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Re: [libvirt] [PATCH 1/2] nwfilter: Fix pointer.



On Mon, Jan 21, 2019 at 03:41:06PM +0000, Richard W.M. Jones wrote:
> On Mon, Jan 21, 2019 at 03:23:59PM +0000, Daniel P. Berrangé wrote:
> > On Mon, Jan 21, 2019 at 03:13:20PM +0000, Richard W.M. Jones wrote:
> > > GCC 9 complains:
> > > 
> > > nwfilter/nwfilter_dhcpsnoop.c: In function 'virNWFilterDHCPSnoopThread':
> > > nwfilter/nwfilter_dhcpsnoop.c:1456:31: error: converting a packed 'virNWFilterSnoopEthHdrPtr' {aka 'struct _virNWFilterSnoopEthHdr *'} pointer (alignment 1) to 'const u_char *' {aka 'const unsigned char *'} (alignment 8) may result in an unaligned pointer value [-Werror=address-of-packed-member]
> > >  1456 |                               (const u_char **)&packet);
> > >       |                               ^
> > 
> > I tend to think this warning is bogus.   We are not de-referencing
> > any packed fields within the sctruct when we call to pcap_next_ex()
> > with the cast.  pcap_next_ex() is just going to fill the entire
> > memory region with a read off the wire, so it would not be triggering
> > unaligned access either.  IOW, I don't think the compiler should be
> > warning there
> > 
> > IIUC gcc X.0.0  versions are not in fact relases, but rather
> > pre-release snapshots.
> > 
> > If so, I think this might be a bug that needs reporting against
> > the GCC pre-release.
> > 
> > 
> > 
> > > nwfilter/nwfilter_dhcpsnoop.c:183:8: note: defined here
> > >   183 | struct _virNWFilterSnoopEthHdr {
> > >       |        ^~~~~~~~~~~~~~~~~~~~~~~
> > > 
> > > However it seems like there's more going on here than just an enhanced
> > > GCC warning.  The function pcap_next_ex is documented as:
> > > 
> > >        the pointer pointed  to  by  the
> > >        pkt_data  argument  is  set  to  point  to the data in the packet
> > > 
> > > We are passing a struct here rather than a pointer.  I changed the
> > > code to pass a pointer instead.
> > 
> > > diff --git a/src/nwfilter/nwfilter_dhcpsnoop.c b/src/nwfilter/nwfilter_dhcpsnoop.c
> > > index 58f0057c3f..45873a542c 100644
> > > --- a/src/nwfilter/nwfilter_dhcpsnoop.c
> > > +++ b/src/nwfilter/nwfilter_dhcpsnoop.c
> > > @@ -1335,7 +1335,7 @@ virNWFilterDHCPSnoopThread(void *req0)
> > >  {
> > >      virNWFilterSnoopReqPtr req = req0;
> > >      struct pcap_pkthdr *hdr;
> > > -    virNWFilterSnoopEthHdrPtr packet;
> > > +    const virNWFilterSnoopEthHdrPtr *packetPtr;
> > 
> > virNWFilterSnoopEthHdrPtr is already a pointer to a virNWFilterSnoopEthHdr.
> > 
> > So this change turns it into a pointer to a pointer....
> 
> Duh you're right there.
> 
> Yup as you say this patch is bogus and more likely indicates
> some bug in GCC.

It was indeed a bug:

  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88664

So we don't need to fix this in libvirt.

Regards,
Daniel
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