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From: Linus Torvalds <torvalds@linux-foundation.org>
To: David Laight <David.Laight@aculab.com>
Cc: Florian Weimer <fweimer@redhat.com>,
	Kees Cook <keescook@chromium.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Randy Dunlap <rdunlap@infradead.org>,
	Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
	Ingo Molnar <mingo@kernel.org>, Ian Abbott <abbotti@mev.co.uk>,
	linux-input <linux-input@vger.kernel.org>,
	linux-btrfs <linux-btrfs@vger.kernel.org>,
	Network Development <netdev@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>
Subject: Re: [PATCH v5 0/2] Remove false-positive VLAs when using max()
Date: Fri, 16 Mar 2018 13:25:26 -0700	[thread overview]
Message-ID: <CA+55aFwzgO9w8Dj1NoCViz6JTwVLWhWabLq_cvzJ1-woEBfJMg@mail.gmail.com> (raw)
In-Reply-To: <a37f7b0da04f4ede9fac2c88f7079a8a@AcuMS.aculab.com>

On Fri, Mar 16, 2018 at 10:44 AM, David Laight <David.Laight@aculab.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

  reply	other threads:[~2018-03-16 20:25 UTC|newest]

Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-03-16  4:25 [PATCH v5 0/2] Remove false-positive VLAs when using max() Kees Cook
2018-03-16  4:25 ` [PATCH v5 1/2] kernel.h: Introduce const_max_t() for VLA removal Kees Cook
2018-03-16  4:25 ` [PATCH v5 2/2] Remove false-positive VLAs when using max() Kees Cook
2018-03-19 10:45   ` Andrey Ryabinin
2018-03-16 11:47 ` [PATCH v5 0/2] " Florian Weimer
2018-03-16 17:29   ` Linus Torvalds
2018-03-16 17:32     ` Florian Weimer
2018-03-16 17:44     ` David Laight
2018-03-16 20:25       ` Linus Torvalds [this message]
2018-03-16 17:55     ` Al Viro
2018-03-16 18:14       ` Al Viro
2018-03-16 19:27       ` Linus Torvalds
2018-03-16 20:03         ` Miguel Ojeda
2018-03-16 20:14           ` Linus Torvalds
2018-03-16 20:19             ` Linus Torvalds
2018-03-17  0:48             ` Miguel Ojeda
2018-03-17  1:49             ` Miguel Ojeda
2018-03-16 20:12         ` Al Viro
2018-03-16 20:15           ` Linus Torvalds
2018-03-16 20:18             ` Al Viro
2018-03-17  7:27         ` Kees Cook
2018-03-17 18:52           ` Linus Torvalds
2018-03-17 20:07             ` Kees Cook
2018-03-17 22:55               ` Josh Poimboeuf
2018-03-20 23:23               ` Linus Torvalds
2018-03-20 23:26                 ` Linus Torvalds
2018-03-21  0:05                   ` Al Viro
2018-03-22 15:01                 ` Kees Cook
2018-03-22 15:13                   ` David Laight
2018-03-22 17:04                   ` Linus Torvalds
2018-03-18 21:13             ` Rasmus Villemoes
2018-03-18 21:33               ` Linus Torvalds
2018-03-18 22:59                 ` Rasmus Villemoes
2018-03-18 23:36                   ` Linus Torvalds
2018-03-19  9:43                     ` David Laight
2018-03-19 23:29                       ` Linus Torvalds
2018-03-20  3:10                         ` Arnd Bergmann

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