[PATCH v3 1/2] printk: Add function to dump printk buffer directly to consoles

[Sreenath Vijayan]

It is useful to be able to dump the printk buffer directly to
consoles in some situations so as to not flood the buffer.
This needs access to private items of printk like PRINTK_MESSAGE_MAX.
Add function in printk.c to accomplish this.

Suggested-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
Signed-off-by: Sreenath Vijayan <sreenath.vijayan@xxxxxxxx>
Signed-off-by: Shimoyashiki Taichi <taichi.shimoyashiki@xxxxxxxx>
---
 include/linux/printk.h |  4 ++++
 kernel/printk/printk.c | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 37 insertions(+)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 8ef499ab3c1e..0896745f31e2 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -192,6 +192,7 @@ void show_regs_print_info(const char *log_lvl);
 extern asmlinkage void dump_stack_lvl(const char *log_lvl) __cold;
 extern asmlinkage void dump_stack(void) __cold;
 void printk_trigger_flush(void);
+void dump_printk_buffer(void);
 #else
 static inline __printf(1, 0)
 int vprintk(const char *s, va_list args)
@@ -271,6 +272,9 @@ static inline void dump_stack(void)
 static inline void printk_trigger_flush(void)
 {
 }
+static inline void dump_printk_buffer(void)
+{
+}
 #endif
 
 #ifdef CONFIG_SMP
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f2444b581e16..5b11fb377f8f 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -4259,6 +4259,39 @@ void kmsg_dump_rewind(struct kmsg_dump_iter *iter)
 }
 EXPORT_SYMBOL_GPL(kmsg_dump_rewind);
 
+/**
+ * Dump the printk ring buffer directly to consoles
+ */
+void dump_printk_buffer(void)
+{
+	struct kmsg_dump_iter iter;
+	struct console *con;
+	char *buf;
+	size_t len;
+	int cookie;
+
+	buf = kmalloc(PRINTK_MESSAGE_MAX, GFP_KERNEL);
+	if (!buf)
+		return;
+
+	kmsg_dump_rewind(&iter);
+	while (kmsg_dump_get_line(&iter, 1, buf, PRINTK_MESSAGE_MAX, &len)) {
+		/*
+		 * Since using printk() or pr_*() will append the message to the
+		 * printk ring buffer, they cannot be used to display the retrieved
+		 * message. Hence console_write() of serial drivers is used.
+		 */
+		console_lock();
+		cookie = console_srcu_read_lock();
+		for_each_console_srcu(con) {
+			if ((console_srcu_read_flags(con) & CON_ENABLED) && con->write)
+				con->write(con, buf, len);
+		}
+		console_srcu_read_unlock(cookie);
+		console_unlock();
+	}
+	kfree(buf);
+}
 #endif
 
 #ifdef CONFIG_SMP
-- 
2.43.0