linux-kernel - Re: Arrays of variable length

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Message-ID: <20170305211254.GA3220@khazad-dum.debian.net>
Date:   Sun, 5 Mar 2017 18:12:54 -0300
From:   Henrique de Moraes Holschuh <hmh@....eng.br>
To:     Måns Rullgård <mans@...sr.com>
Cc:     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, 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.

-- 
  Henrique Holschuh