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Date:   Thu, 21 Nov 2019 21:40:45 +0100 (CET)
From:   Julia Lawall <julia.lawall@lip6.fr>
To:     Michal Kubecek <mkubecek@suse.cz>
cc:     Dan Carpenter <dan.carpenter@oracle.com>,
        jakub.kicinski@netronome.com, ast@kernel.org,
        natechancellor@gmail.com, jiang.xuexin@zte.com.cn,
        cocci <cocci@systeme.lip6.fr>, f.fainelli@gmail.com,
        daniel@iogearbox.net, john.fastabend@gmail.com,
        lirongqing@baidu.com, maxime.chevallier@bootlin.com,
        vivien.didelot@gmail.com, pablo@netfilter.org,
        wang.yi59@zte.com.cn, hawk@kernel.org, arnd@arndb.de,
        jiri@mellanox.com, xue.zhihong@zte.com.cn,
        zhanglin <zhang.lin16@zte.com.cn>,
        Thomas Gleixner <tglx@linutronix.de>, bpf@vger.kernel.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        linyunsheng@huawei.com, Joe Perches <joe@perches.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        davem@davemloft.net
Subject: Re: [Cocci] [PATCH] net: Zeroing the structure ethtool_wolinfo in
 ethtool_get_wol()
In-Reply-To: <20191121133817.GF29650@unicorn.suse.cz>
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On Thu, 21 Nov 2019, Michal Kubecek wrote:

> On Thu, Nov 21, 2019 at 03:07:33PM +0300, Dan Carpenter wrote:
> > On Thu, Nov 21, 2019 at 12:19:17PM +0100, Michal Kubecek wrote:
> > > On Thu, Nov 21, 2019 at 11:23:34AM +0100, Enrico Weigelt, metux IT consult wrote:
> > > > On 26.10.19 21:40, Joe Perches wrote:
> > > > > On Sat, 2019-10-26 at 15:54 +0800, zhanglin wrote:
> > > > >> memset() the structure ethtool_wolinfo that has padded bytes
> > > > >> but the padded bytes have not been zeroed out.
> > > > > []
> > > > >> diff --git a/net/core/ethtool.c b/net/core/ethtool.c
> > > > > []
> > > > >> @@ -1471,11 +1471,13 @@ static int ethtool_reset(struct net_device *dev, char __user *useraddr)
> > > > >>
> > > > >>  static int ethtool_get_wol(struct net_device *dev, char __user *useraddr)
> > > > >>  {
> > > > >> -	struct ethtool_wolinfo wol = { .cmd = ETHTOOL_GWOL };
> > > > >> +	struct ethtool_wolinfo wol;
> > > > >>
> > > > >>  	if (!dev->ethtool_ops->get_wol)
> > > > >>  		return -EOPNOTSUPP;
> > > > >>
> > > > >> +	memset(&wol, 0, sizeof(struct ethtool_wolinfo));
> > > > >> +	wol.cmd = ETHTOOL_GWOL;
> > > > >>  	dev->ethtool_ops->get_wol(dev, &wol);
> > > > >>
> > > > >>  	if (copy_to_user(useraddr, &wol, sizeof(wol)))
> > > > >
> > > > > It seems likely there are more of these.
> > > > >
> > > > > Is there any way for coccinelle to find them?
> > > >
> > > > Just curios: is static struct initialization (on stack) something that
> > > > should be avoided ? I've been under the impression that static
> > > > initialization allows thinner code and gives the compiler better chance
> > > > for optimizations.
> > >
> > > Not in general. The (potential) problem here is that the structure has
> > > padding and it is as a whole (i.e. including the padding) copied to
> > > userspace. While I'm not aware of a compiler that wouldn't actually
> > > initialize the whole data block including the padding in this case, the
> > > C standard provides no guarantee about that so that to be sure we cannot
> > > leak leftover kernel data to userspace, we need to explicitly initialize
> > > the whole block.
> >
> > GCC will not always initialize the struct holes.  This patch fixes a
> > real bug that GCC on my system (v7.4)
>
> Just checked (again) to be sure. No matter if the function is inlined or
> not, gcc 7.4.1 initializes the structure by one movl (of 0x5) and two
> movq (of 0x0), i.e. initializes all sizeof(struct ethtool_wolinfo) = 20
> bytes including the padding.
>
> One could certainly construct examples where a real life compiler would
> only initialize the fields. That's why I said "in this case".

Looking again at the case that I mentioned, I see:

# drivers/gpu/drm/amd/amdgpu/amdgpu_kms.c:691:          struct drm_amdgpu_info_device dev_info = {};
        call    __sanitizer_cov_trace_pc        #
        leaq    840(%rsp), %rdi #, tmp1126
        xorl    %eax, %eax      # tmp1127
        movl    $46, %ecx       #, tmp1128
        rep stosq

So I guess the rep stosq is doing the memset.

julia

>
> Michal Kubecek
>
>
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