linux-kernel - Re: Arrays of variable length

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAFLxGvwmH5QF5hSKG6Dm1Sc_uhwV7pV6mLsOrbYPN-metCKNOg@mail.gmail.com>
Date:   Sun, 5 Mar 2017 22:49:40 +0100
From:   Richard Weinberger <richard.weinberger@...il.com>
To:     Henrique de Moraes Holschuh <hmh@....eng.br>
Cc:     Måns Rullgård <mans@...sr.com>,
        Tomas Winkler <tomasw@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        sparse@...r.kernel.org
Subject: Re: Arrays of variable length

On Sun, Mar 5, 2017 at 10:12 PM, Henrique de Moraes Holschuh
<hmh@....eng.br> wrote:
> 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.

First we'd have to fix all existing offenders which are a few...

-- 
Thanks,
//richard