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

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 6c42061ca20e581b5192b66c6f25aba38d4f8ff8..e638274757927b3d5406b5f5adc6af41779e01ac 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6500,6 +6500,10 @@
                        Memory area to be used by remote processor image,
                        managed by CMA.
 
+       rseq_debug=     [KNL] Enable or disable restartable sequence
+                       debug mode. Defaults to CONFIG_RSEQ_DEBUG_DEFAULT_ENABLE.
+                       Format: <bool>
+
        rt_group_sched= [KNL] Enable or disable SCHED_RR/FIFO group scheduling
                        when CONFIG_RT_GROUP_SCHED=y. Defaults to
                        !CONFIG_RT_GROUP_SCHED_DEFAULT_DISABLED.

diff --git a/include/linux/rseq_entry.h b/include/linux/rseq_entry.h
index ff9080b89be39542c26b01f5e102601309760cb4..ed8e5f89499b392e9d089f0ec4d0d12c782278be 100644 (file)
--- a/include/linux/rseq_entry.h
+++ b/include/linux/rseq_entry.h
@@ -34,6 +34,7 @@ DECLARE_PER_CPU(struct rseq_stats, rseq_stats);
 #endif /* !CONFIG_RSEQ_STATS */
 
 #ifdef CONFIG_RSEQ
+#include <linux/jump_label.h>
 #include <linux/rseq.h>
 
 #include <linux/tracepoint-defs.h>
@@ -64,6 +65,8 @@ static inline void rseq_trace_ip_fixup(unsigned long ip, unsigned long start_ip,
                                       unsigned long offset, unsigned long abort_ip) { }
 #endif /* !CONFIG_TRACEPOINT */
 
+DECLARE_STATIC_KEY_MAYBE(CONFIG_RSEQ_DEBUG_DEFAULT_ENABLE, rseq_debug_enabled);
+
 static __always_inline void rseq_note_user_irq_entry(void)
 {
        if (IS_ENABLED(CONFIG_GENERIC_IRQ_ENTRY))

diff --git a/init/Kconfig b/init/Kconfig
index f39fdfb28797823c7192fe7cb9329f974c135984..bde40ab664e2450d0a60563dc7cec0ee1082acaa 100644 (file)
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1925,10 +1925,24 @@ config RSEQ_STATS
 
          If unsure, say N.
 
+config RSEQ_DEBUG_DEFAULT_ENABLE
+       default n
+       bool "Enable restartable sequences debug mode by default" if EXPERT
+       depends on RSEQ
+       help
+         This enables the static branch for debug mode of restartable
+         sequences.
+
+         This also can be controlled on the kernel command line via the
+         command line parameter "rseq_debug=0/1" and through debugfs.
+
+         If unsure, say N.
+
 config DEBUG_RSEQ
        default n
        bool "Enable debugging of rseq() system call" if EXPERT
        depends on RSEQ && DEBUG_KERNEL
+       select RSEQ_DEBUG_DEFAULT_ENABLE
        help
          Enable extra debugging checks for the rseq system call.
 

diff --git a/kernel/rseq.c b/kernel/rseq.c
index c0dbe2ed26a89dd4bce6126538a7552af2ed494e..679ab8ebdfd30bb19fb1f324cc6ae2f4c87fe6c5 100644 (file)
--- 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)