linux-hardening - Re: [wish] Flexible array members in unions

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Date:   Thu, 11 May 2023 12:07:40 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Alejandro Colomar <alx.manpages@...il.com>
Cc:     GCC <gcc@....gnu.org>, Alejandro Colomar <alx@...nx.com>,
        Andrew Clayton <a.clayton@...nx.com>,
        Andrew Clayton <andrew@...ital-domain.net>,
        linux-hardening@...r.kernel.org
Subject: Re: [wish] Flexible array members in unions

On Thu, May 11, 2023 at 06:29:10PM +0200, Alejandro Colomar wrote:
> On 5/11/23 18:07, Alejandro Colomar wrote:
> [...]
> > Would you allow flexible array members in unions?  Is there any
> > strong reason to disallow them?

Yes please!! And alone in a struct, too.

AFAICT, there is no mechanical/architectural reason to disallow them
(especially since they _can_ be constructed with some fancy tricks,
and they behave as expected.) My understanding is that it's disallowed
due to an overly strict reading of the very terse language that created
flexible arrays in C99.

> [...]
> Currently, the Linux kernel has to go through some hoops due to this
> restriction:
> 
> 
> $ grepc -tm __DECLARE_FLEX_ARRAY *
> include/uapi/linux/stddef.h:42:
> #define __DECLARE_FLEX_ARRAY(TYPE, NAME)	\
> 	struct { \
> 		struct { } __empty_ ## NAME; \
> 		TYPE NAME[]; \
> 	}

Yes, we've had to do this as we eradicate all the fake flexible arrays
in the kernel which cause endless bugs[1]. Additionally, we'll be
using -fstrict-flex-arrays=3 soon to let existing array bounds checking
mitigations gain coverage over trailing arrays. All of this means that
we're converting a lot of code that is happily using dynamically sized
arrays in unions, etc.

[1] https://people.kernel.org/kees/bounded-flexible-arrays-in-c

-- 
Kees Cook