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Subject[PATCH 2/3] printk: Add /sys/consoles/ interface
Date
This adds a new sysfs interface that contains a directory for each
console registered on the system. Each directory contains a single
"loglevel" file for reading and setting the per-console loglevel.

We can let kobject destruction race with console removal: if it does,
loglevel_{show,store}() will safely fail with -ENODEV. This is a little
weird, but avoids embedding the kobject and therefore needing to totally
refactor the way we handle console struct lifetime.

Cc: Petr Mladek <pmladek@suse.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Calvin Owens <calvinowens@fb.com>
---
(V1: https://lkml.org/lkml/2017/4/4/784)

Changes in V2:
* Honor minimum_console_loglevel when setting loglevels
* Added entry in Documentation/ABI/testing

Documentation/ABI/testing/sysfs-consoles | 13 +++++
include/linux/console.h | 1 +
kernel/printk/printk.c | 88 ++++++++++++++++++++++++++++++++
3 files changed, 102 insertions(+)
create mode 100644 Documentation/ABI/testing/sysfs-consoles

diff --git a/Documentation/ABI/testing/sysfs-consoles b/Documentation/ABI/testing/sysfs-consoles
new file mode 100644
index 0000000..6a1593e
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-consoles
@@ -0,0 +1,13 @@
+What: /sys/consoles/
+Date: September 2017
+KernelVersion: 4.15
+Contact: Calvin Owens <calvinowens@fb.com>
+Description: The /sys/consoles tree contains a directory for each console
+ configured on the system. These directories contain the
+ following attributes:
+
+ * "loglevel" Set the per-console loglevel: the kernel uses
+ max(system_loglevel, perconsole_loglevel) when
+ deciding whether to emit a given message. The
+ default is 0, which means max() always yields
+ the system setting in the kernel.printk sysctl.
diff --git a/include/linux/console.h b/include/linux/console.h
index a5b5d79..76840be 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -148,6 +148,7 @@ struct console {
void *data;
struct console *next;
int level;
+ struct kobject *kobj;
};

/*
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 3f1675e..488bda3 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -105,6 +105,8 @@ enum devkmsg_log_masks {

static unsigned int __read_mostly devkmsg_log = DEVKMSG_LOG_MASK_DEFAULT;

+static struct kobject *consoles_dir_kobj;
+
static int __control_devkmsg(char *str)
{
if (!str)
@@ -2371,6 +2373,82 @@ static int __init keep_bootcon_setup(char *str)

early_param("keep_bootcon", keep_bootcon_setup);

+static ssize_t loglevel_show(struct kobject *kobj, struct kobj_attribute *attr,
+ char *buf)
+{
+ struct console *con;
+ ssize_t ret = -ENODEV;
+
+ console_lock();
+ for_each_console(con) {
+ if (con->kobj == kobj) {
+ ret = sprintf(buf, "%d\n", con->level);
+ break;
+ }
+ }
+ console_unlock();
+
+ return ret;
+}
+
+static ssize_t loglevel_store(struct kobject *kobj, struct kobj_attribute *attr,
+ const char *buf, size_t count)
+{
+ struct console *con;
+ ssize_t ret;
+ int tmp;
+
+ ret = kstrtoint(buf, 10, &tmp);
+ if (ret < 0)
+ return ret;
+
+ if (tmp < LOGLEVEL_EMERG)
+ return -ERANGE;
+
+ /*
+ * Mimic the behavior of /dev/kmsg with respect to minimum_loglevel
+ */
+ if (tmp < minimum_console_loglevel)
+ tmp = minimum_console_loglevel;
+
+ ret = -ENODEV;
+ console_lock();
+ for_each_console(con) {
+ if (con->kobj == kobj) {
+ con->level = tmp;
+ ret = count;
+ break;
+ }
+ }
+ console_unlock();
+
+ return ret;
+}
+
+static const struct kobj_attribute console_loglevel_attr =
+ __ATTR(loglevel, 0644, loglevel_show, loglevel_store);
+
+static void console_register_sysfs(struct console *newcon)
+{
+ /*
+ * We might be called very early from register_console(): in that case,
+ * printk_late_init() will take care of this later.
+ */
+ if (!consoles_dir_kobj)
+ return;
+
+ newcon->kobj = kobject_create_and_add(newcon->name, consoles_dir_kobj);
+ if (WARN_ON(!newcon->kobj))
+ return;
+
+ WARN_ON(sysfs_create_file(newcon->kobj, &console_loglevel_attr.attr));
+}
+
+static void console_unregister_sysfs(struct console *oldcon)
+{
+ kobject_put(oldcon->kobj);
+}
+
/*
* The console driver calls this routine during kernel initialization
* to register the console printing procedure with printk() and to
@@ -2495,6 +2573,7 @@ void register_console(struct console *newcon)
* By default, the per-console minimum forces no messages through.
*/
newcon->level = LOGLEVEL_EMERG;
+ newcon->kobj = NULL;

/*
* Put this console in the list - keep the
@@ -2531,6 +2610,7 @@ void register_console(struct console *newcon)
*/
exclusive_console = newcon;
}
+ console_register_sysfs(newcon);
console_unlock();
console_sysfs_notify();

@@ -2597,6 +2677,7 @@ int unregister_console(struct console *console)
console_drivers->flags |= CON_CONSDEV;

console->flags &= ~CON_ENABLED;
+ console_unregister_sysfs(console);
console_unlock();
console_sysfs_notify();
return res;
@@ -2672,6 +2753,13 @@ static int __init printk_late_init(void)
ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN, "printk:online",
console_cpu_notify, NULL);
WARN_ON(ret < 0);
+
+ consoles_dir_kobj = kobject_create_and_add("consoles", NULL);
+ WARN_ON(!consoles_dir_kobj);
+
+ for_each_console(con)
+ console_register_sysfs(con);
+
return 0;
}
late_initcall(printk_late_init);
--
2.9.5
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