Re: socket_send & socket_recv fail when sending non-socket ports

[Thomas Bushnell, BSG]

Good catch. As Roland notes the old code here has the restart race
bug. Also it was probably a leak because I think nothing here wa
counting he new make send instance the way we normally wrap all uses
of make send.

On Sunday, September 19, 2010, Manuel Menal <mmenal@hurdfr.org> wrote:
> On 27/07/2010 17:39, Emilio Pozuelo Monfort wrote:
>> Hi,
>
> Hello,
>
>> While adding support for SCM_RIGHTS to glibc, I've created a testcase that 
>> sends
>> and receives some ports on the result of a socketpair() call. The ports sent
>> were initially the two ports result of another socketpair() call, and it was
>> working fine, but then I tried with the result of a couple of open() calls, 
>> and
>> socket_recv() was returning EMIGSERVERDIED.
> [snip]
>> It's returning because of this:
>>
>>         if (OutP->Head.msgh_id != 26115) {
>>                 if (OutP->Head.msgh_id == MACH_NOTIFY_SEND_ONCE)
>>                 return MIG_SERVER_DIED;
>>
>> But I'm not sure how to interpret it, and why a non-socket port in the ports
>> argument would cause this.
>> Any ideas?
>
> I think you've encountered a little bug in pflocal.
>
> Here's the story: the file descriptor you're sending is, in Mach terms,
> a send right. It gets stored by pflocal (libpipe actually) when you send
> it through socket_send() (see <pflocal/socket.c:S_socket_send>). When
> you call socket_recv(), pflocal retrieves your send right and tries to
> send it to the caller. But it sends it with the MACH_MSG_TYPE_MAKE_SEND
> flag (see <pflocal/socket.c:S_socket_recv>). This can't work, since
> MAKE_SEND only works with receive rights. So mach_msg() fails and you
> get the error code you've mentioned.
>
> Here is a very simple fix for this bug:
>
> diff -Nurp hurd-20100829/pflocal/socket.c
> hurd-20100829.patched//pflocal/socket.c
> --- hurd-20100829/pflocal/socket.c      2010-09-19 12:49:09.000000000 +0200
> +++ hurd-20100829.patched//pflocal/socket.c     2010-09-19
> 12:47:23.000000000 +0200
> @@ -397,7 +397,7 @@ S_socket_recv (struct sock_user *user,
>      /* Setup mach ports for return.  */
>      {
>        *addr_type = MACH_MSG_TYPE_MAKE_SEND;
> -      *ports_type = MACH_MSG_TYPE_MAKE_SEND;
> +      *ports_type = MACH_MSG_TYPE_COPY_SEND;
>        if (source_addr)
>         {
>           *addr = ports_get_right (source_addr);
>
> I've tried several test cases (including yours) using your libc patch
> for sendmsg()/recvmsg() and this fix for pflocal. They all work fine.
>
> As a sidenote, this also makes us able to use sshd with
> UsePrivilegeSeparation set to yes (which used to fail because it sends
> the tty fd through sendmsg() with SCM_RIGHTS, which fails silently
> without your patch).
>
> HTH,
>
> --
> Manuel Menal
>
>