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Message-ID: <CAGXu5j+JiYcsdaC4V9+SmUM2SKW1yRHr18d74G5SM_sSrQm=Nw@mail.gmail.com>
Date: Thu, 8 Mar 2018 16:45:03 -0800
From: Kees Cook <keescook@...omium.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Josh Poimboeuf <jpoimboe@...hat.com>,
Rasmus Villemoes <linux@...musvillemoes.dk>, "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
"Tobin C. Harding" <me@...in.cc>, Steven Rostedt <rostedt@...dmis.org>, Jonathan Corbet <corbet@....net>,
Chris Mason <clm@...com>, Josef Bacik <jbacik@...com>, David Sterba <dsterba@...e.com>,
"David S. Miller" <davem@...emloft.net>, Alexey Kuznetsov <kuznet@....inr.ac.ru>,
Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>, Ingo Molnar <mingo@...nel.org>,
Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>,
Masahiro Yamada <yamada.masahiro@...ionext.com>, Borislav Petkov <bp@...e.de>,
Randy Dunlap <rdunlap@...radead.org>, Ian Abbott <abbotti@....co.uk>,
Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>, Petr Mladek <pmladek@...e.com>,
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
Pantelis Antoniou <pantelis.antoniou@...sulko.com>, 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] kernel.h: Skip single-eval logic on literals in min()/max()
On Thu, Mar 8, 2018 at 3:48 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Mar 8, 2018 at 1:40 PM, Kees Cook <keescook@...omium.org> wrote:
>> +#define __min(t1, t2, x, y) \
>> + __builtin_choose_expr(__builtin_constant_p(x) && \
>> + __builtin_constant_p(y) && \
>> + __builtin_types_compatible_p(t1, t2), \
>> + (t1)(x) < (t2)(y) ? (t1)(x) : (t2)(y), \
>
> I understand why you use __builtin_types_compatible_p(), but please don't.
>
> It will mean that trivial constants like "5" and "sizeof(x)" won't
> simplify, because they have different types.
Rasmus mentioned this too. What I said there was that I was shy to
make that change, since we already can't mix that kind of thing with
the existing min()/max() implementation. The existing min()/max() is
already extremely strict, so there are no instances of this in the
tree. If I explicitly add one, I see this with or without the patch:
In file included from drivers/misc/lkdtm.h:7:0,
from drivers/misc/lkdtm_core.c:33:
drivers/misc/lkdtm_core.c: In function ‘lkdtm_module_exit’:
./include/linux/kernel.h:809:16: warning: comparison of distinct
pointer types lacks a cast
(void) (&max1 == &max2); \
^
./include/linux/kernel.h:818:2: note: in expansion of macro ‘__max’
__max(typeof(x), typeof(y), \
^~~~~
./include/linux/printk.h:308:34: note: in expansion of macro ‘max’
printk(KERN_INFO pr_fmt(fmt), ##__VA_ARGS__)
^~~~~~~~~~~
drivers/misc/lkdtm_core.c:500:2: note: in expansion of macro ‘pr_info’
pr_info("%lu\n", max(16, sizeof(unsigned long)));
^~~~~~~
> The ?: will give the right combined type anyway, and if you want the
> type comparison warning, just add a comma-expression with something
> like like
>
> (t1 *)1 == (t2 *)1
>
> to get the type compatibility warning.
When I tried removing __builtin_types_compatible_p(), I still got the
type-check warning because I think the preprocessor still sees the
"(void) (&min1 == &min2)" before optimizing? So, I technically _can_
drop the __builtin_types_compatible_p(), and still keep the type
warning. :P
> Yeah, yeah, maybe none of the VLA cases triggered that, but it seems
> silly to not just get that obvious constant case right.
>
> Hmm?
So are you saying you _want_ the type enforcement weakened here, or
that I should just not use __builtin_types_compatible_p()?
Thanks!
-Kees
--
Kees Cook
Pixel Security
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