v0.2: prev snapshot of regs, refactored state/prev to share build_register_view

src/target.rs

@@ -52,6 +52,9 @@ pub enum State {
 pub struct Target {
     pid: Pid,
     state: State,
+    /// Snapshot of registers from the previous stop. None until at least one
+    /// step/cont has happened after spawn.
+    prev_regs: Option<libc::user_regs_struct>,
 }
 
 impl Target {
@@ -102,6 +105,7 @@ impl Target {
                     WaitStatus::Stopped(_, Signal::SIGTRAP) => Ok(Target {
                         pid: child,
                         state: State::Stopped(StopReason::Step),
+                        prev_regs: None,
                     }),
                     WaitStatus::Exited(_, code) => {
                         Err(format!("child exited immediately with code {} (exec failed?)", code))
@@ -117,6 +121,8 @@ impl Target {
         if !self.is_stopped() {
             return Err("target is not stopped".into());
         }
+        // Snapshot regs before resuming so `prev` reflects the pre-step state.
+        self.prev_regs = ptrace::getregs(self.pid).ok();
         ptrace::step(self.pid, None).map_err(|e| format!("ptrace step: {}", e))?;
         let reason = self.wait_for_stop()?;
         Ok(reason)
@@ -127,11 +133,16 @@ impl Target {
         if !self.is_stopped() {
             return Err("target is not stopped".into());
         }
+        self.prev_regs = ptrace::getregs(self.pid).ok();
         ptrace::cont(self.pid, None).map_err(|e| format!("ptrace cont: {}", e))?;
         let reason = self.wait_for_stop()?;
         Ok(reason)
     }
 
+    pub fn prev_regs(&self) -> Option<libc::user_regs_struct> {
+        self.prev_regs
+    }
+
     fn wait_for_stop(&mut self) -> Result<StopReason, String> {
         let status = waitpid(self.pid, None).map_err(|e| format!("waitpid: {}", e))?;
         let reason = match status {