Tomas Winkler <tomasw@xxxxxxxxx> writes: > On Thu, Mar 9, 2017 at 4:16 PM, Måns Rullgård <mans@xxxxxxxxx> wrote: >> 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? > > This is a water mark and not actual usage, but maybe I didn't > understand your comment. If there is an upper bound known at compile time, why not simply use that size statically? If there is no upper bound, well, then you have a problem. -- 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
