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

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
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