linux-kernel - Re: Arrays of variable length

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Date:   Thu, 9 Mar 2017 09:54:54 +0200
From:   Tomas Winkler <tomasw@...il.com>
To:     Måns Rullgård <mans@...sr.com>
Cc:     Henrique de Moraes Holschuh <hmh@....eng.br>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-sparse@...r.kernel.org,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: Arrays of variable length

On Mon, Mar 6, 2017 at 2:31 AM, Måns Rullgård <mans@...sr.com> wrote:
> Henrique de Moraes Holschuh <hmh@....eng.br> writes:
>
>> On Sun, 05 Mar 2017, Måns Rullgård wrote:
>>> Tomas Winkler <tomasw@...il.com> 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