Open Radar

 
Summary:
The mach_ports_register() system call was broken on 10.7 with the introduction of libxpc. Prior to 10.7, you could use mach_ports_register() to pass an array of Mach ports to a child process across fork(). The child could then look up the array of ports with mach_ports_lookup().

In 10.7 and higher, however, XPC clobbers this array of ports for its own use, here:

libsystem_kernel.dylib`mach_msg_trap+0xa
libsystem_kernel.dylib`mach_ports_register+0x70
libxpc.dylib`xpc_atfork_prepare+0x2b
libSystem.B.dylib`libSystem_atfork_prepare+0x9
libsystem_c.dylib`fork+0xc
parent`main+0x138
libdyld.dylib`start
parent`0x1

This removes the only non-kludgy way to pass a Mach port from a parent to a child.

Steps to Reproduce:
1. Unzip the attached program
2. `make`
3. ./parent

On 10.6, this works. On 10.7 and higher, it does not.

Expected Results:
The ./parent program should exit cleanly with output like:

Sent message to parent on port 519
Got request: I'm a child: 71489
Child exited:1, status:0

This works on 10.6.

Actual Results:
On 10.7 and 10.8, the parent program hangs in mach_msg_receive() because the child program has sent the message to its bootstrap port. This happens because the port array has been replaced by XPC via xpc_atfork_prepare().

Version:
Works: 10.6.8 10K549
Broken: 10.8.5 12F45

Regressed: 10.7.0

Notes:


Configuration:


Attachments:
'mach_ports_register.zip' was successfully uploaded.