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Re: [libvirt] [PATCH 5/8] Extend RPC protocol to allow FD passing



On Thu, Oct 27, 2011 at 05:15:40PM -0600, Eric Blake wrote:
> On 10/25/2011 10:03 AM, Daniel P. Berrange wrote:
> >From: "Daniel P. Berrange"<berrange redhat com>
> >
> >Define two new RPC message types VIR_NET_CALL_WITH_FDS and
> >VIR_NET_REPLY_WITH_FDS. These message types are equivalent
> >to VIR_NET_CALL and VIR_NET_REPLY, except that between the
> >message header, and payload there is a 32-bit integer field
> >specifying how many file descriptors have been passed.
> >
> >The actual file descriptors are sent/recv'd out of band.
> >
> >* src/rpc/virnetmessage.c, src/rpc/virnetmessage.h,
> >   src/libvirt_private.syms: Add support for handling
> >   passed file descriptors
> >* src/rpc/virnetprotocol.x: Extend protocol for FD
> >   passing
> >---
> >  docs/internals/rpc.html.in |   33 +++++++++++++
> 
> Thanks; this fixes my biggest concern from the last round.
> 
> >      <p>
> >+      With the two packet types that support passing file descriptors, in
> >+      between the header and the payload there will be a 4-byte integer
> >+      specifying the number of file descriptors which are being sent.
> >+      The actual file handles are sent after the payload has been sent.
> >+      Each file handle has a single dummy byte transmitted as a carrier
> >+      for the out of band file descriptor.
> 
> gnulib recvfd() ignores this byte value.  While sendfd() sends NUL,
> it is conceivable that a 3rd-party implementation of the wire
> protocol might pick a different value (or that gnulib might change
> in the future).  Should we add any documentation along the lines of
> 'sender should send 0' and 'receiver must ignore the dummy byte,
> even if it is not 0', just for robustness sake?
> 
> >+
> >+
> >+int virNetMessageDupFD(virNetMessagePtr msg,
> >+                       size_t slot)
> >+{
> >+    int fd;
> >+
> >+    if (slot>= msg->nfds) {
> >+        virNetError(VIR_ERR_INTERNAL_ERROR,
> >+                    _("No FD available at slot %zu"), slot);
> >+        return -1;
> >+    }
> >+
> >+    if ((fd = dup(msg->fds[slot]))<  0) {
> >+        virReportSystemError(errno,
> >+                             _("Unable to duplicate FD %d"),
> >+                             msg->fds[slot]);
> >+        return -1;
> >+    }
> 
> Do we want to be using gnulib's dup_cloexec ("cloexec.h") or
> fcntl(fd, F_DUPFD_CLOEXEC, 0) here, so that our dup doesn't leak
> into a third-party child if we are linked into some larger
> multithreaded app?

dup3() is the new glibc API for this ? Does gnulib support
that yet ?


Daniel
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