linux-kernel - Re: Arrays of variable length

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Message-ID: <yw1xbmtabk6i.fsf@unicorn.mansr.com>
Date:   Thu, 09 Mar 2017 14:26:29 +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 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.

-- 
Måns Rullgård