When terminal is unresponsive, one cannot use dmesg to view printk
ring buffer messages. Also, syslog services may be disabled,
to check them after a reboot, especially on embedded systems.
In this scenario, dump the printk ring buffer messages via sysrq
by pressing sysrq+D.

Signed-off-by: Sreenath Vijayan <sreenath.vijayan@sony.com>
Signed-off-by: Shimoyashiki Taichi <taichi.shimoyashiki@sony.com>
---
V2: - modified kernel ring buffer to printk ring buffer
    - allocated buf dynamically to prevent stack frame size warnings
    - used buf of size 2048 to match PRINTK_MESSAGE_MAX and added comment

 Documentation/admin-guide/sysrq.rst |  2 ++
 drivers/tty/sysrq.c                 | 49 ++++++++++++++++++++++++++++-
 2 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/sysrq.rst b/Documentation/admin-guide/sysrq.rst
index 2f2e5bd440f9..c634e8b4cea2 100644
--- a/Documentation/admin-guide/sysrq.rst
+++ b/Documentation/admin-guide/sysrq.rst
@@ -161,6 +161,8 @@ Command	    Function
             will be printed to your console. (``0``, for example would make
             it so that only emergency messages like PANICs or OOPSes would
             make it to your console.)
+
+``D``	    Dump the printk ring buffer
 =========== ===================================================================
 
 Okay, so what can I use them for?
diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index 02217e3c916b..62b3911f03b5 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -51,6 +51,8 @@
 #include <linux/syscalls.h>
 #include <linux/of.h>
 #include <linux/rcupdate.h>
+#include <linux/kmsg_dump.h>
+#include <linux/console.h>
 
 #include <asm/ptrace.h>
 #include <asm/irq_regs.h>
@@ -450,6 +452,51 @@ static const struct sysrq_key_op sysrq_unrt_op = {
 	.enable_mask	= SYSRQ_ENABLE_RTNICE,
 };
 
+static void dmesg_dump_callback(struct work_struct *work)
+{
+	struct kmsg_dump_iter iter;
+	size_t len;
+	char *buf;
+	struct console *con;
+	int cookie;
+
+	/* Size to be updated if PRINTK_MESSAGE_MAX changes */
+	buf = kzalloc(2048, GFP_KERNEL);
+	if (!buf)
+		return;
+
+	kmsg_dump_rewind(&iter);
+	while (kmsg_dump_get_line(&iter, 1, buf, 2048, &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);
+}
+
+static DECLARE_WORK(sysrq_dmesg_work, dmesg_dump_callback);
+
+static void sysrq_handle_dmesg_dump(u8 key)
+{
+	queue_work(system_unbound_wq, &sysrq_dmesg_work);
+}
+static struct sysrq_key_op sysrq_dmesg_dump_op = {
+	.handler        = sysrq_handle_dmesg_dump,
+	.help_msg       = "dump-dmesg(D)",
+	.action_msg     = "Dump dmesg",
+	.enable_mask    = SYSRQ_ENABLE_DUMP,
+};
+
 /* Key Operations table and lock */
 static DEFINE_SPINLOCK(sysrq_key_table_lock);
 
@@ -505,7 +552,7 @@ static const struct sysrq_key_op *sysrq_key_table[62] = {
 	NULL,				/* A */
 	NULL,				/* B */
 	NULL,				/* C */
-	NULL,				/* D */
+	&sysrq_dmesg_dump_op,		/* D */
 	NULL,				/* E */
 	NULL,				/* F */
 	NULL,				/* G */
-- 
2.43.0

On 2024-01-10, Sreenath Vijayan <sreenath.vijayan@sony.com> wrote:
> When terminal is unresponsive, one cannot use dmesg to view printk
> ring buffer messages. Also, syslog services may be disabled,
> to check them after a reboot, especially on embedded systems.
> In this scenario, dump the printk ring buffer messages via sysrq
> by pressing sysrq+D.

Generally speaking, I like this idea. I also like that you do not
re-flood the kernel buffers and instead just print directly to the
consoles. (I wish sysrq+z with ftrace did that as well.)

Relying on a workqueue will really limit the usefulness of this
feature. Safe efforts could be made to print directly from the interrupt
context. But maybe this approach can be accepted for now as being
"better than nothing", which can be improved upon in the future (for
example, when atomic consoles are available).

Some more comments from me below...

> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
> index 02217e3c916b..62b3911f03b5 100644
> --- a/drivers/tty/sysrq.c
> +++ b/drivers/tty/sysrq.c
> @@ -450,6 +452,51 @@ static const struct sysrq_key_op sysrq_unrt_op = {
>  	.enable_mask	= SYSRQ_ENABLE_RTNICE,
>  };
>  
> +static void dmesg_dump_callback(struct work_struct *work)
> +{
> +	struct kmsg_dump_iter iter;
> +	size_t len;
> +	char *buf;
> +	struct console *con;
> +	int cookie;
> +
> +	/* Size to be updated if PRINTK_MESSAGE_MAX changes */
> +	buf = kzalloc(2048, GFP_KERNEL);
> +	if (!buf)
> +		return;
> +
> +	kmsg_dump_rewind(&iter);
> +	while (kmsg_dump_get_line(&iter, 1, buf, 2048, &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);
> +}

Rather than implementing all this in drivers/tty/sysrq.c it would
probably be better to just call a new function that is implemented in
kernel/printk/printk.c. Then you would have access to printk-private
items (such as the PRINTK_MESSAGE_MAX macro).

For example, sysrq+z just calls ftrace_dump(), which is implemented in
kernel/trace/trace.c.

John Ogness