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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