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

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Date:   Fri, 16 Mar 2018 17:55:02 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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>,
        David Laight <David.Laight@...lab.com>,
        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 10:29:16AM -0700, Linus Torvalds wrote:
>    t.c: In function ‘test’:
>    t.c:6:6: error: argument to variable-length array is too large
> [-Werror=vla-larger-than=]
>      int array[(1,100)];
> 
> Gcc people are crazy.

That's not them, that's C standard regarding ICE.  1,100 is *not* a
constant expression as far as the standard is concerned, and that
type is actually a VLA with the size that can be optimized into
a compiler-calculated value.

Would you argue that in
void foo(char c)
{
	int a[(c<<1) + 10 - c + 2 - c];

a is not a VLA?  Sure, compiler probably would be able to reduce
that expression to 12, but demanding that to be recognized means
that compiler must do a bunch of optimizations in the middle of
typechecking.

expr, constant_expression is not a constant_expression.  And in
this particular case the standard is not insane - the only reason
for using that is typechecking and _that_ can be achieved without
violating 6.6p6:
	sizeof(expr,0) * 0 + ICE
*is* an integer constant expression, and it gives you exact same
typechecking.  So if somebody wants to play odd games, they can
do that just fine, without complicating the logics for compilers...