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Message-ID: <299a3c124f4e0ece36f2a04db2d4d2143e66ebca.1764272407.git.chris@chrisdown.name>
Date: Fri, 28 Nov 2025 03:44:01 +0800
From: Chris Down <chris@...isdown.name>
To: Petr Mladek <pmladek@...e.com>
Cc: linux-kernel@...r.kernel.org,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Sergey Senozhatsky <senozhatsky@...omium.org>,
Steven Rostedt <rostedt@...dmis.org>,
John Ogness <john.ogness@...utronix.de>,
Geert Uytterhoeven <geert@...ux-m68k.org>,
Tony Lindgren <tony.lindgren@...ux.intel.com>, kernel-team@...com
Subject: [PATCH v8 13/21] printk: Toggle ignore_per_console_loglevel via
syslog
The syslog() system call provides SYSLOG_ACTION_CONSOLE_OFF and
SYSLOG_ACTION_CONSOLE_ON to disable and re-enable console logging.
These actions save and restore the global console_loglevel.
With per-console loglevels, simply setting console_loglevel to
minimum_console_loglevel during CONSOLE_OFF is insufficient - consoles
with explicit per-console loglevels would continue printing based on
their local settings.
Augment these actions to also save and restore the state of
ignore_per_console_loglevel. When console logging is disabled via
syslog(), ignore_per_console_loglevel is set to true so that the
minimum_console_loglevel applies to all consoles. When logging is
re-enabled, the previous ignore_per_console_loglevel state is restored.
A forced flag tracks whether this code path actually set
ignore_per_console_loglevel. This prevents blindly restoring stale
state if other contexts (sysrq, module parameter writes) legitimately
modified the flag in parallel.
SYSLOG_ACTION_CONSOLE_LEVEL now emits a one-time warning when
per-console loglevels are active, informing administrators that their
level change only affects consoles using the global loglevel. The
level change also properly restores ignore_per_console_loglevel state
if it was forced by a previous CONSOLE_OFF.
These changes allow userspace tools like dmesg to properly control
console output even in the presence of per-console loglevels.
Reviewed-by: Petr Mladek <pmladek@...e.com>
Signed-off-by: Chris Down <chris@...isdown.name>
---
kernel/printk/printk.c | 53 +++++++++++++++++++++++++++++++++++-------
1 file changed, 45 insertions(+), 8 deletions(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index e5b6a926135e..1d28887e7218 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1869,6 +1869,8 @@ int do_syslog(int type, char __user *buf, int len, int source)
struct printk_info info;
bool clear = false;
static int saved_console_loglevel = LOGLEVEL_DEFAULT;
+ static bool saved_ignore_per_console_loglevel;
+ static bool saved_ignore_per_console_loglevel_forced;
int error;
error = check_syslog_permissions(type, source);
@@ -1910,9 +1912,23 @@ int do_syslog(int type, char __user *buf, int len, int source)
/* Disable logging to console */
case SYSLOG_ACTION_CONSOLE_OFF:
mutex_lock(&syslog_lock);
- if (saved_console_loglevel == LOGLEVEL_DEFAULT)
+ if (saved_console_loglevel == LOGLEVEL_DEFAULT) {
saved_console_loglevel = console_loglevel;
+ saved_ignore_per_console_loglevel = ignore_per_console_loglevel;
+ saved_ignore_per_console_loglevel_forced = false;
+ }
console_loglevel = minimum_console_loglevel;
+ if (!ignore_per_console_loglevel) {
+ /*
+ * Only remember the saved value if this path actually
+ * forced the override. Other contexts (sysrq, module
+ * parameter writes, etc.) can legitimately toggle the
+ * flag in parallel, so blindly restoring it later would
+ * resurrect stale state.
+ */
+ ignore_per_console_loglevel = true;
+ saved_ignore_per_console_loglevel_forced = true;
+ }
mutex_unlock(&syslog_lock);
break;
/* Enable logging to console */
@@ -1920,20 +1936,41 @@ int do_syslog(int type, char __user *buf, int len, int source)
mutex_lock(&syslog_lock);
if (saved_console_loglevel != LOGLEVEL_DEFAULT) {
console_loglevel = saved_console_loglevel;
+ if (saved_ignore_per_console_loglevel_forced)
+ ignore_per_console_loglevel = saved_ignore_per_console_loglevel;
saved_console_loglevel = LOGLEVEL_DEFAULT;
+ saved_ignore_per_console_loglevel_forced = false;
}
mutex_unlock(&syslog_lock);
break;
/* Set level of messages printed to console */
- case SYSLOG_ACTION_CONSOLE_LEVEL:
- if (len < 1 || len > 8)
+ case SYSLOG_ACTION_CONSOLE_LEVEL: {
+ int new_level = len;
+
+ if (!ignore_per_console_loglevel)
+ pr_warn_once(
+ "SYSLOG_ACTION_CONSOLE_LEVEL is ignored by consoles with an explicitly set per-console loglevel, see Documentation/admin-guide/per-console-loglevel.rst\n");
+ if (new_level < 1 || new_level > 8)
return -EINVAL;
- if (len < minimum_console_loglevel)
- len = minimum_console_loglevel;
- console_loglevel = len;
- /* Implicitly re-enable logging to console */
- saved_console_loglevel = LOGLEVEL_DEFAULT;
+ if (new_level < minimum_console_loglevel)
+ new_level = minimum_console_loglevel;
+
+ mutex_lock(&syslog_lock);
+ console_loglevel = new_level;
+ /*
+ * If SYSLOG_ACTION_CONSOLE_OFF forced ignore_per_console_loglevel,
+ * restore the previous setting so per-console loglevels continue
+ * to work after administrators re-enable logging via syslog().
+ */
+ if (saved_console_loglevel != LOGLEVEL_DEFAULT) {
+ if (saved_ignore_per_console_loglevel_forced)
+ ignore_per_console_loglevel = saved_ignore_per_console_loglevel;
+ saved_console_loglevel = LOGLEVEL_DEFAULT;
+ saved_ignore_per_console_loglevel_forced = false;
+ }
+ mutex_unlock(&syslog_lock);
break;
+ }
/* Number of chars in the log buffer */
case SYSLOG_ACTION_SIZE_UNREAD:
mutex_lock(&syslog_lock);
--
2.51.2
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