Return ENOENT when reading /proc/{pid}/task of an exited process

There will be a deadloop when we use getdents to read /proc/{pid}/task
of an exited process

Like this:

Process A is running
                         Process B: open /proc/{pid of A}/task
Process A exits
                         Process B: getdents /proc/{pid of A}/task

Then, process B will fall into deadloop, and return "." and ".."
in loops and never ends.

This patch returns ENOENT when use getdents to read /proc/{pid}/task
if the process is just exited.

Signed-off-by: chris.zn <chris.zn@antfin.com>

pkg/sentry/fs/proc/task.go

@@ -20,6 +20,7 @@ import (
 	"io"
 	"sort"
 	"strconv"
+	"syscall"
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/sentry/context"
@@ -162,6 +163,11 @@ func (f *subtasksFile) Readdir(ctx context.Context, file *fs.File, ser fs.Dentry
 	// subtask to emit.
 	offset := file.Offset()
 
+	tasks := f.t.ThreadGroup().MemberIDs(f.pidns)
+	if len(tasks) == 0 {
+		return offset, syscall.ENOENT
+	}
+
 	if offset == 0 {
 		// Serialize "." and "..".
 		root := fs.RootFromContext(ctx)
@@ -178,7 +184,6 @@ func (f *subtasksFile) Readdir(ctx context.Context, file *fs.File, ser fs.Dentry
 	}
 
 	// Serialize tasks.
-	tasks := f.t.ThreadGroup().MemberIDs(f.pidns)
 	taskInts := make([]int, 0, len(tasks))
 	for _, tid := range tasks {
 		taskInts = append(taskInts, int(tid))

test/syscalls/linux/proc.cc

@@ -1908,6 +1908,25 @@ void CheckDuplicatesRecursively(std::string path) {
 
 TEST(Proc, NoDuplicates) { CheckDuplicatesRecursively("/proc"); }
 
+TEST(Proc, Getdents) {
+  int child_pid = fork();
+  ASSERT_GE(child_pid, 0);
+  if (child_pid == 0) {
+    while(1){
+      sleep(100);
+    }
+  }
+  ASSERT_THAT(child_pid, SyscallSucceeds());
+  char name[100];
+  char buf[1024];
+  int fd;
+  sprintf(name, "/proc/%d/task", child_pid);
+  fd = open(name, O_RDONLY | O_DIRECTORY);
+  ASSERT_THAT(kill(child_pid, SIGKILL), SyscallSucceeds());
+  ASSERT_THAT(waitpid(child_pid, NULL, 0), SyscallSucceedsWithValue(child_pid));
+  ASSERT_THAT(syscall(SYS_getdents, fd, buf, 1024), SyscallFailsWithErrno(ENOENT));
+}
+
 // Most /proc/PID files are owned by the task user with SUID_DUMP_USER.
 TEST(ProcPid, UserDumpableOwner) {
   int before;