fix: stop double counting jailer time in process_startup_time_cpu_us

In `report_cpu_start_time()`, Firecracker computers the CPU time it took
until the API socket was available as

CLOCK_PROCESS_CPUTIME_ID - cpu_start_time + jailer_time

However, here `cpu_start_time` is always zero currently (it's the value
the jailer passes via `--cpu-start-time`), and in the case where the
jailer exec()s into Firecracker (without prior fork() or clone()), this
is wrong: CLOCK_PROCESS_CPUTIME_ID will now be the CPU time since the
_jailer_ process started, and thus adding the jailer_time to it is
nonsense. This all works out if we set cpu_start_time to be the CPU time
right before we exec() into Firecracker: If we did not fork(), then it
will cause the calculation above to correctly discard all time before
exec()-ing into Firecracker (before re-adding specifically the time
spent in the jailer). If we _did_ fork(), then it will be approximately
zero, and thus preserve the current semantics (as the jailer time will
only be the time _until_ we did the last fork(), and
CLOCK_PROCESS_CPUTIME_ID will only be the absolute CPU time _since_ the
last fork()).

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

src/jailer/src/env.rs

@@ -476,7 +476,10 @@ impl Env {
         Command::new(chroot_exec_file)
             .args(["--id", &self.id])
             .args(["--start-time-us", &self.start_time_us.to_string()])
-            .args(["--start-time-cpu-us", &self.start_time_cpu_us.to_string()])
+            .args([
+                "--start-time-cpu-us",
+                &get_time_us(ClockType::ProcessCpu).to_string(),
+            ])
             .args(["--parent-cpu-time-us", &self.jailer_cpu_time_us.to_string()])
             .stdin(Stdio::inherit())
             .stdout(Stdio::inherit())
@@ -698,8 +701,6 @@ impl Env {
         // Compute jailer's total CPU time up to the current time.
         self.jailer_cpu_time_us += get_time_us(ClockType::ProcessCpu);
         self.jailer_cpu_time_us -= self.start_time_cpu_us;
-        // Reset process start time.
-        self.start_time_cpu_us = 0;
 
         // If specified, exec the provided binary into a new PID namespace.
         if self.new_pid_ns {