linux-kernel - Re: VLA removal (was Re: [RFC 2/2] lustre: use VLA

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Date:   Thu, 8 Mar 2018 00:05:27 -0500
From:   Daniel Micay <danielmicay@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Kees Cook <keescook@...omium.org>, Joern Engel <joern@...fs.org>,
        "Tobin C. Harding" <tobin@...orbit.com>,
        "Tobin C. Harding" <me@...in.cc>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Tycho Andersen <tycho@...ho.ws>,
        Oleg Drokin <oleg.drokin@...el.com>,
        Andreas Dilger <andreas.dilger@...el.com>,
        James Simmons <jsimmons@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Gustavo A. R. Silva" <garsilva@...eddedor.com>
Subject: Re: VLA removal (was Re: [RFC 2/2] lustre: use VLA_SAFE)

On 7 March 2018 at 13:09, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> On Wed, Mar 7, 2018 at 9:37 AM, Kees Cook <keescook@...omium.org> wrote:
>>
>> Building with -Wvla, I see 209 unique locations reported in 60 directories:
>> http://paste.ubuntu.com/p/srQxwPQS9s/
>
> Ok, that's not so bad. Maybe Greg could even add it to one of those
> things he encourages new people to do?
>
> Because at least *some* of them are pretty trivial. For example,
> looking at the core code, I was surprised to see something in
> lib/btree.c

Some are probably just the issue of technically having a VLA that's
not really a VLA:

    static const int size = 5;

    void foo(void) {
      int x[size];
    }

% gcc -c -Wvla foo.c
foo.c: In function ‘foo’:
foo.c:4:3: warning: ISO C90 forbids variable length array ‘x’ [-Wvla]
   int x[size];
   ^~~

I don't really understand why the C standard didn't make `static
const` declarations usable as constant expressions like C++. They made
the pointer conversions more painful too.

It would be nice to get rid of those cases to use -Werror=vla though.