From: Rasmus Villemoes <linux@rasmusvillemoes.dk>
To: Josh Poimboeuf <jpoimboe@redhat.com>, Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
linux-kernel@vger.kernel.org, corbet@lwn.net,
gustavo@embeddedor.com, rostedt@goodmis.org,
Chris Mason <clm@fb.com>, Josef Bacik <jbacik@fb.com>,
David Sterba <dsterba@suse.com>,
"David S. Miller" <davem@davemloft.net>,
Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
Ingo Molnar <mingo@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
Masahiro Yamada <yamada.masahiro@socionext.com>,
Borislav Petkov <bp@suse.de>,
Randy Dunlap <rdunlap@infradead.org>,
Ian Abbott <abbotti@mev.co.uk>, "Tobin C. Harding" <me@tobin.cc>,
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Petr Mladek <pmladek@suse.com>,
Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
Pantelis Antoniou <pantelis.antoniou@konsulko.com>,
linux-btrfs@vger.kernel.org, netdev@vger.kernel.org,
kernel-hardening@lists.openwall.com
Subject: Re: [PATCH 0/3] Remove accidental VLA usage
Date: Thu, 8 Mar 2018 20:57:16 +0100 [thread overview]
Message-ID: <78428dec-270e-5d74-0160-83feb5c0ff02@rasmusvillemoes.dk> (raw)
In-Reply-To: <20180308150236.5tysfbm3xdouii5n@treble>
On 2018-03-08 16:02, Josh Poimboeuf wrote:
> On Wed, Mar 07, 2018 at 07:30:44PM -0800, Kees Cook wrote:
>> This series adds SIMPLE_MAX() to be used in places where a stack array
>> is actually fixed, but the compiler still warns about VLA usage due to
>> confusion caused by the safety checks in the max() macro.
>>
>> I'm sending these via -mm since that's where I've introduced SIMPLE_MAX(),
>> and they should all have no operational differences.
>
> What if we instead simplify the max() macro's type checking so that GCC
> can more easily fold the array size constants? The below patch seems to
> work:
>
> +extern long __error_incompatible_types_in_min_macro;
> +extern long __error_incompatible_types_in_max_macro;
> +
> +#define __min(t1, t2, x, y) \
> + __builtin_choose_expr(__builtin_types_compatible_p(t1, t2), \
> + (t1)(x) < (t2)(y) ? (t1)(x) : (t2)(y), \
> + (t1)__error_incompatible_types_in_min_macro)
>
> /**
> * min - return minimum of two values of the same or compatible types
> * @x: first value
> * @y: second value
> */
> -#define min(x, y) \
> - __min(typeof(x), typeof(y), \
> - __UNIQUE_ID(min1_), __UNIQUE_ID(min2_), \
> - x, y)
> +#define min(x, y) __min(typeof(x), typeof(y), x, y) \
>
But this introduces the the-chosen-one-of-x-and-y-gets-evaluated-twice
problem. Maybe we don't care? But until we get a
__builtin_assert_this_has_no_side_effects() I think that's a little
dangerous.
Rasmus
next prev parent reply other threads:[~2018-03-08 19:57 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-03-08 3:30 [PATCH 0/3] Remove accidental VLA usage Kees Cook
2018-03-08 3:30 ` [PATCH v2 1/3] vsprintf: " Kees Cook
2018-03-08 8:25 ` Rasmus Villemoes
2018-03-08 11:21 ` Thomas Gleixner
2018-03-08 3:30 ` [PATCH 2/3] net: Remove accidental VLAs from proc buffers Kees Cook
2018-03-08 3:30 ` [PATCH 3/3] btrfs: tree-checker: Avoid accidental stack VLA Kees Cook
2018-03-08 11:33 ` David Sterba
2018-03-08 15:02 ` [PATCH 0/3] Remove accidental VLA usage Josh Poimboeuf
2018-03-08 18:02 ` Kees Cook
2018-03-08 18:11 ` Josh Poimboeuf
2018-03-08 18:06 ` Steven Rostedt
2018-03-08 19:57 ` Rasmus Villemoes [this message]
2018-03-08 20:39 ` Kees Cook
2018-03-08 22:12 ` Rasmus Villemoes
2018-03-08 23:33 ` Kees Cook
2018-03-08 20:49 ` Andrew Morton
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