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

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....

> 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 -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|