[PATCH v4 1/2] PM: wakeup: Add kfuncs to traverse over wakeup_sources

[Samuel Wu <wusamuel@google.com>]

Iterating through wakeup sources via sysfs or debugfs can be inefficient
or restricted. Introduce BPF kfuncs to allow high-performance and safe
in-kernel traversal of the wakeup_sources list. There is at least a 30x
speedup for walking 150 wakeup sources and all their attributes.

The new kfuncs include:
- bpf_wakeup_sources_get_head() to obtain the list head.
- bpf_wakeup_sources_read_lock/unlock() to manage the SRCU lock.

For verifier safety, the underlying SRCU index is wrapped in an opaque
'struct bpf_ws_lock' pointer. This enables the use of KF_ACQUIRE and
KF_RELEASE flags, allowing the BPF verifier to strictly enforce paired
lock/unlock cycles and prevent resource leaks.

Signed-off-by: Samuel Wu <wusamuel@google.com>
---
 drivers/base/power/power.h  |  7 ++++
 drivers/base/power/wakeup.c | 71 +++++++++++++++++++++++++++++++++++--
 2 files changed, 76 insertions(+), 2 deletions(-)

diff --git a/drivers/base/power/power.h b/drivers/base/power/power.h
index 922ed457db19..8823aceeac8b 100644
--- a/drivers/base/power/power.h
+++ b/drivers/base/power/power.h
@@ -168,3 +168,10 @@ static inline void device_pm_init(struct device *dev)
 	device_pm_sleep_init(dev);
 	pm_runtime_init(dev);
 }
+
+#ifdef CONFIG_BPF_SYSCALL
+struct bpf_ws_lock { };
+struct bpf_ws_lock *bpf_wakeup_sources_read_lock(void);
+void bpf_wakeup_sources_read_unlock(struct bpf_ws_lock *lock);
+void *bpf_wakeup_sources_get_head(void);
+#endif
diff --git a/drivers/base/power/wakeup.c b/drivers/base/power/wakeup.c
index b8e48a023bf0..80b497de2deb 100644
--- a/drivers/base/power/wakeup.c
+++ b/drivers/base/power/wakeup.c
@@ -1168,11 +1168,78 @@ static const struct file_operations wakeup_sources_stats_fops = {
 	.release = seq_release_private,
 };
 
-static int __init wakeup_sources_debugfs_init(void)
+#ifdef CONFIG_BPF_SYSCALL
+#include <linux/btf.h>
+
+__bpf_kfunc_start_defs();
+
+/**
+ * bpf_wakeup_sources_read_lock - Acquire the SRCU lock for wakeup sources
+ *
+ * The underlying SRCU lock returns an integer index. However, the BPF verifier
+ * requires a pointer (PTR_TO_BTF_ID) to strictly track the state of acquired
+ * resources using KF_ACQUIRE and KF_RELEASE semantics. We use an opaque
+ * structure pointer (struct bpf_ws_lock *) to satisfy the verifier while
+ * safely encoding the integer index within the pointer address itself.
+ *
+ * Return: An opaque pointer encoding the SRCU lock index + 1 (to avoid NULL).
+ */
+__bpf_kfunc struct bpf_ws_lock *bpf_wakeup_sources_read_lock(void)
+{
+	return (struct bpf_ws_lock *)(long)(wakeup_sources_read_lock() + 1);
+}
+
+/**
+ * bpf_wakeup_sources_read_unlock - Release the SRCU lock for wakeup sources
+ * @lock: The opaque pointer returned by bpf_wakeup_sources_read_lock()
+ *
+ * The BPF verifier guarantees that @lock is a valid, unreleased pointer from
+ * the acquire function. We decode the pointer back into the integer SRCU index
+ * by subtracting 1 and release the lock.
+ */
+__bpf_kfunc void bpf_wakeup_sources_read_unlock(struct bpf_ws_lock *lock)
+{
+	wakeup_sources_read_unlock((int)(long)lock - 1);
+}
+
+/**
+ * bpf_wakeup_sources_get_head - Get the head of the wakeup sources list
+ *
+ * Return: The head of the wakeup sources list.
+ */
+__bpf_kfunc void *bpf_wakeup_sources_get_head(void)
+{
+	return &wakeup_sources;
+}
+
+__bpf_kfunc_end_defs();
+
+BTF_KFUNCS_START(wakeup_source_kfunc_ids)
+BTF_ID_FLAGS(func, bpf_wakeup_sources_read_lock, KF_ACQUIRE)
+BTF_ID_FLAGS(func, bpf_wakeup_sources_read_unlock, KF_RELEASE)
+BTF_ID_FLAGS(func, bpf_wakeup_sources_get_head)
+BTF_KFUNCS_END(wakeup_source_kfunc_ids)
+
+static const struct btf_kfunc_id_set wakeup_source_kfunc_set = {
+	.set   = &wakeup_source_kfunc_ids,
+};
+
+static void __init wakeup_sources_bpf_init(void)
+{
+	if (register_btf_kfunc_id_set(BPF_PROG_TYPE_SYSCALL, &wakeup_source_kfunc_set))
+		pm_pr_dbg("Wakeup: failed to register BTF kfuncs\n");
+}
+#else
+static inline void wakeup_sources_bpf_init(void) {}
+#endif /* CONFIG_BPF_SYSCALL */
+
+static int __init wakeup_sources_init(void)
 {
 	debugfs_create_file("wakeup_sources", 0444, NULL, NULL,
 			    &wakeup_sources_stats_fops);
+	wakeup_sources_bpf_init();
+
 	return 0;
 }
 
-postcore_initcall(wakeup_sources_debugfs_init);
+postcore_initcall(wakeup_sources_init);
-- 
2.54.0.563.g4f69b47b94-goog