This looks suspect for precise GC:

// On x86 Linux, all the socket calls go through an extra indirection,
// I think because the 5-register system call interface can't handle
// the 6-argument calls like sendto and recvfrom.  Instead the
// arguments to the underlying system call are the number below
// and a pointer to an array of uintptr.  We hide the pointer in the
// socketcall assembly to avoid allocation on every system call.

const (
    // see linux/net.h
    _SOCKET      = 1
    _BIND        = 2
    _CONNECT     = 3
    _LISTEN      = 4
    _ACCEPT      = 5
    _GETSOCKNAME = 6
    _GETPEERNAME = 7
    _SOCKETPAIR  = 8
    _SEND        = 9
    _RECV        = 10
    _SENDTO      = 11
    _RECVFROM    = 12
    _SHUTDOWN    = 13
    _SETSOCKOPT  = 14
    _GETSOCKOPT  = 15
    _SENDMSG     = 16
    _RECVMSG     = 17
    _ACCEPT4     = 18
    _RECVMMSG    = 19
    _SENDMMSG    = 20
)

func socketcall(call int, a0, a1, a2, a3, a4, a5 uintptr) (n int, err Errno)
func rawsocketcall(call int, a0, a1, a2, a3, a4, a5 uintptr) (n int, err Errno)