linux-kernel - Re: Arrays of variable length

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Date:   Thu, 9 Mar 2017 16:29:18 +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 Thu, Mar 9, 2017 at 4:26 PM, Måns Rullgård <mans@...sr.com> wrote:
> Tomas Winkler <tomasw@...il.com> writes:
>
>> On Thu, Mar 9, 2017 at 4:16 PM, Måns Rullgård <mans@...sr.com> wrote:
>>> 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?
>>
>> This is a water mark and not  actual usage, but maybe I didn't
>> understand your comment.
>
> If there is an upper bound known at compile time, why not simply use
> that size statically?  If there is no upper bound, well, then you have a
> problem.

If the compiler can do the job, why not to use this flexibility ?

Tomas