[PATCH 10/X] uprobes: introduce uprobe_deny_signal()

[Oleg Nesterov <oleg@redhat.com>]

A not-UTASK_RUNNING task obviously can't handle the signals, neither it
should stop/freeze/etc. It must not even exit if it was SIGKILL'ed (see
the next changes).

This patch adds the new hook, uprobe_deny_signal(), called by
get_signal_to_deliver(). It simply clears TIF_SIGPENDING to ensure that
this thread can do nothing connected to signals until it becomes
UTASK_RUNNING.

We also change post_xol() path to do recalc_sigpending() before return
to user-mode, this ensures the signal can't be lost.

NOTE! Without the next changes this patch is buggy.
---
 include/linux/uprobes.h |    5 +++++
 kernel/signal.c         |    3 +++
 kernel/uprobes.c        |   23 +++++++++++++++++++++++
 3 files changed, 31 insertions(+), 0 deletions(-)

diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
index 1591c7c..27928e5 100644
--- a/include/linux/uprobes.h
+++ b/include/linux/uprobes.h
@@ -134,6 +134,7 @@ extern unsigned long __weak get_uprobe_bkpt_addr(struct pt_regs *regs);
 extern int uprobe_post_notifier(struct pt_regs *regs);
 extern int uprobe_bkpt_notifier(struct pt_regs *regs);
 extern void uprobe_notify_resume(struct pt_regs *regs);
+extern bool uprobe_deny_signal(void);
 #else /* CONFIG_UPROBES is not defined */
 static inline int register_uprobe(struct inode *inode, loff_t offset,
 				struct uprobe_consumer *consumer)
@@ -154,6 +155,10 @@ static inline void munmap_uprobe(struct vm_area_struct *vma)
 static inline void uprobe_notify_resume(struct pt_regs *regs)
 {
 }
+static inline bool uprobe_deny_signal(void)
+{
+	return false;
+}
 static inline unsigned long get_uprobe_bkpt_addr(struct pt_regs *regs)
 {
 	return 0;
diff --git a/kernel/signal.c b/kernel/signal.c
index 291c970..788b494 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2141,6 +2141,9 @@ int get_signal_to_deliver(siginfo_t *info, struct k_sigaction *return_ka,
 	struct signal_struct *signal = current->signal;
 	int signr;
 
+	if (unlikely(uprobe_deny_signal()))
+		return 0;
+
 relock:
 	/*
 	 * We'll jump back here after any time we were stopped in TASK_STOPPED.
diff --git a/kernel/uprobes.c b/kernel/uprobes.c
index 5fd72b8..d6f4508 100644
--- a/kernel/uprobes.c
+++ b/kernel/uprobes.c
@@ -1320,6 +1320,25 @@ static int pre_ssout(struct uprobe *uprobe, struct pt_regs *regs,
 	return -EFAULT;
 }
 
+bool uprobe_deny_signal(void)
+{
+	struct task_struct *tsk = current;
+	struct uprobe_task *utask = tsk->utask;
+
+	if (likely(!utask || !utask->active_uprobe))
+		return false;
+
+	WARN_ON_ONCE(utask->state != UTASK_SSTEP);
+
+	if (signal_pending(tsk)) {
+		spin_lock_irq(&tsk->sighand->siglock);
+		clear_tsk_thread_flag(tsk, TIF_SIGPENDING);
+		spin_unlock_irq(&tsk->sighand->siglock);
+	}
+
+	return true;
+}
+
 /*
  * uprobe_notify_resume gets called in task context just before returning
  * to userspace.
@@ -1378,6 +1397,10 @@ void uprobe_notify_resume(struct pt_regs *regs)
 		utask->state = UTASK_RUNNING;
 		user_disable_single_step(current);
 		xol_free_insn_slot(current);
+
+		spin_lock_irq(&current->sighand->siglock);
+		recalc_sigpending(); /* see uprobe_deny_signal() */
+		spin_unlock_irq(&current->sighand->siglock);
 	}
 	return;
 
-- 
1.5.5.1