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Message-ID: <CAGXu5j+6rPgn1biniy6Xju5w7=n9ogiOZvHs_kvT55LT9u9y2Q@mail.gmail.com> Date: Thu, 8 Mar 2018 14:49:59 -0800 From: Kees Cook <keescook@...omium.org> To: Andrew Morton <akpm@...ux-foundation.org> Cc: 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>, LKML <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 2:18 PM, Andrew Morton <akpm@...ux-foundation.org> wrote: > On Thu, 8 Mar 2018 13:40:45 -0800 Kees Cook <keescook@...omium.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. I just went with symmetry. It seems like an ugly risk to implement min and mix differently. :) In theory it may produce smaller code for rare min() uses, but I haven't actually verified that. I will send a v2 with the two nits mentioned... -Kees -- Kees Cook Pixel Security
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