[PATCH v6 09/11] printk: Add sysctl interface to set global loglevels

[Chris Down <chris@chrisdown.name>]

Introduce two new sysctl interfaces for configuring global loglevels:

- kernel.console_loglevel: Sets the global console loglevel, determining
  the minimum priority of messages printed to consoles. Messages with a
  loglevel lower than this value will be printed.
- kernel.default_message_loglevel: Sets the default loglevel for
  messages that do not specify an explicit loglevel.

The kernel.printk sysctl was previously used to set multiple loglevel
parameters simultaneously, but it was confusing and lacked proper
validation. By introducing these dedicated sysctl interfaces, we provide
a clearer and more granular way to configure the loglevels.

Signed-off-by: Chris Down <chris@chrisdown.name>
---
 Documentation/admin-guide/sysctl/kernel.rst | 17 ++++++++-
 kernel/printk/sysctl.c                      | 42 +++++++++++++++++++++
 2 files changed, 57 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index f8bc1630eba0..8019779b27f6 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -1044,6 +1044,20 @@ otherwise the 'doze' mode will be used.
 
 ==============================================================
 
+Some of these settings may be overridden per-console, see
+Documentation/admin-guide/per-console-loglevel.rst. See ``man 2 syslog`` for
+more information on the different loglevels.
+
+console_loglevel
+================
+
+Messages with a higher priority than this will be printed to consoles.
+
+default_message_loglevel
+========================
+
+Messages without an explicit priority will be printed with this priority.
+
 printk
 ======
 
@@ -1052,8 +1066,7 @@ The four values in printk denote: ``console_loglevel``,
 ``default_console_loglevel`` respectively.
 
 These values influence printk() behavior when printing or
-logging error messages. See '``man 2 syslog``' for more info on
-the different loglevels.
+logging error messages.
 
 ======================== =====================================
 console_loglevel         messages with a higher priority than
diff --git a/kernel/printk/sysctl.c b/kernel/printk/sysctl.c
index f5072dc85f7a..3bce8b89dacc 100644
--- a/kernel/printk/sysctl.c
+++ b/kernel/printk/sysctl.c
@@ -11,6 +11,9 @@
 
 static const int ten_thousand = 10000;
 
+static int min_msg_loglevel = LOGLEVEL_EMERG;
+static int max_msg_loglevel = LOGLEVEL_DEBUG;
+
 static int proc_dointvec_minmax_sysadmin(const struct ctl_table *table, int write,
 				void *buffer, size_t *lenp, loff_t *ppos)
 {
@@ -20,6 +23,29 @@ static int proc_dointvec_minmax_sysadmin(const struct ctl_table *table, int writ
 	return proc_dointvec_minmax(table, write, buffer, lenp, ppos);
 }
 
+static int printk_console_loglevel(const struct ctl_table *table, int write,
+				   void *buffer, size_t *lenp, loff_t *ppos)
+{
+	struct ctl_table ltable = *table;
+	int ret, level;
+
+	if (!write)
+		return proc_dointvec(table, write, buffer, lenp, ppos);
+
+	ltable.data = &level;
+
+	ret = proc_dointvec(&ltable, write, buffer, lenp, ppos);
+	if (ret)
+		return ret;
+
+	if (level != -1 && level != clamp_loglevel(level))
+		return -ERANGE;
+
+	console_loglevel = level;
+
+	return 0;
+}
+
 static struct ctl_table printk_sysctls[] = {
 	{
 		.procname	= "printk",
@@ -76,6 +102,22 @@ static struct ctl_table printk_sysctls[] = {
 		.extra1		= SYSCTL_ZERO,
 		.extra2		= SYSCTL_TWO,
 	},
+	{
+		.procname	= "console_loglevel",
+		.data		= &console_loglevel,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= printk_console_loglevel,
+	},
+	{
+		.procname	= "default_message_loglevel",
+		.data		= &default_message_loglevel,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= &min_msg_loglevel,
+		.extra2		= &max_msg_loglevel,
+	},
 };
 
 void __init printk_sysctl_init(void)
-- 
2.46.0