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

[Laurent Vivier]

Le mardi 01 janvier 2013 à 19:45 +0000, Peter Maydell a écrit :
> 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).

OK, I will... but I think we will fall back to my original patch ;-)

Regards,
Laurent

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