Re: Arrays of variable length

[Måns Rullgård]

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