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? -- 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
