From 3657484eee1290672f10b251ac32b2f0a634483b Mon Sep 17 00:00:00 2001
From: Shambhavi Srivastava <shambhavisr@google.com>
Date: Thu, 17 Aug 2023 11:39:07 -0700
Subject: [PATCH] Adding /proc/[pid]/task/[tid]/children

PiperOrigin-RevId: 557888186
---
 pkg/sentry/fsimpl/proc/task.go       |  2 ++
 pkg/sentry/fsimpl/proc/task_files.go | 34 ++++++++++++++++++++++++++++
 pkg/sentry/kernel/threads.go         | 13 +++++++++++
 test/syscalls/linux/proc.cc          | 32 ++++++++++++++++++++++++++
 4 files changed, 81 insertions(+)

diff --git a/pkg/sentry/fsimpl/proc/task.go b/pkg/sentry/fsimpl/proc/task.go
index c15c23ca0..8e03c09a4 100644
--- a/pkg/sentry/fsimpl/proc/task.go
+++ b/pkg/sentry/fsimpl/proc/task.go
@@ -90,6 +90,8 @@ func (fs *filesystem) newTaskInode(ctx context.Context, task *kernel.Task, pidns
 	}
 	if isThreadGroup {
 		contents["task"] = fs.newSubtasks(ctx, task, pidns, fakeCgroupControllers)
+	} else {
+		contents["children"] = fs.newTaskOwnedInode(ctx, task, fs.NextIno(), 0644, &childrenData{task: task, pidns: pidns})
 	}
 	if len(fakeCgroupControllers) > 0 {
 		contents["cgroup"] = fs.newTaskOwnedInode(ctx, task, fs.NextIno(), 0444, newFakeCgroupData(fakeCgroupControllers))
diff --git a/pkg/sentry/fsimpl/proc/task_files.go b/pkg/sentry/fsimpl/proc/task_files.go
index 06a7cf013..c9047d4e0 100644
--- a/pkg/sentry/fsimpl/proc/task_files.go
+++ b/pkg/sentry/fsimpl/proc/task_files.go
@@ -18,6 +18,7 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"sort"
 	"strconv"
 	"strings"
 
@@ -1440,3 +1441,36 @@ func (d *taskCgroupData) Generate(ctx context.Context, buf *bytes.Buffer) error
 	d.task.GenerateProcTaskCgroup(buf)
 	return nil
 }
+
+// childrenData implements vfs.DynamicBytesSource for /proc/[pid]/task/[tid]/children.
+//
+// +stateify savable
+type childrenData struct {
+	kernfs.DynamicBytesFile
+
+	task *kernel.Task
+
+	// pidns is the PID namespace associated with the proc filesystem that
+	// includes the file using this childrenData.
+	pidns *kernel.PIDNamespace
+}
+
+// Generate implements vfs.DynamicBytesSource.Generate.
+func (d *childrenData) Generate(ctx context.Context, buf *bytes.Buffer) error {
+	children := d.task.Children()
+	var childrenTIDs []int
+	for childTask := range children {
+		childrenTIDs = append(childrenTIDs, int(d.pidns.IDOfTask(childTask)))
+	}
+
+	// The TIDs need to be in sorted order in accordance with the Linux implementation.
+	sort.Ints(childrenTIDs)
+
+	for _, childrenTID := range childrenTIDs {
+		// It contains a space-separated list of child tasks of the `task`.
+		// Each task is represented by its TID.
+		fmt.Fprintf(buf, "%d ", childrenTID)
+	}
+
+	return nil
+}
diff --git a/pkg/sentry/kernel/threads.go b/pkg/sentry/kernel/threads.go
index e54da8ef9..a0b2cbc38 100644
--- a/pkg/sentry/kernel/threads.go
+++ b/pkg/sentry/kernel/threads.go
@@ -545,3 +545,16 @@ func (t *Task) ThreadID() ThreadID {
 func (t *Task) TGIDInRoot() ThreadID {
 	return t.tg.pidns.owner.Root.IDOfThreadGroup(t.tg)
 }
+
+// Children returns children of this task.
+func (t *Task) Children() map[*Task]struct{} {
+	t.tg.pidns.owner.mu.RLock()
+	defer t.tg.pidns.owner.mu.RUnlock()
+
+	children := make(map[*Task]struct{}, len(t.children))
+	for child, val := range t.children {
+		children[child] = val
+	}
+
+	return children
+}
diff --git a/test/syscalls/linux/proc.cc b/test/syscalls/linux/proc.cc
index 78edcf186..3afc6126d 100644
--- a/test/syscalls/linux/proc.cc
+++ b/test/syscalls/linux/proc.cc
@@ -2390,6 +2390,38 @@ TEST(ProcTask, VerifyTaskDir) {
       DirContains(absl::StrCat("/proc/self/task/", getpid()), {}, {"task"}));
 }
 
+TEST(ProcTask, VerifyTaskChildren) {
+  auto path = JoinPath("/proc", absl::StrCat(getpid()), "task",
+                       absl::StrCat(gettid()), "children");
+  EXPECT_THAT(access(path.c_str(), F_OK), SyscallSucceeds());
+
+  int pid1 = -1, status1 = -1;
+  auto cleanup1 =
+      ForkAndExec("/bin/sleep", {"sleep", "100"}, {}, nullptr, &pid1, &status1);
+  ASSERT_GT(pid1, 0);
+  ASSERT_EQ(status1, 0);
+
+  auto proc_children_file = ASSERT_NO_ERRNO_AND_VALUE(GetContents(path));
+  EXPECT_EQ(absl::StrCat(pid1, " "), proc_children_file);
+
+  int pid2 = -1, status2 = -1;
+  auto cleanup2 =
+      ForkAndExec("/bin/sleep", {"sleep", "100"}, {}, nullptr, &pid2, &status2);
+  ASSERT_GT(pid2, 0);
+  ASSERT_EQ(status2, 0);
+
+  proc_children_file = ASSERT_NO_ERRNO_AND_VALUE(GetContents(path));
+
+  // /children contains space-separated sorted list of thread Ids of children.
+  std::string expectedContent;
+  if (pid1 < pid2) {
+    expectedContent = absl::StrCat(pid1, " ", pid2, " ");
+  } else {
+    expectedContent = absl::StrCat(pid2, " ", pid1, " ");
+  }
+  EXPECT_EQ(expectedContent, proc_children_file);
+}
+
 TEST(ProcTask, TaskDirCannotBeDeleted) {
   // Drop capabilities that allow us to override file and directory permissions.
   AutoCapability cap(CAP_DAC_OVERRIDE, false);