linux-kernel - Re: Arrays of variable length

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Message-ID: <yw1xfuimbkml.fsf@unicorn.mansr.com>
Date:   Thu, 09 Mar 2017 14:16:50 +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 Thu, Mar 9, 2017 at 3:02 PM, Måns Rullgård <mans@...sr.com> wrote:
>> 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?
>
> One option is to add '-Wvla-larger-than=n'

If you know the upper bound, why use VLAs in the first place?

-- 
Måns Rullgård