linux-hardening - Re: [PATCH 01/64] media: omap3isp: Extract struct group for memcpy() region

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Message-ID: <202107300937.C7016A82@keescook>
Date:   Fri, 30 Jul 2021 09:44:00 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     dsterba@...e.cz, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Bart Van Assche <bvanassche@....org>,
        linux-hardening@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Keith Packard <keithpac@...zon.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-staging@...ts.linux.dev, linux-block@...r.kernel.org,
        linux-kbuild@...r.kernel.org, clang-built-linux@...glegroups.com,
        nborisov@...e.com
Subject: Re: [PATCH 01/64] media: omap3isp: Extract struct group for memcpy()
 region

On Fri, Jul 30, 2021 at 12:00:54PM +0300, Dan Carpenter wrote:
> On Fri, Jul 30, 2021 at 10:38:45AM +0200, David Sterba wrote:
> > Then is explicit memset the only reliable way accross all compiler
> > flavors and supported versions?
> > 
> 
> The = { } initializer works.  It's only when you start partially
> initializing the struct that it doesn't initialize holes.

No, partial works. It's when you _fully_ initialize the struct where the
padding doesn't get initialized. *sob*

struct foo {
	u8 flag;
	/* padding */
	void *ptr;
};

These are fine:

struct foo ok1 = { };
struct foo ok2 = { .flag = 7 };
struct foo ok3 = { .ptr = NULL };

This is not:

struct foo bad = { .flag = 7, .ptr = NULL };

(But, of course, it depends on padding size, compiler version, and
architecture. i.e. things remain unreliable.)

-- 
Kees Cook