Re: [Qemu-devel] [PATCH 2/2] linux-user: SOCK_PACKET uses network endian to encode protocol in socket()

[Peter Maydell]

On 31 December 2012 22:19, Laurent Vivier <address@hidden> wrote:
> Le lundi 31 décembre 2012 à 21:32 +0000, Peter Maydell a écrit :
>> Also it seems rather involved since we swap things three times and
>> have an entirely new abi_* function. Either I'm completely confused
>> or it should be enough to just have
>>
>> if (type == SOCK_PACKET) {
>>       protocol = tswap16(protocol);
>> }

Looking more carefully at packet(7) this is actually the wrong
guard anyway. You need to check for
 (domain == AF_PACKET) || (type == SOCK_PACKET)

since SOCK_PACKET is the obsolete Linux 2.0 way of doing packet sockets.

> works... sometime. In fact, work if target endianess is network endianess.
>
> Correct me if I'm wrong.
>
> target          host
> little endian / big endian
>
> memory   00 00 00 03

Syscall arguments aren't generally passed in memory, they're
in registers (and if they were pased in memory for some architecture
then that arch would do a load-and-swap-from-memory in main.c).
So the value you see in do_socket() is always "the integer passed
as a syscall parameter, as a host-order integer".

So in this case, with a simple guest program:
#include <sys/socket.h>
#include <netpacket/packet.h>
#include <net/ethernet.h>
#include <arpa/inet.h>

int main(void) {
   return socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
}

you will find that do_socket() in QEMU is passed either 0x3 [if the
guest is bigendian and the guest htons() is a no-op] or 0x0300
[if the guest is littleendian]. Since what we want to pass to the
host socket() call is 0x3 if the host is bigendian and 0x0300 if
the host is little endian, this amounts to needing to do a 16 bit
byteswap if the host and guest are different endianness, which
is exactly what tswap16() does. I checked with i386-to-i386
that do_socket() gets passed 0x300 and we correctly send it
through to the host socket().

-- PMM