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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
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