linux-kernel - Re: [PATCH v5 0/2] Remove false-positive VLAs when using max()

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Date:   Fri, 16 Mar 2018 13:25:26 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Florian Weimer <fweimer@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Randy Dunlap <rdunlap@...radead.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Ingo Molnar <mingo@...nel.org>, Ian Abbott <abbotti@....co.uk>,
        linux-input <linux-input@...r.kernel.org>,
        linux-btrfs <linux-btrfs@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v5 0/2] Remove false-positive VLAs when using max()

On Fri, Mar 16, 2018 at 10:44 AM, David Laight <David.Laight@...lab.com> wrote:
>
> I looked at the generated code for one of the constant sized VLA that
> the compiler barfed at.
> It seemed to subtract constants from %sp separately for the VLA.
> So it looks like the compiler treats them as VLA even though it
> knows the size.
> That is probably missing optimisation.

Looking at the code is definitely an option.

In fact, instead of depending on -Wvla, we could just make 'objtool'
warn about real variable-sized stack frames.

That said, if that "sizeof()" trick of Al's actually works with older
gcc versions too (it *should*, but it's not like
__builtin_choose_expr() and __builtin_constant_p() have well-defined
rules in the standard), that may just be the solution.

And if gcc ends up generating bad code for those "constant sized vlas"
anyway, then -Wvla would effectively warn about that code generation
problem.

              Linus