linux-kernel - Re: Arrays of variable length

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Message-ID: <CA+i0qc4VOXbHBSbhTOGEZ6D04X-tQsMmtAoj8JtzEf6TCOm=Pg@mail.gmail.com>
Date:   Thu, 9 Mar 2017 16:21:26 +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 4:16 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.

Tomas