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
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