Re: [Cocci] [PATCH] net: Zeroing the structure ethtool_wolinfo in ethtool_get_wol()

Julia Lawall
2019-11-21T20:40:51Z

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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