[lxc-devel] [PATCH 1/4] coverity: don't reference beyond structure

[Dwight Engen]

On Fri, 3 May 2013 11:17:08 -0500
Serge Hallyn <serge.hallyn at ubuntu.com> wrote:

> can that be correct?  That changes the value of the
> pointer.  NLMSG_DATA adds aligned sizeof(struct nlmsghdr).

The current code is relying on the fact that our genlmsg looks like
this:

struct genlmsg {
	struct nlmsghdr nlmsghdr;
	struct genlmsghdr genlmsghdr;
};

so NLMSG_DATA(&request->nlmsghdr) == genlmsghdr (I think sizeof struct
nlmsghdr will make genlmsghdr be aligned anyway?). Right now lxc doesn't
use this code so I cannot test it in lxc, but let me write a test
program that dumps the pointers using the old way and the new way. We
do have a 2*PAGE_SIZE buffer these point to so I'm sure its not
overflowing, so maybe we do just want to ignore coverity.
 
> I suspect lxc is doing the right thing in practice, but could
> do some sort of casting somewhere to tell coverity to lighten
> up.  But I haven't yet figured out what that is.
> 
> Quoting Dwight Engen (dwight.engen at oracle.com):
> > Signed-off-by: Dwight Engen <dwight.engen at oracle.com>
> > ---
> >  src/lxc/genl.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/src/lxc/genl.c b/src/lxc/genl.c
> > index 873b084..8fcb6b7 100644
> > --- a/src/lxc/genl.c
> > +++ b/src/lxc/genl.c
> > @@ -55,7 +55,7 @@ static int genetlink_resolve_family(const char
> > *family) request->nlmsghdr.nlmsg_flags = NLM_F_REQUEST | NLM_F_ACK;
> >          request->nlmsghdr.nlmsg_type = GENL_ID_CTRL;
> >  
> > -	genlmsghdr = NLMSG_DATA(&request->nlmsghdr);
> > +	genlmsghdr = &request->genlmsghdr;
> >          genlmsghdr->cmd = CTRL_CMD_GETFAMILY;
> >  
> >  	ret = netlink_open(&handler, NETLINK_GENERIC);
> > @@ -72,7 +72,7 @@ static int genetlink_resolve_family(const char
> > *family) if (ret < 0)
> >  		goto out_close;
> >  
> > -	genlmsghdr = NLMSG_DATA(&reply->nlmsghdr);
> > +	genlmsghdr = &reply->genlmsghdr;
> >  	len = reply->nlmsghdr.nlmsg_len;
> >  
> >  	ret = -ENOMSG;
> > -- 
> > 1.8.1.4
> > 
> > 
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