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. 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
