rseq: Provide static branch for runtime debugging

Config based debug is rarely turned on and is not available easily when
things go wrong.

Provide a static branch to allow permanent integration of debug mechanisms
along with the usual toggles in Kconfig, command line and debugfs.

Requested-by: Peter Zijlstra <peterz@infradead.org>
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/20251027084307.089270547@linutronix.de

1 files changed, 69 insertions, 4 deletions

diff --git a/kernel/rseq.c b/kernel/rseq.c
index c0dbe2ed26a8..679ab8ebdfd3 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -95,6 +95,27 @@
 				  RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL | \
 				  RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE)
 
+DEFINE_STATIC_KEY_MAYBE(CONFIG_RSEQ_DEBUG_DEFAULT_ENABLE, rseq_debug_enabled);
+
+static inline void rseq_control_debug(bool on)
+{
+	if (on)
+		static_branch_enable(&rseq_debug_enabled);
+	else
+		static_branch_disable(&rseq_debug_enabled);
+}
+
+static int __init rseq_setup_debug(char *str)
+{
+	bool on;
+
+	if (kstrtobool(str, &on))
+		return -EINVAL;
+	rseq_control_debug(on);
+	return 1;
+}
+__setup("rseq_debug=", rseq_setup_debug);
+
 #ifdef CONFIG_TRACEPOINTS
 /*
  * Out of line, so the actual update functions can be in a header to be
@@ -112,10 +133,11 @@ void __rseq_trace_ip_fixup(unsigned long ip, unsigned long start_ip,
 }
 #endif /* CONFIG_TRACEPOINTS */
 
+#ifdef CONFIG_DEBUG_FS
 #ifdef CONFIG_RSEQ_STATS
 DEFINE_PER_CPU(struct rseq_stats, rseq_stats);
 
-static int rseq_debug_show(struct seq_file *m, void *p)
+static int rseq_stats_show(struct seq_file *m, void *p)
 {
 	struct rseq_stats stats = { };
 	unsigned int cpu;
@@ -140,14 +162,56 @@ static int rseq_debug_show(struct seq_file *m, void *p)
 	return 0;
 }
 
+static int rseq_stats_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, rseq_stats_show, inode->i_private);
+}
+
+static const struct file_operations stat_ops = {
+	.open		= rseq_stats_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= single_release,
+};
+
+static int __init rseq_stats_init(struct dentry *root_dir)
+{
+	debugfs_create_file("stats", 0444, root_dir, NULL, &stat_ops);
+	return 0;
+}
+#else
+static inline void rseq_stats_init(struct dentry *root_dir) { }
+#endif /* CONFIG_RSEQ_STATS */
+
+static int rseq_debug_show(struct seq_file *m, void *p)
+{
+	bool on = static_branch_unlikely(&rseq_debug_enabled);
+
+	seq_printf(m, "%d\n", on);
+	return 0;
+}
+
+static ssize_t rseq_debug_write(struct file *file, const char __user *ubuf,
+			    size_t count, loff_t *ppos)
+{
+	bool on;
+
+	if (kstrtobool_from_user(ubuf, count, &on))
+		return -EINVAL;
+
+	rseq_control_debug(on);
+	return count;
+}
+
 static int rseq_debug_open(struct inode *inode, struct file *file)
 {
 	return single_open(file, rseq_debug_show, inode->i_private);
 }
 
-static const struct file_operations dfs_ops = {
+static const struct file_operations debug_ops = {
 	.open		= rseq_debug_open,
 	.read		= seq_read,
+	.write		= rseq_debug_write,
 	.llseek		= seq_lseek,
 	.release	= single_release,
 };
@@ -156,11 +220,12 @@ static int __init rseq_debugfs_init(void)
 {
 	struct dentry *root_dir = debugfs_create_dir("rseq", NULL);
 
-	debugfs_create_file("stats", 0444, root_dir, NULL, &dfs_ops);
+	debugfs_create_file("debug", 0644, root_dir, NULL, &debug_ops);
+	rseq_stats_init(root_dir);
 	return 0;
 }
 __initcall(rseq_debugfs_init);
-#endif /* CONFIG_RSEQ_STATS */
+#endif /* CONFIG_DEBUG_FS */
 
 #ifdef CONFIG_DEBUG_RSEQ
 static struct rseq *rseq_kernel_fields(struct task_struct *t)