linux-kernel - Re: Arrays of variable length

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Message-ID: <yw1xk27ybo30.fsf@unicorn.mansr.com>
Date:   Thu, 09 Mar 2017 13:02:11 +0000
From:   Måns Rullgård <mans@...sr.com>
To:     Tomas Winkler <tomasw@...il.com>
Cc:     Henrique de Moraes Holschuh <hmh@....eng.br>,
        "linux-kernel\@vger.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

Tomas Winkler <tomasw@...il.com> writes:

> 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.

And what happens if the requested size is insane?

-- 
Måns Rullgård