From 8aba07be92768aea58aca079307432371a9f7feb Mon Sep 17 00:00:00 2001
From: Leonardo Forchini <leonardo.forchini@nutanix.com>
Date: Fri, 25 Sep 2026 08:44:34 +0000
Subject: [PATCH] Read /proc through the mock-root path

Host CPU and per-task samples were opening the real /proc, so
IO_THREAD_CONTROLLER_ROOT_PATH could not redirect them at a test
filesystem.

Signed-off-by: Leonardo Forchini <leonardo.forchini@nutanix.com>
---
 src/backends/qemu/client.rs  |  3 ++-
 src/backends/qemu/helpers.rs |  7 +++++--
 src/controller.rs            |  7 +++++--
 src/instance.rs              |  2 +-
 src/util.rs                  | 10 ++++++++++
 5 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/src/backends/qemu/client.rs b/src/backends/qemu/client.rs
index e618623..eae7a0b 100644
--- a/src/backends/qemu/client.rs
+++ b/src/backends/qemu/client.rs
@@ -36,6 +36,7 @@ use crate::{
         },
     },
     instance::{InstanceClient, ThreadPoolSnapshot},
+    util::Path,
 };
 
 /// 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::new_with_root(Path::proc_pid(first_tid))?
             .status()
             .map_err(|e| BackendClientError::InvalidState(format!("pid resolution: {e}")))?
             .tgid;
diff --git a/src/backends/qemu/helpers.rs b/src/backends/qemu/helpers.rs
index 34b5479..59dcccb 100644
--- a/src/backends/qemu/helpers.rs
+++ b/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::Path,
+};
 
 /// Errors produced while querying or reconfiguring a QEMU vm.
 #[derive(Debug, Error)]
@@ -200,7 +203,7 @@ impl CpuSample {
             wall: Instant::now(),
         };
 
-        let p = Process::new(pid)?;
+        let p = Process::new_with_root(Path::proc_pid(pid))?;
         for &tid in iothread_tids.values() {
             let stat = p.task_from_tid(tid)?.stat()?;
             out.jiffies.insert(tid, stat.utime + stat.stime);
diff --git a/src/controller.rs b/src/controller.rs
index 09b64de..c534a60 100644
--- a/src/controller.rs
+++ b/src/controller.rs
@@ -12,7 +12,7 @@ use std::{
 };
 
 use futures_util::future::join_all;
-use procfs::{CurrentSI, ProcError};
+use procfs::{FromReadSI, ProcError};
 use serde::{Deserialize, Serialize};
 use statistical::median;
 use thiserror::Error;
@@ -25,6 +25,7 @@ use crate::{
     instance::{Instance, InstanceStatus},
     rolling::format_1_5_15,
     state::{StateError, VmOwnership, VmStateStore},
+    util::Path,
 };
 /// Wire-facing container for the D-Bus `GetSnapshot` reply.
 #[derive(Debug, Clone, Default, Serialize, Deserialize)]
@@ -804,7 +805,9 @@ impl Controller {
 
 /// Read aggregate host CPU counters.
 fn read_host_cpu_sample() -> Result<HostCpuSample, ControllerError> {
-    let total = procfs::KernelStats::current()?.total;
+    let total =
+        procfs::KernelStats::from_file(Path::new("/proc/stat"), procfs::current_system_info())?
+            .total;
     let idle_ticks = total.idle.saturating_add(total.iowait.unwrap_or(0));
     let total_ticks = total
         .user
diff --git a/src/instance.rs b/src/instance.rs
index 82fffe2..b31cf49 100644
--- a/src/instance.rs
+++ b/src/instance.rs
@@ -523,7 +523,7 @@ fn read_cpu_sample(
     pid: i32,
     filter: &crate::instance::ThreadNameFilter,
 ) -> Result<CpuSample, CpuSampleError> {
-    let process = Process::new(pid)?;
+    let process = Process::new_with_root(Path::proc_pid(pid))?;
     let tasks = process.tasks()?;
     let mut cpu_ticks = 0u64;
     let mut thread_count = 0u32;
diff --git a/src/util.rs b/src/util.rs
index 68bac0e..edfc89c 100644
--- a/src/util.rs
+++ b/src/util.rs
@@ -90,6 +90,16 @@ impl Path {
     pub fn glob(&self) -> glob::Paths {
         glob::glob(&self.path.to_string_lossy()).expect("valid glob pattern")
     }
+
+    /// `/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 path to
+    /// `Process::new_with_root`.
+    pub fn proc_pid(pid: i32) -> std::path::PathBuf {
+        let path = Self::new(&format!("/proc/{pid}"));
+        std::path::PathBuf::from(std::path::Path::as_os_str(&path))
+    }
 }
 
 impl From<&str> for Path {