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

3 files changed, 70 insertions, 2 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 817a89f4..b7edf991 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -26,6 +26,9 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - Added `cfmakeraw`, `cfsetspeed`, and `tcgetsid`. ([#527](https://github.com/nix-rust/nix/pull/527))
 - Added "bad none", "bad write_ptr", "bad write_int", and "bad readwrite" variants to the `ioctl!`
   macro. ([#670](https://github.com/nix-rust/nix/pull/670))
+- On Linux and Android, added support for receiving `PTRACE_O_TRACESYSGOOD`
+  events from `wait` and `waitpid` using `WaitStatus::PtraceSyscall`
+  ([#566](https://github.com/nix-rust/nix/pull/566)).
 
 ### Changed
 - Changed `ioctl!(write ...)` into `ioctl!(write_ptr ...)` and `ioctl!(write_int ..)` variants
diff --git a/src/sys/wait.rs b/src/sys/wait.rs
index ee0beade..7033422d 100644
--- a/src/sys/wait.rs
+++ b/src/sys/wait.rs
@@ -47,6 +47,8 @@ pub enum WaitStatus {
     Stopped(Pid, Signal),
     #[cfg(any(target_os = "linux", target_os = "android"))]
     PtraceEvent(Pid, Signal, c_int),
+    #[cfg(any(target_os = "linux", target_os = "android"))]
+    PtraceSyscall(Pid),
     Continued(Pid),
     StillAlive
 }
@@ -56,6 +58,7 @@ 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
@@ -82,7 +85,17 @@ mod status {
     }
 
     pub fn stop_signal(status: i32) -> Signal {
-        Signal::from_c_int((status & 0xFF00) >> 8).unwrap()
+        // Keep only 7 bits of the signal: the high bit
+        // is used to indicate syscall stops, below.
+        Signal::from_c_int((status & 0x7F00) >> 8).unwrap()
+    }
+
+    pub fn syscall_stop(status: i32) -> bool {
+        // From ptrace(2), setting PTRACE_O_TRACESYSGOOD has the effect
+        // of delivering SIGTRAP | 0x80 as the signal number for syscall
+        // stops. This allows easily distinguishing syscall stops from
+        // genuine SIGTRAP signals.
+        ((status & 0xFF00) >> 8) == SIGTRAP | 0x80
     }
 
     pub fn stop_additional(status: i32) -> c_int {
@@ -196,7 +209,9 @@ fn decode(pid : Pid, status: i32) -> WaitStatus {
             if #[cfg(any(target_os = "linux", target_os = "android"))] {
                 fn decode_stopped(pid: Pid, 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))
diff --git a/test/sys/test_wait.rs b/test/sys/test_wait.rs
index 2e28d9e7..5f6c9231 100644
--- a/test/sys/test_wait.rs
+++ b/test/sys/test_wait.rs
@@ -34,3 +34,53 @@ fn test_wait_exit() {
       Err(_) => panic!("Error: Fork Failed")
     }
 }
+
+#[cfg(any(target_os = "linux", target_os = "android"))]
+// FIXME: qemu-user doesn't implement ptrace on most arches
+#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+mod ptrace {
+    use nix::sys::ptrace::*;
+    use nix::sys::ptrace::ptrace::*;
+    use nix::sys::signal::*;
+    use nix::sys::wait::*;
+    use nix::unistd::*;
+    use nix::unistd::ForkResult::*;
+    use std::{ptr, process};
+
+    fn ptrace_child() -> ! {
+        let _ = ptrace(PTRACE_TRACEME, Pid::from_raw(0), ptr::null_mut(), ptr::null_mut());
+        // As recommended by ptrace(2), raise SIGTRAP to pause the child
+        // until the parent is ready to continue
+        let _ = raise(SIGTRAP);
+        process::exit(0)
+    }
+
+    fn ptrace_parent(child: Pid) {
+        // Wait for the raised SIGTRAP
+        assert_eq!(waitpid(child, None), Ok(WaitStatus::Stopped(child, SIGTRAP)));
+        // We want to test a syscall stop and a PTRACE_EVENT stop
+        assert!(ptrace_setoptions(child, PTRACE_O_TRACESYSGOOD | PTRACE_O_TRACEEXIT).is_ok());
+
+        // First, stop on the next system call, which will be exit()
+        assert!(ptrace(PTRACE_SYSCALL, child, ptr::null_mut(), ptr::null_mut()).is_ok());
+        assert_eq!(waitpid(child, None), Ok(WaitStatus::PtraceSyscall(child)));
+        // Then get the ptrace event for the process exiting
+        assert!(ptrace(PTRACE_CONT, child, ptr::null_mut(), ptr::null_mut()).is_ok());
+        assert_eq!(waitpid(child, None), Ok(WaitStatus::PtraceEvent(child, SIGTRAP, PTRACE_EVENT_EXIT)));
+        // Finally get the normal wait() result, now that the process has exited
+        assert!(ptrace(PTRACE_CONT, child, ptr::null_mut(), ptr::null_mut()).is_ok());
+        assert_eq!(waitpid(child, None), Ok(WaitStatus::Exited(child, 0)));
+    }
+
+    #[test]
+    fn test_wait_ptrace() {
+        #[allow(unused_variables)]
+        let m = ::FORK_MTX.lock().expect("Mutex got poisoned by another test");
+
+        match fork() {
+            Ok(Child) => ptrace_child(),
+            Ok(Parent { child }) => ptrace_parent(child),
+            Err(_) => panic!("Error: Fork Failed")
+        }
+    }
+}