Tomas Winkler <tomasw@xxxxxxxxx> writes: > 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' If you know the upper bound, why use VLAs in the first place? -- Måns Rullgård -- 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
