implement CLOCK_MONOTONIC on Linux

Author: RalfJung

src/machine.rs

@@ -5,6 +5,7 @@ use std::borrow::Cow;
 use std::cell::RefCell;
 use std::num::NonZeroU64;
 use std::rc::Rc;
+use std::time::Instant;
 
 use rand::rngs::StdRng;
 
@@ -164,6 +165,9 @@ pub struct Evaluator<'tcx> {
     /// the call to `miri_start_panic` (the panic payload) when unwinding.
     /// This is pointer-sized, and matches the `Payload` type in `src/libpanic_unwind/miri.rs`.
     pub(crate) panic_payload: Option<Scalar<Tag>>,
+
+    /// The "time anchor" for this machine's monotone clock (for `Instant` simulation).
+    pub(crate) time_anchor: Instant,
 }
 
 impl<'tcx> Evaluator<'tcx> {
@@ -182,6 +186,7 @@ impl<'tcx> Evaluator<'tcx> {
             file_handler: Default::default(),
             dir_handler: Default::default(),
             panic_payload: None,
+            time_anchor: Instant::now(),
         }
     }
 }

src/shims/time.rs

@@ -1,14 +1,9 @@
-use std::time::{Duration, SystemTime};
+use std::time::{Duration, SystemTime, Instant};
 
 use crate::stacked_borrows::Tag;
 use crate::*;
 use helpers::immty_from_int_checked;
 
-// Returns the time elapsed between now and the unix epoch as a `Duration`.
-fn get_time<'tcx>() -> InterpResult<'tcx, Duration> {
-    system_time_to_duration(&SystemTime::now())
-}
-
 /// Returns the time elapsed between the provided time and the unix epoch as a `Duration`.
 pub fn system_time_to_duration<'tcx>(time: &SystemTime) -> InterpResult<'tcx, Duration> {
     time.duration_since(SystemTime::UNIX_EPOCH)
@@ -28,15 +23,20 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         this.check_no_isolation("clock_gettime")?;
 
         let clk_id = this.read_scalar(clk_id_op)?.to_i32()?;
-        if clk_id != this.eval_libc_i32("CLOCK_REALTIME")? {
+        let tp = this.deref_operand(tp_op)?;
+
+        let duration = if clk_id == this.eval_libc_i32("CLOCK_REALTIME")? {
+            system_time_to_duration(&SystemTime::now())?
+        } else if clk_id == this.eval_libc_i32("CLOCK_MONOTONIC")? {
+            // Absolute time does not matter, only relative time does, so we can just
+            // use our own time anchor here.
+            Instant::now().duration_since(this.machine.time_anchor)
+        } else {
             let einval = this.eval_libc("EINVAL")?;
             this.set_last_error(einval)?;
             return Ok(-1);
-        }
-
-        let tp = this.deref_operand(tp_op)?;
+        };
 
-        let duration = get_time()?;
         let tv_sec = duration.as_secs();
         let tv_nsec = duration.subsec_nanos();
 
@@ -68,7 +68,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
 
         let tv = this.deref_operand(tv_op)?;
 
-        let duration = get_time()?;
+        let duration = system_time_to_duration(&SystemTime::now())?;
         let tv_sec = duration.as_secs();
         let tv_usec = duration.subsec_micros();
 

tests/run-pass/clock.rs

@@ -0,0 +1,18 @@
+// ignore-windows: TODO clock shims are not implemented on Windows
+// compile-flags: -Zmiri-disable-isolation
+
+use std::time::{SystemTime, Instant};
+
+fn main() {
+    let now1 = SystemTime::now();
+    // Do some work to make time pass.
+    for _ in 0..10 { drop(vec![42]); }
+    let now2 = SystemTime::now();
+    assert!(now2 > now1);
+
+    let now1 = Instant::now();
+    // Do some work to make time pass.
+    for _ in 0..10 { drop(vec![42]); }
+    let now2 = Instant::now();
+    assert!(now2 > now1);
+}