On Thu, Mar 9, 2017 at 3:02 PM, Måns Rullgård <mans@xxxxxxxxx> wrote: > Tomas Winkler <tomasw@xxxxxxxxx> writes: > >> On Mon, Mar 6, 2017 at 2:31 AM, Måns Rullgård <mans@xxxxxxxxx> wrote: >>> Henrique de Moraes Holschuh <hmh@xxxxxxxxxx> writes: >>> >>>> On Sun, 05 Mar 2017, Måns Rullgård wrote: >>>>> Tomas Winkler <tomasw@xxxxxxxxx> writes: >>>>> > Sparse complains for arrays declared with variable length >>>>> > >>>>> > 'warning: Variable length array is used' >>>>> > >>>>> > Prior to c99 this was not allowed but lgcc (c99) doesn't have problem >>>>> > with that https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html. >>>>> > And also Linux kernel compilation with W=1 doesn't complain. >>>>> > >>>>> > Since sparse is used extensively would like to ask what is the correct >>>>> > usage of arrays of variable length >>>>> > within Linux Kernel. >>>>> >>>>> Variable-length arrays are a very bad idea. Don't use them, ever. >>>>> If the size has a sane upper bound, just use that value statically. >>>>> Otherwise, you have a stack overflow waiting to happen and should be >>>>> using some kind of dynamic allocation instead. >>>>> >>>>> Furthermore, use of VLAs generally results in less efficient code. For >>>>> instance, it forces gcc to waste a register for the frame pointer, and >>>>> it often prevents inlining. >>>> >>>> Well, if we're going to forbid VLAs in the kernel, IMHO the kernel build >>>> system should call gcc with -Werror=vla to get that point across early, >>>> and flush out any offenders. >>> >>> If it were up to me, that's exactly what I'd do. >> >>> >> Some parts of the kernel depends on VLA such as ___ON_STACK macros in >> include/crypto/hash.h >> It's actually pretty neat implementation, maybe it's too harsh to >> disable VLA completely. > > And what happens if the requested size is insane? One option is to add '-Wvla-larger-than=n' other option is to selectively shut down the warning on ON_STACK macros using #pragma warning(disable:) though this looks rather ugly. Just a thought Tomas -- To unsubscribe from this list: send the line "unsubscribe linux-sparse" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
