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

[Peter Maydell]

On 1 January 2013 18:37, Laurent Vivier <address@hidden> wrote:
> Le mardi 01 janvier 2013 à 18:27 +0100, Laurent Vivier a écrit :
>> Le mardi 01 janvier 2013 à 15:03 +0000, Peter Maydell a écrit :
>> > On 31 December 2012 22:19, Laurent Vivier <address@hidden> wrote:
>> > > 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".
>>
>> Yes, I missed that.
>
> But, in fact, for socketcall(), they are read from memory :

Yes, this is because socketcall is weird. The actual kernel
implementation also reads them from memory:
  http://lxr.linux.no/#linux+v3.7.1/net/socket.c#L2443
as an array of unsigned longs. So as long as qemu also reads
them out of memory as an array of target abi_ulongs (which as
you can see we do) then we'll retrieve the same value (0x3 or
0x300) to pass to do_socket() as the guest program wrote into
its guest view of memory (since it should have written an
unsigned long). (What is happening here is that the guest
binary writes the protocol value to memory as an unsigned
long, so it goes in as 4 bytes in whichever order the guest uses;
qemu's get_user_ual() then rereads those 4 bytes, swapping
the value back so we get the same integer value the guest
program stored. Note that the guest doesn't write the protocol
argument as a 2 byte value!)

I would encourage you to write some simple test programs
and check them using strace (both of the native program and
of qemu running the program).

-- PMM