linux-hardening - Re: [PATCH] overflow: Change DEFINE_FLEX to take __counted

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Date: Wed, 6 Mar 2024 15:52:13 -0800
From: Kees Cook <keescook@...omium.org>
To: Przemek Kitszel <przemyslaw.kitszel@...el.com>
Cc: "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
	Jesse Brandeburg <jesse.brandeburg@...el.com>,
	Tony Nguyen <anthony.l.nguyen@...el.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
	linux-hardening@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] overflow: Change DEFINE_FLEX to take __counted_by member

On Wed, Mar 06, 2024 at 08:06:29AM +0100, Przemek Kitszel wrote:
> On 3/6/24 04:25, Gustavo A. R. Silva wrote:
> > 
> > 
> > On 05/03/24 19:07, Kees Cook wrote:
> > > The norm should be flexible array structures with __counted_by
> > > annotations, so DEFINE_FLEX() is updated to expect that. Rename
> > > the non-annotated version to DEFINE_RAW_FLEX(), and update the few
> > > existing users. Additionally add self-tests to validate syntax and
> > > size calculations.
> > > 
> > > Signed-off-by: Kees Cook <keescook@...omium.org>
> > > ---
> > 
> > [..]
> 
> Just a note that ice changes are purely mechanical, so this seems ok
> to go via linux-hardening tree. And changes per-se are fine too :)

Thanks!

> 
> > 
> > > +/**
> > > + * DEFINE_FLEX() - Define an on-stack instance of structure with a
> > > trailing
> > > + * flexible array member.
> > > + *
> > > + * @TYPE: structure type name, including "struct" keyword.
> > > + * @NAME: Name for a variable to define.
> > > + * @COUNTER: Name of the __counted_by member.
> > > + * @MEMBER: Name of the array member.
> > > + * @COUNT: Number of elements in the array; must be compile-time const.
> > > + *
> > > + * Define a zeroed, on-stack, instance of @TYPE structure with a
> > > trailing
> > > + * flexible array member.
> > > + * Use __struct_size(@NAME) to get compile-time size of it afterwards.
> > > + */
> > > +#define DEFINE_FLEX(TYPE, NAME, COUNTER, MEMBER, COUNT)    \
> > 
> > Probably, swapping COUNTER and MEMBER is better?
> 
> right now we have usage scenario (from Kunits):
> 	DEFINE_FLEX(struct foo, eight, counter, array, 8);
> 
> > 
> >      DEFINE_FLEX(TYPE, NAME, MEMBER, COUNTER, COUNT)
> 
> usage would become:
> 	DEFINE_FLEX(struct foo, eight, array, counter, 8);
> 
> which reads a bit better indeed, with the added benefit that we
> go from broader to more specific:
> whole struct -> array -> array size variable -> given array size
> 
> so +1 from me for the params swap

Sounds good. You and Gustavo have convinced me. :) I've sent a v2 now.

-- 
Kees Cook