[PATCH] livepatch: test_printk() patch

[Petr Mladek]

!!!!IMPORTANT!!!

The patch is a bit ugly because cmdline_proc_show() can be called
also by some other code. So, you might get the crash earlier than
you expect.
---
 include/linux/printk.h               |  3 +++
 kernel/livepatch/core.c              |  1 +
 kernel/printk/printk.c               | 23 +++++++++++++++++++++
 samples/livepatch/livepatch-sample.c | 39 ++++++++++++++++++++++++++++++++++++
 4 files changed, 66 insertions(+)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 9ccbdf2c1453..ffe0ceb56972 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -125,6 +125,9 @@ void early_printk(const char *s, ...) { }
 typedef __printf(1, 0) int (*printk_func_t)(const char *fmt, va_list args);
 
 #ifdef CONFIG_PRINTK
+int vprintk_default(const char *fmt, va_list args);
+int test_printk(const char *fmt, ...);
+
 asmlinkage __printf(5, 0)
 int vprintk_emit(int facility, int level,
 		 const char *dict, size_t dictlen,
diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index e2dbf0127f0f..7d0a6029043c 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -615,6 +615,7 @@ static ssize_t enabled_show(struct kobject *kobj,
 	struct klp_patch *patch;
 
 	patch = container_of(kobj, struct klp_patch, kobj);
+	printk("patch enabled: %d\n", patch->state);
 	return snprintf(buf, PAGE_SIZE-1, "%d\n", patch->state);
 }
 
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index c963ba534a78..9f785bfbb3fd 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1920,6 +1920,29 @@ asmlinkage __visible int printk(const char *fmt, ...)
 }
 EXPORT_SYMBOL(printk);
 
+int test_printk(const char *fmt, ...)
+{
+	printk_func_t vprintk_func;
+	va_list args;
+	int r;
+
+	va_start(args, fmt);
+
+	/*
+	 * If a caller overrides the per_cpu printk_func, then it needs
+	 * to disable preemption when calling printk(). Otherwise
+	 * the printk_func should be set to the default. No need to
+	 * disable preemption here.
+	 */
+	vprintk_func = this_cpu_read(printk_func);
+	r = vprintk_func(fmt, args);
+
+	va_end(args);
+
+	return r;
+}
+EXPORT_SYMBOL(test_printk);
+
 #else /* CONFIG_PRINTK */
 
 #define LOG_LINE_MAX		0
diff --git a/samples/livepatch/livepatch-sample.c b/samples/livepatch/livepatch-sample.c
index fb8c8614e728..d5c09bc629e8 100644
--- a/samples/livepatch/livepatch-sample.c
+++ b/samples/livepatch/livepatch-sample.c
@@ -40,16 +40,53 @@
  */
 
 #include <linux/seq_file.h>
+#include <linux/printk.h>
 static int livepatch_cmdline_proc_show(struct seq_file *m, void *v)
 {
+	static int count;
+
+	if (!count++)
+		trace_printk("%s\n", "this has been live patched");
+	else
+		printk("%s\n", "this has been live patched");
+
 	seq_printf(m, "%s\n", "this has been live patched");
 	return 0;
 }
 
+asmlinkage static int livepatch_printk(const char *fmt, ...)
+{
+	va_list args, args2;
+	int r = 0;
+
+	va_start(args, fmt);
+
+	/*
+	 * If a caller overrides the per_cpu printk_func, then it needs
+	 * to disable preemption when calling printk(). Otherwise
+	 * the printk_func should be set to the default. No need to
+	 * disable preemption here.
+	 */
+	vprintk_default(fmt, args);
+
+	va_end(args);
+
+	va_start(args2, fmt);
+	ftrace_vprintk(fmt, args2);
+	va_end(args2);
+
+
+	return r;
+}
+
 static struct klp_func funcs[] = {
 	{
 		.old_name = "cmdline_proc_show",
 		.new_func = livepatch_cmdline_proc_show,
+	},
+	{
+		.old_name = "printk",
+		.new_func = livepatch_printk,
 	}, { }
 };
 
@@ -77,6 +114,8 @@ static int livepatch_init(void)
 		WARN_ON(klp_unregister_patch(&patch));
 		return ret;
 	}
+	/* Make sure that trace_printk buffers are allocated. */
+	trace_printk("LivePatch sample loaded\n");
 	return 0;
 }
 
-- 
1.8.5.6