linux-hardening - Re: [PATCH v2 3/5] treewide: Replace 0-element memcpy() destinations with flexible arrays

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Message-ID: <202108252250.C1DAEE5@keescook>
Date:   Wed, 25 Aug 2021 22:51:20 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Keith Packard <keithp@...thp.com>
Cc:     linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Nilesh Javali <njavali@...vell.com>,
        Manish Rangankar <mrangankar@...vell.com>,
        GR-QLogic-Storage-Upstream@...vell.com,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Larry Finger <Larry.Finger@...inger.net>,
        Phillip Potter <phil@...lpotter.co.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Florian Schilhabel <florian.c.schilhabel@...glemail.com>,
        Johannes Berg <johannes@...solutions.net>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Fabio Aiuto <fabioaiuto83@...il.com>,
        Ross Schmidt <ross.schm.dev@...il.com>,
        Marco Cesati <marcocesati@...il.com>,
        ath10k@...ts.infradead.org, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, linux-scsi@...r.kernel.org,
        linux-staging@...ts.linux.dev,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Dan Williams <dan.j.williams@...el.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        clang-built-linux@...glegroups.com, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2 3/5] treewide: Replace 0-element memcpy() destinations
 with flexible arrays

On Wed, Aug 25, 2021 at 10:24:18PM -0700, Keith Packard wrote:
> Kees Cook <keescook@...omium.org> writes:
> 
> > In some cases, use of the flex_array() helper is needed when a flexible
> > array is part of a union.
> 
> The code below seems to show that the helper is also needed when the
> flexible array is the only member of a struct? Or is this just an
> extension of the 'part of a union' clause?

That's correct. I have that documented in the DECLARE_FLEX_ARRAY macro
itself, but I mis-spoke in this changelog here (the uses were for "alone
in a struct"). I've adjusted the changelog now. :)

Thanks!

-Kees

> 
> > @@ -160,7 +160,7 @@ struct bmi_cmd {
> >  
> >  union bmi_resp {
> >  	struct {
> > -		u8 payload[0];
> > +		DECLARE_FLEX_ARRAY(u8, payload);
> >  	} read_mem;
> >  	struct {
> >  		__le32 result;
> 
> -- 
> -keith



-- 
Kees Cook