[PATCH v3 11/11] sched,signal,ptrace: Rework TASK_TRACED, TASK_STOPPED state

Eric W. Biederman
2022-05-04T22:40:58Z

From: Peter Zijlstra <peterz@infradead.org>

Currently ptrace_stop() / do_signal_stop() rely on the special states
TASK_TRACED and TASK_STOPPED resp. to keep unique state. That is, this
state exists only in task->__state and nowhere else.

There's two spots of bother with this:

- PREEMPT_RT has task->saved_state which complicates matters,
meaning task_is_{traced,stopped}() needs to check an additional
variable.

- An alternative freezer implementation that itself relies on a
special TASK state would loose TASK_TRACED/TASK_STOPPED and will
result in misbehaviour.

As such, add additional state to task->jobctl to track this state
outside of task->__state.

NOTE: this doesn't actually fix anything yet, just adds extra state.

--EWB
* didn't add a unnecessary newline in signal.h
* Update t->jobctl in signal_wake_up and ptrace_signal_wake_up
instead of in signal_wake_up_state. This prevents the clearing
of TASK_STOPPED and TASK_TRACED from getting lost.
* Added warnings if JOBCTL_STOPPED or JOBCTL_TRACED are not cleared

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220421150654.757693825@infradead.org
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
---
include/linux/sched.h | 8 +++-----
include/linux/sched/jobctl.h | 6 ++++++
include/linux/sched/signal.h | 19 +++++++++++++++----
kernel/ptrace.c | 16 +++++++++++++---
kernel/signal.c | 10 ++++++++--
5 files changed, 45 insertions(+), 14 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 610f2fdb1e2c..cbe5c899599c 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -118,11 +118,9 @@ struct task_group;

#define task_is_running(task) (READ_ONCE((task)->__state) = TASK_RUNNING)

-#define task_is_traced(task) ((READ_ONCE(task->__state) & __TASK_TRACED) != 0)
-
-#define task_is_stopped(task) ((READ_ONCE(task->__state) & __TASK_STOPPED) != 0)
-
-#define task_is_stopped_or_traced(task) ((READ_ONCE(task->__state) & (__TASK_STOPPED | __TASK_TRACED)) != 0)
+#define task_is_traced(task) ((READ_ONCE(task->jobctl) & JOBCTL_TRACED) != 0)
+#define task_is_stopped(task) ((READ_ONCE(task->jobctl) & JOBCTL_STOPPED) != 0)
+#define task_is_stopped_or_traced(task) ((READ_ONCE(task->jobctl) & (JOBCTL_STOPPED | JOBCTL_TRACED)) != 0)

/*
* Special states are those that do not use the normal wait-loop pattern. See
diff --git a/include/linux/sched/jobctl.h b/include/linux/sched/jobctl.h
index d556c3425963..68876d0a7ef9 100644
--- a/include/linux/sched/jobctl.h
+++ b/include/linux/sched/jobctl.h
@@ -21,6 +21,9 @@ struct task_struct;
#define JOBCTL_TRAP_FREEZE_BIT 23 /* trap for cgroup freezer */
#define JOBCTL_PTRACE_FROZEN_BIT 24 /* frozen for ptrace */

+#define JOBCTL_STOPPED_BIT 26 /* do_signal_stop() */
+#define JOBCTL_TRACED_BIT 27 /* ptrace_stop() */
+
#define JOBCTL_STOP_DEQUEUED (1UL << JOBCTL_STOP_DEQUEUED_BIT)
#define JOBCTL_STOP_PENDING (1UL << JOBCTL_STOP_PENDING_BIT)
#define JOBCTL_STOP_CONSUME (1UL << JOBCTL_STOP_CONSUME_BIT)
@@ -31,6 +34,9 @@ struct task_struct;
#define JOBCTL_TRAP_FREEZE (1UL << JOBCTL_TRAP_FREEZE_BIT)
#define JOBCTL_PTRACE_FROZEN (1UL << JOBCTL_PTRACE_FROZEN_BIT)

+#define JOBCTL_STOPPED (1UL << JOBCTL_STOPPED_BIT)
+#define JOBCTL_TRACED (1UL << JOBCTL_TRACED_BIT)
+
#define JOBCTL_TRAP_MASK (JOBCTL_TRAP_STOP | JOBCTL_TRAP_NOTIFY)
#define JOBCTL_PENDING_MASK (JOBCTL_STOP_PENDING | JOBCTL_TRAP_MASK)

diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index e66948abbee4..07ba3404fcde 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -294,8 +294,10 @@ static inline int kernel_dequeue_signal(void)
static inline void kernel_signal_stop(void)
{
spin_lock_irq(&current->sighand->siglock);
- if (current->jobctl & JOBCTL_STOP_DEQUEUED)
+ if (current->jobctl & JOBCTL_STOP_DEQUEUED) {
+ current->jobctl |= JOBCTL_STOPPED;
set_special_state(TASK_STOPPED);
+ }
spin_unlock_irq(&current->sighand->siglock);

schedule();
@@ -437,12 +439,21 @@ extern void signal_wake_up_state(struct task_struct *t, unsigned int state);

static inline void signal_wake_up(struct task_struct *t, bool fatal)
{
- fatal = fatal && !(t->jobctl & JOBCTL_PTRACE_FROZEN);
- signal_wake_up_state(t, fatal ? TASK_WAKEKILL | __TASK_TRACED : 0);
+ unsigned int state = 0;
+ if (fatal && !(t->jobctl & JOBCTL_PTRACE_FROZEN)) {
+ t->jobctl &= ~(JOBCTL_STOPPED | JOBCTL_TRACED);
+ state = TASK_WAKEKILL | __TASK_TRACED;
+ }
+ signal_wake_up_state(t, state);
}
static inline void ptrace_signal_wake_up(struct task_struct *t, bool resume)
{
- signal_wake_up_state(t, resume ? __TASK_TRACED : 0);
+ unsigned int state = 0;
+ if (resume) {
+ t->jobctl &= ~JOBCTL_TRACED;
+ state = __TASK_TRACED;
+ }
+ signal_wake_up_state(t, state);
}

void task_join_group_stop(struct task_struct *task);
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 36a5b7a00d2f..328a34a99124 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -185,7 +185,12 @@ static bool looks_like_a_spurious_pid(struct task_struct *task)
return true;
}

-/* Ensure that nothing can wake it up, even SIGKILL */
+/*
+ * Ensure that nothing can wake it up, even SIGKILL
+ *
+ * A task is switched to this state while a ptrace operation is in progress;
+ * such that the ptrace operation is uninterruptible.
+ */
static bool ptrace_freeze_traced(struct task_struct *task)
{
bool ret = false;
@@ -216,8 +221,10 @@ static void ptrace_unfreeze_traced(struct task_struct *task)
*/
if (lock_task_sighand(task, &flags)) {
task->jobctl &= ~JOBCTL_PTRACE_FROZEN;
- if (__fatal_signal_pending(task))
+ if (__fatal_signal_pending(task)) {
+ task->jobctl &= ~TASK_TRACED;
wake_up_state(task, __TASK_TRACED);
+ }
unlock_task_sighand(task, &flags);
}
}
@@ -462,8 +469,10 @@ static int ptrace_attach(struct task_struct *task, long request,
* in and out of STOPPED are protected by siglock.
*/
if (task_is_stopped(task) &&
- task_set_jobctl_pending(task, JOBCTL_TRAP_STOP | JOBCTL_TRAPPING))
+ task_set_jobctl_pending(task, JOBCTL_TRAP_STOP | JOBCTL_TRAPPING)) {
+ task->jobctl &= ~JOBCTL_STOPPED;
signal_wake_up_state(task, __TASK_STOPPED);
+ }

spin_unlock(&task->sighand->siglock);

@@ -875,6 +884,7 @@ static int ptrace_resume(struct task_struct *child, long request,
*/
spin_lock_irq(&child->sighand->siglock);
child->exit_code = data;
+ child->jobctl &= ~JOBCTL_TRACED;
wake_up_state(child, __TASK_TRACED);
spin_unlock_irq(&child->sighand->siglock);

diff --git a/kernel/signal.c b/kernel/signal.c
index e0b416b21ad3..80108017783d 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -762,7 +762,10 @@ static int dequeue_synchronous_signal(kernel_siginfo_t *info)
*/
void signal_wake_up_state(struct task_struct *t, unsigned int state)
{
+ lockdep_assert_held(&t->sighand->siglock);
+
set_tsk_thread_flag(t, TIF_SIGPENDING);
+
/*
* TASK_WAKEKILL also means wake it up in the stopped/traced/killable
* case. We don't check t->state here because there is a race with it
@@ -930,9 +933,10 @@ static bool prepare_signal(int sig, struct task_struct *p, bool force)
for_each_thread(p, t) {
flush_sigqueue_mask(&flush, &t->pending);
task_clear_jobctl_pending(t, JOBCTL_STOP_PENDING);
- if (likely(!(t->ptrace & PT_SEIZED)))
+ if (likely(!(t->ptrace & PT_SEIZED))) {
+ t->jobctl &= ~JOBCTL_STOPPED;
wake_up_state(t, __TASK_STOPPED);
- else
+ } else
ptrace_trap_notify(t);
}

@@ -2218,6 +2222,7 @@ static int ptrace_stop(int exit_code, int why, unsigned long message,
return exit_code;

set_special_state(TASK_TRACED);
+ current->jobctl |= JOBCTL_TRACED;

/*
* We're committing to trapping. TRACED should be visible before
@@ -2437,6 +2442,7 @@ static bool do_signal_stop(int signr)
if (task_participate_group_stop(current))
notify = CLD_STOPPED;

+ current->jobctl |= JOBCTL_STOPPED;
set_special_state(TASK_STOPPED);
spin_unlock_irq(&current->sighand->siglock);

--
2.35.3