rseq: Introduce struct rseq_data

In preparation for a major rewrite of this code, provide a data structure
for rseq management.

Put all the rseq related data into it (except for the debug part), which
allows to simplify fork/execve by using memset() and memcpy() instead of
adding new fields to initialize over and over.

Create a storage struct for event management as well and put the
sched_switch event and a indicator for RSEQ on a task into it as a
start. That uses a union, which allows to mask and clear the whole lot
efficiently.

The indicators are explicitly not a bit field. Bit fields generate abysmal
code.

The boolean members are defined as u8 as that actually guarantees that it
fits. There seem to be strange architecture ABIs which need more than 8
bits for a boolean.

The has_rseq member is redundant vs. task::rseq, but it turns out that
boolean operations and quick checks on the union generate better code than
fiddling with separate entities and data types.

This struct will be extended over time to carry more information.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251027084306.527086690@linutronix.de

1 files changed, 22 insertions, 26 deletions

diff --git a/include/linux/rseq.h b/include/linux/rseq.h
index c6267f70c746..ab91b1e6bb4a 100644
--- a/include/linux/rseq.h
+++ b/include/linux/rseq.h
@@ -9,22 +9,22 @@ void __rseq_handle_notify_resume(struct ksignal *sig, struct pt_regs *regs);
 
 static inline void rseq_handle_notify_resume(struct pt_regs *regs)
 {
-	if (current->rseq)
+	if (current->rseq.event.has_rseq)
 		__rseq_handle_notify_resume(NULL, regs);
 }
 
 static inline void rseq_signal_deliver(struct ksignal *ksig, struct pt_regs *regs)
 {
-	if (current->rseq) {
-		current->rseq_event_pending = true;
+	if (current->rseq.event.has_rseq) {
+		current->rseq.event.sched_switch = true;
 		__rseq_handle_notify_resume(ksig, regs);
 	}
 }
 
 static inline void rseq_sched_switch_event(struct task_struct *t)
 {
-	if (t->rseq) {
-		t->rseq_event_pending = true;
+	if (t->rseq.event.has_rseq) {
+		t->rseq.event.sched_switch = true;
 		set_tsk_thread_flag(t, TIF_NOTIFY_RESUME);
 	}
 }
@@ -32,8 +32,9 @@ static inline void rseq_sched_switch_event(struct task_struct *t)
 static __always_inline void rseq_exit_to_user_mode(void)
 {
 	if (IS_ENABLED(CONFIG_DEBUG_RSEQ)) {
-		if (WARN_ON_ONCE(current->rseq && current->rseq_event_pending))
-			current->rseq_event_pending = false;
+		if (WARN_ON_ONCE(current->rseq.event.has_rseq &&
+				 current->rseq.event.events))
+			current->rseq.event.events = 0;
 	}
 }
 
@@ -49,35 +50,30 @@ static __always_inline void rseq_exit_to_user_mode(void)
  */
 static inline void rseq_virt_userspace_exit(void)
 {
-	if (current->rseq_event_pending)
+	if (current->rseq.event.sched_switch)
 		set_tsk_thread_flag(current, TIF_NOTIFY_RESUME);
 }
 
+static inline void rseq_reset(struct task_struct *t)
+{
+	memset(&t->rseq, 0, sizeof(t->rseq));
+}
+
+static inline void rseq_execve(struct task_struct *t)
+{
+	rseq_reset(t);
+}
+
 /*
  * If parent process has a registered restartable sequences area, the
  * child inherits. Unregister rseq for a clone with CLONE_VM set.
  */
 static inline void rseq_fork(struct task_struct *t, u64 clone_flags)
 {
-	if (clone_flags & CLONE_VM) {
-		t->rseq = NULL;
-		t->rseq_len = 0;
-		t->rseq_sig = 0;
-		t->rseq_event_pending = false;
-	} else {
+	if (clone_flags & CLONE_VM)
+		rseq_reset(t);
+	else
 		t->rseq = current->rseq;
-		t->rseq_len = current->rseq_len;
-		t->rseq_sig = current->rseq_sig;
-		t->rseq_event_pending = current->rseq_event_pending;
-	}
-}
-
-static inline void rseq_execve(struct task_struct *t)
-{
-	t->rseq = NULL;
-	t->rseq_len = 0;
-	t->rseq_sig = 0;
-	t->rseq_event_pending = false;
 }
 
 #else /* CONFIG_RSEQ */