[PATCH 18/19] ptrace: add JOBCTL_BLOCK_NOTIFY

[Tejun Heo <tj@kernel.org>]

For to-be-added notification retraps, other tasks need to be able to
tell whether ptrace request is currently in progress while tracee is
in STOP trap.  This patch adds JOBCTL_BLOCK_NOTIFY which is set on
ptrace_check_attach() if the request requires tracee to be trapped and
it's trapped for STOP, and cleared when ptrace syscall finishes.

This flag isn't used yet.

-v2: ptrace_put_task_struct() reorganized per Oleg's comment.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 include/linux/ptrace.h |    2 +
 include/linux/sched.h  |    1 +
 kernel/ptrace.c        |   54 ++++++++++++++++++++++++++++++++++++++++++-----
 3 files changed, 51 insertions(+), 6 deletions(-)

diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index 1e84960..2e39f81 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -73,6 +73,8 @@
 #define PTRACE_EVENT_EXIT	6
 #define PTRACE_EVENT_STOP	7
 
+#define PTRACE_STOP_SI_CODE	(__SI_TRAP | SIGTRAP | PTRACE_EVENT_STOP << 8)
+
 /* flags in siginfo.si_pt_flags from PTRACE_GETSIGINFO */
 #define PTRACE_SI_STOPPED	0x00000001 /* tracee is job control stopped */
 
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 9a0e1bc..9298f97 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1801,6 +1801,7 @@ extern void thread_group_times(struct task_struct *p, cputime_t *ut, cputime_t *
 #define JOBCTL_STOP_CONSUME	(1 << 18) /* consume group stop count */
 #define JOBCTL_TRAP_STOP	(1 << 19) /* trap for STOP */
 #define JOBCTL_TRAPPING		(1 << 21) /* switching to TRACED */
+#define JOBCTL_BLOCK_NOTIFY	(1 << 22) /* block NOTIFY re-traps */
 
 #define JOBCTL_TRAP_MASK	JOBCTL_TRAP_STOP
 #define JOBCTL_PENDING_MASK	(JOBCTL_STOP_PENDING | JOBCTL_TRAP_MASK)
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index a9b3c67..1982d7a 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -128,11 +128,13 @@ void __ptrace_unlink(struct task_struct *child)
 	spin_lock(&child->sighand->siglock);
 
 	/*
-	 * Clear all pending traps and TRAPPING.  TRAPPING should be
-	 * cleared regardless of JOBCTL_STOP_PENDING.  Do it explicitly.
+	 * Clear all pending traps, TRAPPING and BLOCK_NOTIFY.  TRAPPING
+	 * should be cleared regardless of JOBCTL_STOP_PENDING.  Do it
+	 * explicitly.
 	 */
 	task_clear_jobctl_pending(child, JOBCTL_TRAP_MASK);
 	task_clear_jobctl_trapping(child);
+	child->jobctl &= ~JOBCTL_BLOCK_NOTIFY;
 
 	/*
 	 * Reinstate JOBCTL_STOP_PENDING if group stop is in effect and
@@ -191,10 +193,24 @@ int ptrace_check_attach(struct task_struct *child, bool ignore_state)
 		 */
 		spin_lock_irq(&child->sighand->siglock);
 		WARN_ON_ONCE(task_is_stopped(child));
-		if (task_is_traced(child) || ignore_state)
+
+		if (ignore_state) {
+			ret = 0;
+		} else if (task_is_traced(child)) {
+			siginfo_t *si = child->last_siginfo;
+
+			/*
+			 * If STOP trapped, ptrace notification may cause
+			 * re-traps, which we don't want while ptrace
+			 * request is in progress.  Block notification.
+			 */
+			if (si && si->si_code == PTRACE_STOP_SI_CODE)
+				child->jobctl |= JOBCTL_BLOCK_NOTIFY;
 			ret = 0;
-		else if (ptrace_wait_trapping(child))
+		} else if (ptrace_wait_trapping(child)) {
 			return restart_syscall();
+		}
+
 		spin_unlock_irq(&child->sighand->siglock);
 	}
 	read_unlock(&tasklist_lock);
@@ -887,6 +903,32 @@ static struct task_struct *ptrace_get_task_struct(pid_t pid)
 #define arch_ptrace_attach(child)	do { } while (0)
 #endif
 
+/**
+ * ptrace_put_task_struct - ptrace request processing done, put child
+ * @child: child task struct to put
+ *
+ * ptrace request processing for @child is finished.  Clean up and put
+ * @child.  This function clears %JOBCTL_BLOCK_NOTIFY which can be set by
+ * ptrace_check_attach().  @child might not be being ptraced by %current.
+ */
+static void ptrace_put_task_struct(struct task_struct *child)
+{
+	unsigned long flags;
+
+	if (!(child->jobctl & JOBCTL_BLOCK_NOTIFY))
+		goto out_put;
+
+	if (unlikely(!(child->ptrace & PT_PTRACED) || child->parent != current))
+		goto out_put;
+
+	if (likely(lock_task_sighand(child, &flags))) {
+		child->jobctl &= ~JOBCTL_BLOCK_NOTIFY;
+		unlock_task_sighand(child, &flags);
+	}
+out_put:
+	put_task_struct(child);
+}
+
 SYSCALL_DEFINE4(ptrace, long, request, long, pid, unsigned long, addr,
 		unsigned long, data)
 {
@@ -925,7 +967,7 @@ SYSCALL_DEFINE4(ptrace, long, request, long, pid, unsigned long, addr,
 	ret = arch_ptrace(child, request, addr, data);
 
  out_put_task_struct:
-	put_task_struct(child);
+	ptrace_put_task_struct(child);
  out:
 	return ret;
 }
@@ -1066,7 +1108,7 @@ asmlinkage long compat_sys_ptrace(compat_long_t request, compat_long_t pid,
 		ret = compat_arch_ptrace(child, request, addr, data);
 
  out_put_task_struct:
-	put_task_struct(child);
+	ptrace_put_task_struct(child);
  out:
 	return ret;
 }
-- 
1.7.1