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Date: Thu, 22 Mar 2018 08:01:25 -0700
From: Kees Cook <>
To: Linus Torvalds <>
Cc: Al Viro <>, Florian Weimer <>, 
	Andrew Morton <>, Josh Poimboeuf <>, 
	Rasmus Villemoes <>, Randy Dunlap <>, 
	Miguel Ojeda <>, Ingo Molnar <>, 
	David Laight <>, Ian Abbott <>, 
	linux-input <>, linux-btrfs <>, 
	Network Development <>, 
	Linux Kernel Mailing List <>, 
	Kernel Hardening <>
Subject: Re: [PATCH v5 0/2] Remove false-positive VLAs when using max()

On Tue, Mar 20, 2018 at 4:23 PM, Linus Torvalds
<> wrote:
> On Sat, Mar 17, 2018 at 1:07 PM, Kees Cook <> wrote:
>> No luck! :( gcc 4.4 refuses to play along. And, hilariously, not only
>> does it not change the complaint about __builtin_choose_expr(), it
>> also thinks that's a VLA now.
> Hmm. So thanks to the diseased mind of Martin Uecker, there's a better
> test for "__is_constant()":
>   /* Glory to Martin Uecker <> */
>   #define __is_constant(a) \
>         (sizeof(int) == sizeof(*(1 ? ((void*)((a) * 0l)) : (int*)1)))
> that is actually *specified* by the C standard to work, and doesn't
> even depend on any gcc extensions.

I feel we risk awakening Cthulhu with this. :)

> The reason is some really subtle pointer conversion rules, where the
> type of the ternary operator will depend on whether one of the
> pointers is NULL or not.
> And the definition of NULL, in turn, very much depends on "integer
> constant expression that has the value 0".
> Are you willing to do one final try on a generic min/max? Same as my
> last patch, but using the above __is_constant() test instead of
> __builtin_constant_p?

So, this time it's not a catastrophic failure with gcc 4.4. Instead it
fails in 11 distinct places:

$ grep "first argument to ‘__builtin_choose_expr’ not a constant" log
| cut -d: -f1-2

Seems like it doesn't like void * arguments:

                void *ptr;
                base = min(ptr, base);

static void scan_large_block(void *start, void *end)
                next = min(start + MAX_SCAN_SIZE, end);

I'll poke a bit more...


Kees Cook
Pixel Security

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