[RFC][PATCH] ptrace: Don't change __state

["Eric W. Biederman" <ebiederm@xmission.com>]

I was thinking about this and I have an approach from a different
direction.  In particular it removes the need for ptrace_freeze_attach
and ptrace_unfreeze_attach to change __state.  Instead a jobctl
bit is used to suppress waking up a process with TASK_WAKEKILL.

I think this would be a good technique to completely decouple
PREEMPT_RT from the work that ptrace_freeze_attach does.

Comments?

Eric

---

The games ptrace_freeze_traced and ptrace_unfreeze_traced have
been playing with __state to remove TASK_WAKEKILL from __state
while a ptrace command is executing.

Instead add JOBCTL_DELAY_WAKEILL and set it in ptrace_freeze_task and
clear it in ptrace_unfreeze_task and the final exit of ptrace_stop.

Then in signal_wake_up_state drop TASK_WAKEKILL if it is sent while
JOBCTL_DELAY_WAKEKILL is set.

As signal_wake_up is the only function that sets TASK_WAKEKILL when
waking a process this achives the same effect with less messing
with state.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

As a proof of concept this seems to work most of the time,
unfortunately I have a bug somewhere (a memory stomp?) and I am
hitting a BUG_ON that asserts __send_signal is not holding
the siglock.

[   97.000168] ------------[ cut here ]------------
[   97.001782] kernel BUG at kernel/signal.c:1086!
[   97.002539] invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
[   97.002846] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.18.0-rc1userns+ #449
[   97.003135] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
[   97.003480] RIP: 0010:__send_signal+0x256/0x580
[   97.003812] Code: f0 ff ff 49 89 c7 48 85 c0 0f 85 a8 fe ff ff 41 bf 04 00 00 00 44 8d 45 ff e9 f8 fe ff ff 45 31 ff 40
[   97.003812] RSP: 0018:ffffc900000c8e88 EFLAGS: 00000046
[   97.003812] RAX: 0000000000000000 RBX: ffff88800875df00 RCX: 0000000000000001
[   97.003812] RDX: ffff88800875df00 RSI: 0000000000000001 RDI: 000000000000000e
[   97.003812] RBP: 000000000000000e R08: 0000000000000001 R09: 0000000000000000
[   97.003812] R10: 0000000000000000 R11: ffffc900000c8ff8 R12: 0000000000000000
[   97.003812] R13: 0000000000000001 R14: 0000000000000001 R15: ffffffff8115f3f3
[   97.003812] FS:  0000000000000000(0000) GS:ffff8880bcb00000(0000) knlGS:0000000000000000
[   97.003812] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   97.003812] CR2: 00007ffff739c830 CR3: 00000000094d6000 CR4: 00000000000006e0
[   97.003812] Call Trace:
[   97.003812]  <IRQ>
[   97.003812]  group_send_sig_info+0xe1/0x190
[   97.003812]  kill_pid_info+0x78/0x150
[   97.003812]  it_real_fn+0x35/0xe0
[   97.003812]  ? __x64_sys_setitimer+0x150/0x150
[   97.003812]  __hrtimer_run_queues+0x1ca/0x3f0
[   97.003812]  hrtimer_interrupt+0x106/0x220
[   97.003812]  __sysvec_apic_timer_interrupt+0x96/0x260
[   97.003812]  sysvec_apic_timer_interrupt+0x9d/0xd0
[   97.003812]  </IRQ>
[   97.003812]  <TASK>
[   97.003812]  asm_sysvec_apic_timer_interrupt+0x12/0x20
[   97.003812] RIP: 0010:default_idle+0x10/0x20
[   97.003812] Code: 8b 04 25 00 b0 01 00 f0 80 60 02 df c3 0f ae f0 0f ae 38 0f ae f0 eb b9 66 90 0f 1f 44 00 00 eb 07 03
[   97.003812] RSP: 0018:ffffc90000093ef8 EFLAGS: 00000246
[   97.003812] RAX: ffffffff823fafb0 RBX: ffff8880056f97c0 RCX: 0000000000000000
[   97.003812] RDX: ffffffff81193e1d RSI: ffffffff82da5301 RDI: ffffffff823fb111
[   97.003812] RBP: 0000000000000001 R08: 0000000000000000 R09: 0000000000000000
[   97.003812] R10: 0000000000000000 R11: 0000000000000001 R12: 0000000000000000
[   97.003812] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
[   97.003812]  ? mwait_idle+0x70/0x70
[   97.003812]  ? nohz_balance_enter_idle+0x6d/0x220
[   97.003812]  ? default_idle_call+0x21/0x90
[   97.003812]  ? mwait_idle+0x70/0x70
[   97.003812]  default_idle_call+0x59/0x90
[   97.003812]  do_idle+0x1f3/0x260
[   97.003812]  ? finish_task_switch.isra.0+0xef/0x350
[   97.003812]  cpu_startup_entry+0x19/0x20
[   97.003812]  secondary_startup_64_no_verify+0xc3/0xcb
[   97.003812]  </TASK>
[   97.003812] Modules linked in:
[   97.003812] ---[ end trace 0000000000000000 ]---
[   97.003812] RIP: 0010:__send_signal+0x256/0x580
[   97.003812] Code: f0 ff ff 49 89 c7 48 85 c0 0f 85 a8 fe ff ff 41 bf 04 00 00 00 44 8d 45 ff e9 f8 fe ff ff 45 31 ff 40
[   97.003812] RSP: 0018:ffffc900000c8e88 EFLAGS: 00000046
[   97.003812] RAX: 0000000000000000 RBX: ffff88800875df00 RCX: 0000000000000001
[   97.003812] RDX: ffff88800875df00 RSI: 0000000000000001 RDI: 000000000000000e
[   97.003812] RBP: 000000000000000e R08: 0000000000000001 R09: 0000000000000000
[   97.003812] R10: 0000000000000000 R11: ffffc900000c8ff8 R12: 0000000000000000
[   97.003812] R13: 0000000000000001 R14: 0000000000000001 R15: ffffffff8115f3f3
[   97.003812] FS:  0000000000000000(0000) GS:ffff8880bcb00000(0000) knlGS:0000000000000000
[   97.003812] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   97.003812] CR2: 00007ffff739c830 CR3: 00000000094d6000 CR4: 00000000000006e0
[   97.003812] Kernel panic - not syncing: Fatal exception in interrupt
[   97.003812] Kernel Offset: disabled
[   97.003812] ---[ end Kernel panic - not syncing: Fatal exception in interrupt ]---
---
 include/linux/sched/jobctl.h |  2 ++
 kernel/ptrace.c              | 32 ++++++++++++++------------------
 kernel/signal.c              |  5 +++++
 3 files changed, 21 insertions(+), 18 deletions(-)

diff --git a/include/linux/sched/jobctl.h b/include/linux/sched/jobctl.h
index fa067de9f1a9..4e154ad8205f 100644
--- a/include/linux/sched/jobctl.h
+++ b/include/linux/sched/jobctl.h
@@ -19,6 +19,7 @@ struct task_struct;
 #define JOBCTL_TRAPPING_BIT	21	/* switching to TRACED */
 #define JOBCTL_LISTENING_BIT	22	/* ptracer is listening for events */
 #define JOBCTL_TRAP_FREEZE_BIT	23	/* trap for cgroup freezer */
+#define JOBCTL_DELAY_WAKEKILL_BIT	24	/* delay killable wakeups */
 
 #define JOBCTL_STOP_DEQUEUED	(1UL << JOBCTL_STOP_DEQUEUED_BIT)
 #define JOBCTL_STOP_PENDING	(1UL << JOBCTL_STOP_PENDING_BIT)
@@ -28,6 +29,7 @@ struct task_struct;
 #define JOBCTL_TRAPPING		(1UL << JOBCTL_TRAPPING_BIT)
 #define JOBCTL_LISTENING	(1UL << JOBCTL_LISTENING_BIT)
 #define JOBCTL_TRAP_FREEZE	(1UL << JOBCTL_TRAP_FREEZE_BIT)
+#define JOBCTL_DELAY_WAKEKILL	(1UL << JOBCTL_DELAY_WAKEKILL_BIT)
 
 #define JOBCTL_TRAP_MASK	(JOBCTL_TRAP_STOP | JOBCTL_TRAP_NOTIFY)
 #define JOBCTL_PENDING_MASK	(JOBCTL_STOP_PENDING | JOBCTL_TRAP_MASK)
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 4f78d0d5940c..a55320a0e8e3 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -197,7 +197,8 @@ static bool ptrace_freeze_traced(struct task_struct *task)
 	spin_lock_irq(&task->sighand->siglock);
 	if (task_is_traced(task) && !looks_like_a_spurious_pid(task) &&
 	    !__fatal_signal_pending(task)) {
-		WRITE_ONCE(task->__state, __TASK_TRACED);
+		//WARN_ON((task->jobctl & JOBCTL_DELAY_WAKEKILL));
+		task->jobctl |= JOBCTL_DELAY_WAKEKILL;
 		ret = true;
 	}
 	spin_unlock_irq(&task->sighand->siglock);
@@ -207,7 +208,7 @@ static bool ptrace_freeze_traced(struct task_struct *task)
 
 static void ptrace_unfreeze_traced(struct task_struct *task)
 {
-	if (READ_ONCE(task->__state) != __TASK_TRACED)
+	if (!task_is_traced(task))
 		return;
 
 	WARN_ON(!task->ptrace || task->parent != current);
@@ -216,13 +217,11 @@ static void ptrace_unfreeze_traced(struct task_struct *task)
 	 * PTRACE_LISTEN can allow ptrace_trap_notify to wake us up remotely.
 	 * Recheck state under the lock to close this race.
 	 */
-	spin_lock_irq(&task->sighand->siglock);
-	if (READ_ONCE(task->__state) == __TASK_TRACED) {
-		if (__fatal_signal_pending(task))
-			wake_up_state(task, __TASK_TRACED);
-		else
-			WRITE_ONCE(task->__state, TASK_TRACED);
-	}
+	spin_unlock_irq(&task->sighand->siglock);
+	WARN_ON(!(task->jobctl & JOBCTL_DELAY_WAKEKILL));
+	task->jobctl &= ~JOBCTL_DELAY_WAKEKILL;
+	if (fatal_signal_pending(task))
+		wake_up_state(task, TASK_WAKEKILL);
 	spin_unlock_irq(&task->sighand->siglock);
 }
 
@@ -256,7 +255,7 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state)
 	 */
 	read_lock(&tasklist_lock);
 	if (child->ptrace && child->parent == current) {
-		WARN_ON(READ_ONCE(child->__state) == __TASK_TRACED);
+		WARN_ON(child->jobctl & JOBCTL_DELAY_WAKEKILL);
 		/*
 		 * child->sighand can't be NULL, release_task()
 		 * does ptrace_unlink() before __exit_signal().
@@ -267,13 +266,13 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state)
 	read_unlock(&tasklist_lock);
 
 	if (!ret && !ignore_state) {
-		if (!wait_task_inactive(child, __TASK_TRACED)) {
+		if (!wait_task_inactive(child, TASK_TRACED)) {
 			/*
 			 * This can only happen if may_ptrace_stop() fails and
 			 * ptrace_stop() changes ->state back to TASK_RUNNING,
-			 * so we should not worry about leaking __TASK_TRACED.
+			 * so we should not worry about leaking JOBCTL_DELAY_WAKEKILL.
 			 */
-			WARN_ON(READ_ONCE(child->__state) == __TASK_TRACED);
+			WARN_ON(!(child->jobctl & JOBCTL_DELAY_WAKEKILL));
 			ret = -ESRCH;
 		}
 	}
@@ -892,7 +891,6 @@ static int ptrace_resume(struct task_struct *child, long request,
 	 * status and clears the code too; this can't race with the tracee, it
 	 * takes siglock after resume.
 	 */
-	need_siglock = data && !thread_group_empty(current);
 	if (need_siglock)
 		spin_lock_irq(&child->sighand->siglock);
 	child->exit_code = data;
@@ -1325,8 +1323,7 @@ SYSCALL_DEFINE4(ptrace, long, request, long, pid, unsigned long, addr,
 		goto out_put_task_struct;
 
 	ret = arch_ptrace(child, request, addr, data);
-	if (ret || request != PTRACE_DETACH)
-		ptrace_unfreeze_traced(child);
+	ptrace_unfreeze_traced(child);
 
  out_put_task_struct:
 	put_task_struct(child);
@@ -1468,8 +1465,7 @@ COMPAT_SYSCALL_DEFINE4(ptrace, compat_long_t, request, compat_long_t, pid,
 				  request == PTRACE_INTERRUPT);
 	if (!ret) {
 		ret = compat_arch_ptrace(child, request, addr, data);
-		if (ret || request != PTRACE_DETACH)
-			ptrace_unfreeze_traced(child);
+		ptrace_unfreeze_traced(child);
 	}
 
  out_put_task_struct:
diff --git a/kernel/signal.c b/kernel/signal.c
index 30cd1ca43bcd..a1277580c92e 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -762,6 +762,10 @@ static int dequeue_synchronous_signal(kernel_siginfo_t *info)
  */
 void signal_wake_up_state(struct task_struct *t, unsigned int state)
 {
+	/* Suppress wakekill? */
+	if (t->jobctl & JOBCTL_DELAY_WAKEKILL)
+		state &= ~TASK_WAKEKILL;
+
 	set_tsk_thread_flag(t, TIF_SIGPENDING);
 	/*
 	 * TASK_WAKEKILL also means wake it up in the stopped/traced/killable
@@ -2328,6 +2332,7 @@ static int ptrace_stop(int exit_code, int why, int clear_code,
 
 	/* LISTENING can be set only during STOP traps, clear it */
 	current->jobctl &= ~JOBCTL_LISTENING;
+	current->jobctl &= ~JOBCTL_DELAY_WAKEKILL;
 
 	/*
 	 * Queued signals ignored us while we were stopped for tracing.
-- 
2.35.3