[PATCH v1 1/2] PM: wakeup: Add kfuncs to lock/unlock wakeup_sources

[Samuel Wu <wusamuel@google.com>]

Add kfuncs to lock/unlock for safe traversal of wakeup sources.

Currently, a traversal of wakeup sources require going through
/sys/class/wakeup/* or /d/wakeup_sources/*. The repeated syscalls to
query sysfs is inefficient, as there can be hundreds of wakeup_sources,
with each wakeup source also having multiple attributes. debugfs is
unstable and insecure.

Adding kfuncs to lock/unlock wakeup sources allows BPF program to safely
traverse the wakeup sources list. A new structure, bpf_ws_lock, acts as
an opaque wrapper for the SRCU index. The head address of wakeup_sources
can be safely resolved through BPF helper functions or variable
attributes.

Doing the traversal in BPF is significantly more performant, and has an
output in a format that the user specifies; this solves all the
drawbacks of current interfaces.

Signed-off-by: Samuel Wu <wusamuel@google.com>
---
 drivers/base/power/wakeup.c | 63 +++++++++++++++++++++++++++++++++++--
 1 file changed, 61 insertions(+), 2 deletions(-)

diff --git a/drivers/base/power/wakeup.c b/drivers/base/power/wakeup.c
index b8e48a023bf0..7fc12ce125bc 100644
--- a/drivers/base/power/wakeup.c
+++ b/drivers/base/power/wakeup.c
@@ -1168,11 +1168,70 @@ 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>
+
+struct bpf_ws_lock { };
+
+__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_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_KFUNCS_END(wakeup_source_kfunc_ids)
+
+static const struct btf_kfunc_id_set wakeup_source_kfunc_set = {
+	.owner = THIS_MODULE,
+	.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);
-- 
2.53.0.959.g497ff81fa9-goog