[RFC PATCH v2 09/13] entry: Carry ordinary-hardirq provenance through kernel return

[Karl Mehltretter <kmehltretter@gmail.com>]

The entry state can cross hardirq teardown and a possible reschedule before
the architecture completes its return tail. At that point preempt_count()
no longer identifies whether the original entry was an ordinary hardirq, a
synchronous exception, or an NMI-like event.

Initialize the hardirq state in every entry path so irqentry_state_t
distinguishes ordinary hardirqs from other entries across kernel return
while keeping the two-byte generic state layout. A later return tail can
restore hardirq classification only for entries which established the
pre-counted hardirq lifecycle.

Assisted-by: LLM
---
 include/linux/irq-entry-common.h | 6 ++++--
 kernel/entry/common.c            | 6 +++++-
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/include/linux/irq-entry-common.h b/include/linux/irq-entry-common.h
index e1fd9155410c1..6f649f6613312 100644
--- a/include/linux/irq-entry-common.h
+++ b/include/linux/irq-entry-common.h
@@ -341,8 +341,8 @@ enum irqentry_hardirq_state {
  *            exit path has to invoke ct_irq_exit().
  * @lockdep: Used exclusively in the irqentry_nmi_*() calls; ensures that
  *           lockdep state is restored correctly on exit from nmi.
- * @hardirq: Encodes whether the current staged hardirq-count attempt was
- *           applied.
+ * @hardirq: Encodes ordinary-hardirq provenance and whether the current
+ *           staged hardirq-count attempt was applied.
  *
  * This opaque object is filled in by the irqentry_*_enter() functions and
  * must be passed back into the corresponding irqentry_*_exit() functions
@@ -412,6 +412,8 @@ __irqentry_enter_from_kernel_mode(struct pt_regs *regs, bool hardirq)
 {
 	irqentry_state_t ret = {
 		.exit_rcu = false,
+		.hardirq = hardirq ? IRQENTRY_HARDIRQ_APPLIED :
+				      IRQENTRY_HARDIRQ_NONE,
 	};
 
 	/*
diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 9d207c37c5707..c7814c51bb35d 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -112,6 +112,7 @@ noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs)
 	if (user_mode(regs)) {
 		irqentry_state_t ret = {
 			.exit_rcu = false,
+			.hardirq = IRQENTRY_HARDIRQ_NONE,
 		};
 
 		irqentry_enter_from_user_mode(regs);
@@ -144,6 +145,7 @@ noinstr irqentry_state_t irqentry_hardirq_enter(struct pt_regs *regs)
 	if (user_mode(regs)) {
 		ret = (irqentry_state_t) {
 			.exit_rcu = false,
+			.hardirq = IRQENTRY_HARDIRQ_APPLIED,
 		};
 		irqentry_hardirq_count_state_set(&ret, cookie);
 
@@ -193,7 +195,9 @@ noinstr void irqentry_exit(struct pt_regs *regs, irqentry_state_t state)
 
 irqentry_state_t noinstr irqentry_nmi_enter(struct pt_regs *regs)
 {
-	irqentry_state_t irq_state;
+	irqentry_state_t irq_state = {
+		.hardirq = IRQENTRY_HARDIRQ_NONE,
+	};
 
 	irq_state.lockdep = lockdep_hardirqs_enabled_raw();
 
-- 
2.39.5 (Apple Git-154)