Read /proc through the mock-root path

src/backends/qemu/client.rs

@@ -36,6 +36,7 @@ use crate::{
         },
     },
     instance::{InstanceClient, ThreadPoolSnapshot},
+    util::ProcessExt,
 };
 
 /// Wire client for an QEMU-managed VM.
@@ -261,7 +262,7 @@ impl InstanceClient for QemuInstanceClient {
                 "qemu topology reported no iothread TIDs".into(),
             ));
         }
-        let pid = Process::new(first_tid)?
+        let pid = Process::from_pid(first_tid)?
             .status()
             .map_err(|e| BackendClientError::InvalidState(format!("pid resolution: {e}")))?
             .tgid;

src/backends/qemu/helpers.rs

@@ -29,7 +29,10 @@ use regex::Regex;
 use rustix::param;
 use thiserror::Error;
 
-use crate::backends::{BackendClientError, VqMapping};
+use crate::{
+    backends::{BackendClientError, VqMapping},
+    util::ProcessExt,
+};
 
 /// Errors produced while querying or reconfiguring a QEMU vm.
 #[derive(Debug, Error)]
@@ -229,7 +232,7 @@ impl CpuSample {
             wall: Instant::now(),
         };
 
-        let p = Process::new(pid)?;
+        let p = Process::from_pid(pid)?;
         for &tid in iothread_tids.values() {
             let stat = p.task_from_tid(tid)?.stat()?;
             out.jiffies.insert(tid, stat.utime + stat.stime);

src/controller.rs

@@ -12,7 +12,9 @@ use std::{
 };
 
 use futures_util::future::join_all;
-use procfs::{CpuPressure, Current, CurrentSI, IoPressure, MemoryPressure, ProcError};
+use procfs::{
+    CpuPressure, Current, FromReadSI, IoPressure, KernelStats, MemoryPressure, ProcError,
+};
 use serde::{Deserialize, Serialize};
 use statistical::median;
 use thiserror::Error;
@@ -27,6 +29,7 @@ use crate::{
     instance::{CgroupError, Instance, InstanceStatus, SnapshotLatency},
     per_vm_log,
     state::{StateError, VmOwnership, VmStateStore},
+    util::Path,
 };
 /// Wire-facing container for the D-Bus `GetSnapshot` reply.
 #[derive(Debug, Clone, Default, Serialize, Deserialize)]
@@ -1008,7 +1011,9 @@ fn read_psi_sample() -> PsiSample {
 
 /// Read aggregate host CPU counters.
 fn read_host_cpu_sample() -> Result<HostCpuSample, ControllerError> {
-    let total = procfs::KernelStats::current()?.total;
+    let current_si = procfs::current_system_info();
+    let total = KernelStats::from_file(Path::new("/proc/stat"), current_si)?.total;
+
     let idle_ticks = total.idle.saturating_add(total.iowait.unwrap_or(0));
     let total_ticks = total
         .user

src/instance.rs

@@ -27,7 +27,7 @@ use tokio::sync::RwLock;
 use crate::{
     backends::{BackendClientError, IoThreadProperties, VqMapping},
     rolling::RollingMetrics,
-    util::Path,
+    util::{Path, ProcessExt},
 };
 
 #[derive(Debug, Error)]
@@ -647,7 +647,7 @@ fn read_cpu_sample(
     pid: i32,
     filter: &crate::instance::ThreadNameFilter,
 ) -> Result<CpuSample, CpuSampleError> {
-    let process = Process::new(pid)?;
+    let process = Process::from_pid(pid)?;
     let tasks = process.tasks()?;
     let mut cpu_ticks = 0u64;
     let mut thread_count = 0u32;
@@ -702,7 +702,7 @@ fn read_cpu_sample(
 
 /// Read cgroup v2 CPU quota and throttle counters for `pid`.
 fn read_cgroup_sample(pid: i32) -> Result<CgroupSample, CgroupError> {
-    let process = Process::new(pid)?;
+    let process = Process::from_pid(pid)?;
     let cgroups = process.cgroups()?;
     let path = cgroups
         .0

src/util.rs

@@ -142,6 +142,22 @@ macro_rules! path {
     };
 }
 
+pub(crate) trait ProcessExt {
+    fn from_pid(pid: i32) -> procfs::ProcResult<procfs::process::Process>;
+}
+
+impl ProcessExt for procfs::process::Process {
+    /// `/proc/<pid>` after applying [`IO_THREAD_CONTROLLER_ROOT_PATH`].
+    ///
+    /// `procfs::process::Process::new` always opens the real `/proc`. Callers
+    /// that should honor the mock root pass this to create the process.
+    fn from_pid(pid: i32) -> procfs::ProcResult<procfs::process::Process> {
+        let path = Path::new(&format!("/proc/{pid}"));
+        let path = std::path::PathBuf::from(std::path::Path::as_os_str(&path));
+        Self::new_with_root(path)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;