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.
Rich.
> > int ifindex = 0;
> > int errcount = 0;
> > int tmp = -1, rv, n, pollTo;
> > @@ -1453,7 +1453,7 @@ virNWFilterDHCPSnoopThread(void *req0)
> > n--;
> >
> > rv = pcap_next_ex(pcapConf[i].handle, &hdr,
> > - (const u_char **)&packet);
> > + (const u_char **)&packetPtr);
>
> And then into a pointer to a pointer to a pointer.
>
>
> Regards,
> Daniel
> --
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Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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