. > > The type can then be checked to find out it 'p' is constant 0. > A non-zero constant 'p' can be multiples by 0. > > I need to replace the definition with (the more portable): > #define __if_constexpr(cond, if_const, if_not_const) \ > _Generic(0 ? (void *)((long)(cond) * 0) : (char *)0, \ > char *: (if_const), \ > void *: (if_not_const)) > which is arguably less cryptic. > > #define __is_constexpr(cond) __if_constexpr(cond, 1, 0) > > So that I can write: > #define is_non_neg_const(x) (__if_constexpr(x, x , -1) >= 0) > and avoid the compiler bleating about some comparisons > in unreachable code. > > David > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales) >
