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* [PATCH] srcutiny: Add reader flavor checking for atomic SRCU
@ 2026-09-13 13:43 Bradley Morgan
  2026-09-13 20:37 ` Paul E. McKenney
  0 siblings, 1 reply; 2+ messages in thread
From: Bradley Morgan @ 2026-09-13 13:43 UTC (permalink / raw)
  To: Paul E. McKenney
  Cc: jiangshanlai, josh, work, rostedt, mathieu.desnoyers, rcu,
	linux-kernel, brads

Tiny SRCU stubs out srcu_check_read_flavor() as a nop.  So if you
DEFINE_SRCU_ATOMIC() a struct and then call srcu_read_lock() on it
by mistake, nothing warns you.  Tree SRCU has
__srcu_check_read_flavor() which splats when the flavor changes.

Add a srcu_reader_flavor field to Tiny's srcu_struct, gated on
CONFIG_PROVE_RCU.  DEFINE_SRCU_ATOMIC() bakes in the flavor at
build time via __SRCU_STRUCT_INIT_ATOMIC(), and the runtime init
paths set it too.

The check itself is simple: warn if the flavor changes after first
use.  Tiny is UP so there is no race on first write, no cmpxchg
needed.

This catches the case where a normal reader sneaks into an atomic
domain.  synchronize_srcu_atomic() would then return early while
that reader is still in its section, which is a UAF waiting to
happen.

Signed-off-by: Bradley Morgan <brads@mainlining.org>
---
 include/linux/srcu.h     |  4 ++--
 include/linux/srcutiny.h | 39 +++++++++++++++++++++++++++++++++++----
 kernel/rcu/srcutiny.c    | 36 ++++++++++++++++++++++++++++++++++++
 3 files changed, 73 insertions(+), 6 deletions(-)

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 1a8a465a5650..649a44d1cd00 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -36,9 +36,9 @@ static inline int __init_srcu_struct(struct srcu_struct *ssp, const char *name,
 int __init_srcu_struct_fast(struct srcu_struct *ssp, const char *name, struct lock_class_key *key);
 int __init_srcu_struct_fast_updown(struct srcu_struct *ssp, const char *name,
 				   struct lock_class_key *key);
+#endif // #ifndef CONFIG_TINY_SRCU
 int __init_srcu_struct_atomic(struct srcu_struct *ssp, const char *name,
 			      struct lock_class_key *key);
-#endif // #ifndef CONFIG_TINY_SRCU
 
 #define init_srcu_struct_fast(ssp) \
 ({ \
@@ -73,8 +73,8 @@ static inline int __init_srcu_struct(struct srcu_struct *ssp, const char *name,
 #ifndef CONFIG_TINY_SRCU
 int init_srcu_struct_fast(struct srcu_struct *ssp);
 int init_srcu_struct_fast_updown(struct srcu_struct *ssp);
-int init_srcu_struct_atomic(struct srcu_struct *ssp);
 #endif // #ifndef CONFIG_TINY_SRCU
+int init_srcu_struct_atomic(struct srcu_struct *ssp);
 
 #define __SRCU_DEP_MAP_INIT(srcu_name)
 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
index 47a368f945e3..6b14511d09f2 100644
--- a/include/linux/srcutiny.h
+++ b/include/linux/srcutiny.h
@@ -20,6 +20,9 @@ struct srcu_struct {
 	u8 srcu_gp_running;		/* GP workqueue running? */
 	u8 srcu_gp_waiting;		/* GP waiting for readers? */
 	u8 srcu_atomic_gp_flag;		/* Serialize atomic GP work.*/
+#ifdef CONFIG_PROVE_RCU
+	u8 srcu_reader_flavor;		/* Reader flavor for srcu_struct? */
+#endif /* #ifdef CONFIG_PROVE_RCU */
 	unsigned long srcu_idx;		/* Current reader array element in bit 0x2. */
 	unsigned long srcu_idx_max;	/* Furthest future srcu_idx request. */
 	struct swait_queue_head srcu_wq;
@@ -39,6 +42,12 @@ void srcu_drive_gp(struct work_struct *wp);
 void srcu_tiny_irq_work(struct irq_work *irq_work);
 void srcu_defer_drain(struct irq_work *irq_work);
 
+#ifdef CONFIG_PROVE_RCU
+#define __SRCU_READER_FLAVOR_INIT	.srcu_reader_flavor = SRCU_READ_FLAVOR_ATOMIC,
+#else
+#define __SRCU_READER_FLAVOR_INIT
+#endif
+
 #define __SRCU_STRUCT_INIT(name, __ignored, ___ignored, ____ignored)	\
 {									\
 	.srcu_wq = __SWAIT_QUEUE_HEAD_INITIALIZER(name.srcu_wq),	\
@@ -51,6 +60,19 @@ void srcu_defer_drain(struct irq_work *irq_work);
 	__SRCU_DEP_MAP_INIT(name)					\
 }
 
+#define __SRCU_STRUCT_INIT_ATOMIC(name)					\
+{									\
+	.srcu_wq = __SWAIT_QUEUE_HEAD_INITIALIZER(name.srcu_wq),	\
+	.srcu_cb_tail = &name.srcu_cb_head,				\
+	.srcu_work = __WORK_INITIALIZER(name.srcu_work, srcu_drive_gp),	\
+	.srcu_irq_work = { .func = srcu_tiny_irq_work },		\
+	.defer_cbs = LLIST_HEAD_INIT(name.defer_cbs),			\
+	.defer_iw = { .node = { .u_flags = IRQ_WORK_HARD_IRQ },		\
+		      .func = srcu_defer_drain },			\
+	__SRCU_READER_FLAVOR_INIT					\
+	__SRCU_DEP_MAP_INIT(name)					\
+}
+
 /*
  * This odd _STATIC_ arrangement is needed for API compatibility with
  * Tree SRCU, which needs some per-CPU data.
@@ -65,20 +87,19 @@ void srcu_defer_drain(struct irq_work *irq_work);
 #define DEFINE_SRCU_FAST_UPDOWN(name) DEFINE_SRCU(name)
 #define DEFINE_STATIC_SRCU_FAST_UPDOWN(name) \
 	static struct srcu_struct name = __SRCU_STRUCT_INIT(name, name, name, name)
-#define DEFINE_SRCU_ATOMIC(name) DEFINE_SRCU(name)
+#define DEFINE_SRCU_ATOMIC(name) \
+	struct srcu_struct name = __SRCU_STRUCT_INIT_ATOMIC(name)
 #define DEFINE_STATIC_SRCU_ATOMIC(name) \
-	static struct srcu_struct name = __SRCU_STRUCT_INIT(name, name, name, name)
+	static struct srcu_struct name = __SRCU_STRUCT_INIT_ATOMIC(name)
 
 // Dummy structure for srcu_notifier_head.
 struct srcu_usage { };
 #define __SRCU_USAGE_INIT(name) { }
 #define __init_srcu_struct_fast __init_srcu_struct
 #define __init_srcu_struct_fast_updown __init_srcu_struct
-#define __init_srcu_struct_atomic __init_srcu_struct
 #ifndef CONFIG_DEBUG_LOCK_ALLOC
 #define init_srcu_struct_fast init_srcu_struct
 #define init_srcu_struct_fast_updown init_srcu_struct
-#define init_srcu_struct_atomic init_srcu_struct
 #endif // #ifndef CONFIG_DEBUG_LOCK_ALLOC
 
 void synchronize_srcu(struct srcu_struct *ssp);
@@ -147,7 +168,17 @@ static inline void synchronize_srcu_expedited(struct srcu_struct *ssp)
 void srcu_barrier(struct srcu_struct *ssp);
 
 static inline void srcu_expedite_current(struct srcu_struct *ssp) { }
+
+#ifdef CONFIG_PROVE_RCU
+void __srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor);
+
+static inline void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor)
+{
+	__srcu_check_read_flavor(ssp, read_flavor);
+}
+#else /* #ifdef CONFIG_PROVE_RCU */
 #define srcu_check_read_flavor(ssp, read_flavor) do { } while (0)
+#endif /* #else #ifdef CONFIG_PROVE_RCU */
 
 /* Defined here to avoid size increase for non-torture kernels. */
 static inline void srcu_torture_stats_print(struct srcu_struct *ssp,
diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
index 76411562366f..76a276de07ee 100644
--- a/kernel/rcu/srcutiny.c
+++ b/kernel/rcu/srcutiny.c
@@ -52,6 +52,25 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp)
 	return 0;
 }
 
+#ifdef CONFIG_PROVE_RCU
+int __init_srcu_struct_atomic(struct srcu_struct *ssp, const char *name,
+			      struct lock_class_key *key)
+{
+	ssp->srcu_reader_flavor = SRCU_READ_FLAVOR_ATOMIC;
+	return __init_srcu_struct(ssp, name, key);
+}
+EXPORT_SYMBOL_GPL(__init_srcu_struct_atomic);
+
+#ifndef CONFIG_DEBUG_LOCK_ALLOC
+int init_srcu_struct_atomic(struct srcu_struct *ssp)
+{
+	ssp->srcu_reader_flavor = SRCU_READ_FLAVOR_ATOMIC;
+	return init_srcu_struct_generic(ssp);
+}
+EXPORT_SYMBOL_GPL(init_srcu_struct_atomic);
+#endif /* #ifndef CONFIG_DEBUG_LOCK_ALLOC */
+#endif /* #ifdef CONFIG_PROVE_RCU */
+
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 
 int init_srcu_struct_lockdep(struct srcu_struct *ssp, const char *name,
@@ -212,6 +231,23 @@ void srcu_tiny_irq_work(struct irq_work *irq_work)
 }
 EXPORT_SYMBOL_GPL(srcu_tiny_irq_work);
 
+#ifdef CONFIG_PROVE_RCU
+/*
+ * Check for consistent reader flavor.  Tiny SRCU is UP only, so no
+ * cmpxchg is needed to set the flavor on first use.
+ */
+void __srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor)
+{
+	int old_read_flavor = ssp->srcu_reader_flavor;
+
+	WARN_ON_ONCE(read_flavor & (read_flavor - 1));
+	WARN_ON_ONCE(old_read_flavor && read_flavor != old_read_flavor);
+	if (!old_read_flavor)
+		ssp->srcu_reader_flavor = read_flavor;
+}
+EXPORT_SYMBOL_GPL(__srcu_check_read_flavor);
+#endif /* #ifdef CONFIG_PROVE_RCU */
+
 static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
 {
 	unsigned long cookie;
-- 
2.47.3


^ permalink raw reply	[flat|nested] 2+ messages in thread
end of thread, other threads:[~2026-09-13 23:16 UTC | newest]

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2026-09-13 13:43 [PATCH] srcutiny: Add reader flavor checking for atomic SRCU Bradley Morgan
2026-09-13 20:37 ` Paul E. McKenney

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