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Date: Thu, 15 Mar 2018 15:23:16 -0700
From: Linus Torvalds <>
To: Kees Cook <>
Cc: 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 v4 1/2] kernel.h: Introduce const_max() for VLA removal

On Thu, Mar 15, 2018 at 3:16 PM, Kees Cook <> wrote:
> size_t __error_not_const_arg(void) \
> __compiletime_error("const_max() used with non-compile-time constant arg");
> #define const_max(x, y)                                         \
>         __builtin_choose_expr(__builtin_constant_p(x) &&        \
>                               __builtin_constant_p(y),          \
>                               (typeof(x))(x) > (typeof(y))(y) ? \
>                                         (x) : (y),              \
>                               __error_not_const_arg())
> Is typeof() forcing enums to int? Regardless, I'll put this through
> larger testing. How does that look?

Ok, that alleviates my worry about one class of insane behavior, but
it does raise a few other questions:

 - what drugs is gcc on where (typeof(x)(x)) makes a difference? Funky.

 - this does have the usual "what happen if you do


where the comparison will now be done in 'size_t', and -1 ends up
being a very very big unsigned integer.

Is there no way to get that type checking inserted? Maybe now is a
good point for that __builtin_types_compatible(), and add it to the
constness checking (and change the name of that error case function)?


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