Fix seek on /proc/pid/cmdline when task is zombie.

PiperOrigin-RevId: 348056159

pkg/sentry/fsimpl/proc/task_files.go

@@ -57,9 +57,6 @@ func getMM(task *kernel.Task) *mm.MemoryManager {
 // MemoryManager's users count is incremented, and must be decremented by the
 // caller when it is no longer in use.
 func getMMIncRef(task *kernel.Task) (*mm.MemoryManager, error) {
-	if task.ExitState() == kernel.TaskExitDead {
-		return nil, syserror.ESRCH
-	}
 	var m *mm.MemoryManager
 	task.WithMuLocked(func(t *kernel.Task) {
 		m = t.MemoryManager()
@@ -111,9 +108,13 @@ var _ dynamicInode = (*auxvData)(nil)
 
 // Generate implements vfs.DynamicBytesSource.Generate.
 func (d *auxvData) Generate(ctx context.Context, buf *bytes.Buffer) error {
+	if d.task.ExitState() == kernel.TaskExitDead {
+		return syserror.ESRCH
+	}
 	m, err := getMMIncRef(d.task)
 	if err != nil {
-		return err
+		// Return empty file.
+		return nil
 	}
 	defer m.DecUsers(ctx)
 
@@ -157,9 +158,13 @@ var _ dynamicInode = (*cmdlineData)(nil)
 
 // Generate implements vfs.DynamicBytesSource.Generate.
 func (d *cmdlineData) Generate(ctx context.Context, buf *bytes.Buffer) error {
+	if d.task.ExitState() == kernel.TaskExitDead {
+		return syserror.ESRCH
+	}
 	m, err := getMMIncRef(d.task)
 	if err != nil {
-		return err
+		// Return empty file.
+		return nil
 	}
 	defer m.DecUsers(ctx)
 
@@ -472,7 +477,7 @@ func (fd *memFD) PRead(ctx context.Context, dst usermem.IOSequence, offset int64
 	}
 	m, err := getMMIncRef(fd.inode.task)
 	if err != nil {
-		return 0, nil
+		return 0, err
 	}
 	defer m.DecUsers(ctx)
 	// Buffer the read data because of MM locks

test/syscalls/linux/proc.cc

@@ -1802,6 +1802,33 @@ TEST(ProcPidCmdline, SubprocessForkSameCmdline) {
   }
 }
 
+TEST(ProcPidCmdline, SubprocessSeekCmdline) {
+  FileDescriptor fd;
+  ASSERT_NO_ERRNO(WithSubprocess(
+      [&](int pid) -> PosixError {
+        // Running. Open /proc/pid/cmdline.
+        ASSIGN_OR_RETURN_ERRNO(
+            fd, Open(absl::StrCat("/proc/", pid, "/cmdline"), O_RDONLY));
+        return NoError();
+      },
+      [&](int pid) -> PosixError {
+        // Zombie, but seek should still succeed.
+        int ret = lseek(fd.get(), 0x801, 0);
+        if (ret < 0) {
+          return PosixError(errno);
+        }
+        return NoError();
+      },
+      [&](int pid) -> PosixError {
+        // Exited.
+        int ret = lseek(fd.get(), 0x801, 0);
+        if (ret < 0) {
+          return PosixError(errno);
+        }
+        return NoError();
+      }));
+}
+
 // Test whether /proc/PID/ symlinks can be read for a running process.
 TEST(ProcPidSymlink, SubprocessRunning) {
   char buf[1];