Add WaitStatus::PtraceSyscall for use with PTRACE_O_TRACESYSGOOD

The recommended way to trace syscalls with ptrace is to set the
PTRACE_O_TRACESYSGOOD option, to distinguish syscall stops from
receiving an actual SIGTRAP. In C, this would cause WSTOPSIG to return
SIGTRAP | 0x80, but nix wants to parse that as an actual signal.

Add another wait status type for syscall stops (in the language of the
ptrace(2) manpage, "PTRACE_EVENT stops" and "Syscall-stops" are
different things), and mask out bit 0x80 from signals before trying to
parse it.

Closes #550

src/sys/wait.rs

@@ -46,6 +46,8 @@ pub enum WaitStatus {
     Stopped(pid_t, Signal),
     #[cfg(any(target_os = "linux", target_os = "android"))]
     PtraceEvent(pid_t, Signal, c_int),
+    #[cfg(any(target_os = "linux", target_os = "android"))]
+    PtraceSyscall(pid_t),
     Continued(pid_t),
     StillAlive
 }
@@ -55,6 +57,7 @@ pub enum WaitStatus {
 mod status {
     use sys::signal::Signal;
     use libc::c_int;
+    use libc::SIGTRAP;
 
     pub fn exited(status: i32) -> bool {
         (status & 0x7F) == 0
@@ -81,7 +84,11 @@ mod status {
     }
 
     pub fn stop_signal(status: i32) -> Signal {
-        Signal::from_c_int((status & 0xFF00) >> 8).unwrap()
+        Signal::from_c_int((status & 0x7F00) >> 8).unwrap()
+    }
+
+    pub fn syscall_stop(status: i32) -> bool {
+        ((status & 0xFF00) >> 8) == SIGTRAP | 0x80
     }
 
     pub fn stop_additional(status: i32) -> c_int {
@@ -195,7 +202,9 @@ fn decode(pid : pid_t, status: i32) -> WaitStatus {
             if #[cfg(any(target_os = "linux", target_os = "android"))] {
                 fn decode_stopped(pid: pid_t, status: i32) -> WaitStatus {
                     let status_additional = status::stop_additional(status);
-                    if status_additional == 0 {
+                    if status::syscall_stop(status) {
+                        WaitStatus::PtraceSyscall(pid)
+                    } else if status_additional == 0 {
                         WaitStatus::Stopped(pid, status::stop_signal(status))
                     } else {
                         WaitStatus::PtraceEvent(pid, status::stop_signal(status), status::stop_additional(status))