[PATCH 2/4] kernel: add support for TIF_NOTIFY_SIGNAL

Jens Axboe
2020-10-26T20:32:50Z

This adds TIF_NOTIFY_SIGNAL handling in the generic code, which if set,
will return true if signal_pending() is used in a wait loop. That causes
an exit of the loop so that notify_signal tracehooks can be run. If the
wait loop is currently inside a system call, the system call is restarted
once task_work has been processed.

In preparation for only having arch_do_signal() handle syscall restarts
if _TIF_SIGPENDING isn't set, rename it to arch_do_signal_or_restart().
Pass in a boolean that tells the arch signal handler if it should attempt
to get a signal, or just process a potential syscall restart.

For !CONFIG_GENERIC_ENTRY archs, we add the TIF_NOTIFY_SIGNAL handling
to get_signal(). This is done to minimize the needed arch changes to
support this feature.

Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
arch/x86/kernel/signal.c | 4 ++--
include/linux/entry-common.h | 11 ++++++++---
include/linux/entry-kvm.h | 4 ++--
include/linux/sched/signal.h | 11 ++++++++++-
include/linux/tracehook.h | 27 +++++++++++++++++++++++++++
kernel/entry/common.c | 14 +++++++++++---
kernel/entry/kvm.c | 3 +++
kernel/signal.c | 14 ++++++++++++++
8 files changed, 77 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index be0d7d4152ec..ea794a083c44 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -804,11 +804,11 @@ static inline unsigned long get_nr_restart_syscall(const struct pt_regs *regs)
* want to handle. Thus you cannot kill init even with a SIGKILL even by
* mistake.
*/
-void arch_do_signal(struct pt_regs *regs)
+void arch_do_signal_or_restart(struct pt_regs *regs, bool has_signal)
{
struct ksignal ksig;

- if (get_signal(&ksig)) {
+ if (has_signal && get_signal(&ksig)) {
/* Whee! Actually deliver the signal. */
handle_signal(&ksig, regs);
return;
diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
index 474f29638d2c..b9711e813ec2 100644
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -37,6 +37,10 @@
# define _TIF_UPROBE (0)
#endif

+#ifndef _TIF_NOTIFY_SIGNAL
+# define _TIF_NOTIFY_SIGNAL (0)
+#endif
+
/*
* TIF flags handled in syscall_enter_from_user_mode()
*/
@@ -69,7 +73,7 @@

#define EXIT_TO_USER_MODE_WORK \
(_TIF_SIGPENDING | _TIF_NOTIFY_RESUME | _TIF_UPROBE | \
- _TIF_NEED_RESCHED | _TIF_PATCH_PENDING | \
+ _TIF_NEED_RESCHED | _TIF_PATCH_PENDING | _TIF_NOTIFY_SIGNAL | \
ARCH_EXIT_TO_USER_MODE_WORK)

/**
@@ -259,12 +263,13 @@ static __always_inline void arch_exit_to_user_mode(void) { }
#endif

/**
- * arch_do_signal - Architecture specific signal delivery function
+ * arch_do_signal_or_restart - Architecture specific signal delivery function
* @regs: Pointer to currents pt_regs
+ * @has_signal: actual signal to handle
*
* Invoked from exit_to_user_mode_loop().
*/
-void arch_do_signal(struct pt_regs *regs);
+void arch_do_signal_or_restart(struct pt_regs *regs, bool has_signal);

/**
* arch_syscall_exit_tracehook - Wrapper around tracehook_report_syscall_exit()
diff --git a/include/linux/entry-kvm.h b/include/linux/entry-kvm.h
index 0cef17afb41a..9b93f8584ff7 100644
--- a/include/linux/entry-kvm.h
+++ b/include/linux/entry-kvm.h
@@ -11,8 +11,8 @@
# define ARCH_XFER_TO_GUEST_MODE_WORK (0)
#endif

-#define XFER_TO_GUEST_MODE_WORK \
- (_TIF_NEED_RESCHED | _TIF_SIGPENDING | \
+#define XFER_TO_GUEST_MODE_WORK \
+ (_TIF_NEED_RESCHED | _TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL | \
_TIF_NOTIFY_RESUME | ARCH_XFER_TO_GUEST_MODE_WORK)

struct kvm_vcpu;
diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 404145dc536e..bd5afa076189 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -360,6 +360,15 @@ static inline int task_sigpending(struct task_struct *p)

static inline int signal_pending(struct task_struct *p)
{
+#if defined(TIF_NOTIFY_SIGNAL)
+ /*
+ * TIF_NOTIFY_SIGNAL isn't really a signal, but it requires the same
+ * behavior in terms of ensuring that we break out of wait loops
+ * so that notify signal callbacks can be processed.
+ */
+ if (unlikely(test_tsk_thread_flag(p, TIF_NOTIFY_SIGNAL)))
+ return 1;
+#endif
return task_sigpending(p);
}

@@ -507,7 +516,7 @@ extern int set_user_sigmask(const sigset_t __user *umask, size_t sigsetsize);
static inline void restore_saved_sigmask_unless(bool interrupted)
{
if (interrupted)
- WARN_ON(!test_thread_flag(TIF_SIGPENDING));
+ WARN_ON(!signal_pending(current));
else
restore_saved_sigmask();
}
diff --git a/include/linux/tracehook.h b/include/linux/tracehook.h
index b480e1a07ed8..f7d82e4fafd6 100644
--- a/include/linux/tracehook.h
+++ b/include/linux/tracehook.h
@@ -198,4 +198,31 @@ static inline void tracehook_notify_resume(struct pt_regs *regs)
blkcg_maybe_throttle_current();
}

+/*
+ * called by exit_to_user_mode_loop() if ti_work & _TIF_NOTIFY_SIGNAL. This
+ * is currently used by TWA_SIGNAL based task_work, which requires breaking
+ * wait loops to ensure that task_work is noticed and run.
+ */
+static inline void tracehook_notify_signal(void)
+{
+#if defined(TIF_NOTIFY_SIGNAL)
+ clear_thread_flag(TIF_NOTIFY_SIGNAL);
+ smp_mb__after_atomic();
+ if (current->task_works)
+ task_work_run();
+#endif
+}
+
+/*
+ * Called when we have work to process from exit_to_user_mode_loop()
+ */
+static inline void set_notify_signal(struct task_struct *task)
+{
+#if defined(TIF_NOTIFY_SIGNAL)
+ if (!test_and_set_tsk_thread_flag(task, TIF_NOTIFY_SIGNAL) &&
+ !wake_up_state(task, TASK_INTERRUPTIBLE))
+ kick_process(task);
+#endif
+}
+
#endif /* <linux/tracehook.h> */
diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 2b8366693d5c..54c07d386aeb 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -135,7 +135,15 @@ static __always_inline void exit_to_user_mode(void)
}

/* Workaround to allow gradual conversion of architecture code */
-void __weak arch_do_signal(struct pt_regs *regs) { }
+void __weak arch_do_signal_or_restart(struct pt_regs *regs, bool has_signal) { }
+
+static void handle_signal_work(struct pt_regs *regs, unsigned long ti_work)
+{
+ if (ti_work & _TIF_NOTIFY_SIGNAL)
+ tracehook_notify_signal();
+
+ arch_do_signal_or_restart(regs, ti_work & _TIF_SIGPENDING);
+}

static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
unsigned long ti_work)
@@ -157,8 +165,8 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
if (ti_work & _TIF_PATCH_PENDING)
klp_update_patch_state(current);

- if (ti_work & _TIF_SIGPENDING)
- arch_do_signal(regs);
+ if (ti_work & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL))
+ handle_signal_work(regs, ti_work);

if (ti_work & _TIF_NOTIFY_RESUME) {
tracehook_notify_resume(regs);
diff --git a/kernel/entry/kvm.c b/kernel/entry/kvm.c
index b6678a5e3cf6..49972ee99aff 100644
--- a/kernel/entry/kvm.c
+++ b/kernel/entry/kvm.c
@@ -8,6 +8,9 @@ static int xfer_to_guest_mode_work(struct kvm_vcpu *vcpu, unsigned long ti_work)
do {
int ret;

+ if (ti_work & _TIF_NOTIFY_SIGNAL)
+ tracehook_notify_signal();
+
if (ti_work & _TIF_SIGPENDING) {
kvm_handle_signal_exit(vcpu);
return -EINTR;
diff --git a/kernel/signal.c b/kernel/signal.c
index 9f86246a8637..f67ea9a08ac0 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2529,6 +2529,20 @@ bool get_signal(struct ksignal *ksig)
struct signal_struct *signal = current->signal;
int signr;

+ /*
+ * For non-generic architectures, check for TIF_NOTIFY_SIGNAL so
+ * that the arch handlers don't all have to do it. If we get here
+ * without TIF_SIGPENDING, just exit after running signal work.
+ */
+#ifdef TIF_NOTIFY_SIGNAL
+ if (!IS_ENABLED(CONFIG_GENERIC_ENTRY)) {
+ if (test_thread_flag(TIF_NOTIFY_SIGNAL))
+ tracehook_notify_signal();
+ if (!task_sigpending(current))
+ return false;
+ }
+#endif
+
if (unlikely(uprobe_deny_signal()))
return false;

--
2.29.0