From: Andrew Morton <akpm@linux-foundation.org>
To: Kees Cook <keescook@chromium.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>,
Rasmus Villemoes <linux@rasmusvillemoes.dk>,
"Gustavo A. R. Silva" <gustavo@embeddedor.com>,
"Tobin C. Harding" <me@tobin.cc>,
rostedt@goodmis.org, corbet@lwn.net, 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>,
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,
linux-kernel@vger.kernel.org,
kernel-hardening@lists.openwall.com
Subject: Re: [PATCH] kernel.h: Skip single-eval logic on literals in min()/max()
Date: Thu, 8 Mar 2018 14:18:33 -0800 [thread overview]
Message-ID: <20180308141833.3fb57913bceae38f18db2bf1@linux-foundation.org> (raw)
In-Reply-To: <20180308214045.GA6787@beast>
On Thu, 8 Mar 2018 13:40:45 -0800 Kees Cook <keescook@chromium.org> wrote:
> When max() is used in stack array size calculations from literal values
> (e.g. "char foo[max(sizeof(struct1), sizeof(struct2))]", the compiler
> thinks this is a dynamic calculation due to the single-eval logic, which
> is not needed in the literal case. This change removes several accidental
> stack VLAs from an x86 allmodconfig build:
>
> $ diff -u before.txt after.txt | grep ^-
> -drivers/input/touchscreen/cyttsp4_core.c:871:2: warning: ISO C90 forbids variable length array ‘ids’ [-Wvla]
> -fs/btrfs/tree-checker.c:344:4: warning: ISO C90 forbids variable length array ‘namebuf’ [-Wvla]
> -lib/vsprintf.c:747:2: warning: ISO C90 forbids variable length array ‘sym’ [-Wvla]
> -net/ipv4/proc.c:403:2: warning: ISO C90 forbids variable length array ‘buff’ [-Wvla]
> -net/ipv6/proc.c:198:2: warning: ISO C90 forbids variable length array ‘buff’ [-Wvla]
> -net/ipv6/proc.c:218:2: warning: ISO C90 forbids variable length array ‘buff64’ [-Wvla]
>
> Based on an earlier patch from Josh Poimboeuf.
>
> ...
>
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -787,37 +787,57 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { }
> * strict type-checking.. See the
> * "unnecessary" pointer comparison.
> */
> -#define __min(t1, t2, min1, min2, x, y) ({ \
> +#define __single_eval_min(t1, t2, min1, min2, x, y) ({ \
> t1 min1 = (x); \
> t2 min2 = (y); \
> (void) (&min1 == &min2); \
> min1 < min2 ? min1 : min2; })
>
> +/*
> + * In the case of builtin constant values, there is no need to do the
> + * double-evaluation protection, so the raw comparison can be made.
> + * This allows min()/max() to be used in stack array allocations and
> + * avoid the compiler thinking it is a dynamic value leading to an
> + * accidental VLA.
> + */
> +#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), \
> + __single_eval_min(t1, t2, \
> + __UNIQUE_ID(max1_), \
> + __UNIQUE_ID(max2_), \
> + x, y))
> +
Holy crap.
I suppose gcc will one day be fixed and we won't need this.
Is there a good reason to convert min()? Surely nobody will be using
min to dimension an array - always max? Just for symmetry, I guess.
next prev parent reply other threads:[~2018-03-08 22:18 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-03-08 21:40 [PATCH] kernel.h: Skip single-eval logic on literals in min()/max() Kees Cook
2018-03-08 21:59 ` Ian Campbell
2018-03-08 22:18 ` Andrew Morton [this message]
2018-03-08 22:49 ` Kees Cook
2018-03-08 23:48 ` Linus Torvalds
2018-03-09 0:45 ` Kees Cook
2018-03-09 1:35 ` Linus Torvalds
2018-03-09 1:46 ` Kees Cook
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