Re: [PATCH v8 18/21] printk: Deconstruct kernel.printk into discrete sysctl controls
From: Petr Mladek
Date: Fri Dec 12 2025 - 10:24:21 EST
Adding Joel into Cc. Joel, see the original patch at
https://lore.kernel.org/all/c3e5cc507eb3fd7db0a002f31d7e47d764cad176.1764272407.git.chris@xxxxxxxxxxxxxx/
On Fri 2025-11-28 03:44:25, Chris Down wrote:
> 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.
>
> Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
> Tested-by: Petr Mladek <pmladek@xxxxxxxx>
> Signed-off-by: Chris Down <chris@xxxxxxxxxxxxxx>
This patch requires non-trivial changes when rebased on top of v6.19.
There have been some conflicting changes in the sysctl API, see
https://lore.kernel.org/lkml/20251016-jag-sysctl_conv-v2-0-a2f16529acc4@xxxxxxxxxx/
> --- a/include/linux/sysctl.h
> +++ b/include/linux/sysctl.h
> @@ -235,6 +235,13 @@ extern struct ctl_table_header *register_sysctl_mount_point(const char *path);
>
> void do_sysctl_args(void);
> bool sysctl_is_alias(char *param);
> +int do_proc_dointvec_conv(bool *negp, unsigned long *lvalp, int *valp,
> + int write, void *data);
> +int do_proc_dointvec(const struct ctl_table *table, int write,
> + void *buffer, size_t *lenp, loff_t *ppos,
> + int (*conv)(bool *negp, unsigned long *lvalp, int *valp,
> + int write, void *data),
> + void *data);
> int do_proc_douintvec(const struct ctl_table *table, int write,
> void *buffer, size_t *lenp, loff_t *ppos,
> int (*conv)(unsigned long *lvalp,
This hunk can be removed in v6.19. There is another interface in 6.19,
see below.
> diff --git a/kernel/printk/sysctl.c b/kernel/printk/sysctl.c
> index da77f3f5c1fe..034739939a61 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 +23,50 @@ static int proc_dointvec_minmax_sysadmin(const struct ctl_table *table, int writ
> return proc_dointvec_minmax(table, write, buffer, lenp, ppos);
> }
The new way is to define the needed helpers using macros. We need this:
static SYSCTL_USER_TO_KERN_INT_CONV(, SYSCTL_CONV_IDENTITY)
static SYSCTL_KERN_TO_USER_INT_CONV(, SYSCTL_CONV_IDENTITY)
static SYSCTL_INT_CONV_CUSTOM(, sysctl_user_to_kern_int_conv,
sysctl_kern_to_user_int_conv, false)
> +static int do_proc_dointvec_console_loglevel(bool *negp, unsigned long *lvalp,
> + int *valp,
> + int write, void *data)
There parameters have got renamed and @table is passed instead or @data:
static int do_proc_dointvec_console_loglevel(bool *negp, unsigned long *u_ptr,
int *k_ptr, int dir,
const struct ctl_table *table)
> +{
> + int level, ret;
> +
> + /*
> + * If writing, first do so via a temporary local int so we can
> + * bounds-check it before touching *valp.
> + */
> + int *intp = write ? &level : valp;
The direction is newly checked by macro:
int *int_ptr = SYSCTL_USER_TO_KERN(dir) ? &level : k_ptr;
> + ret = do_proc_dointvec_conv(negp, lvalp, intp, write, data);
The following helper is defined by the above mentioned macros:
ret = do_proc_int_conv(negp, u_ptr, int_ptr, dir, table);
> + if (ret)
> + return ret;
> +
> + if (write) {
The new way:
if (SYSCTL_USER_TO_KERN(dir)) {
> + if (level != console_clamp_loglevel(level))
> + return -ERANGE;
> +
> + /*
> + * Honour the administrator-configured minimum console
> + * loglevel (third element of kernel.printk). This mirrors
> + * the syslog() and sysfs control paths so that once the floor
> + * is raised we do not let this sysctl silently bypass it.
> + */
> + if (minimum_console_loglevel > CONSOLE_LOGLEVEL_MIN &&
> + level < minimum_console_loglevel)
> + level = minimum_console_loglevel;
> +
> + WRITE_ONCE(*valp, level);
New parameter name:
WRITE_ONCE(*k_ptr, level);
> + }
> +
> + return 0;
> +}
> +
> +static int proc_dointvec_console_loglevel(const struct ctl_table *table,
> + int write, void *buffer, size_t *lenp,
> + loff_t *ppos)
> +{
> + return do_proc_dointvec(table, write, buffer, lenp, ppos,
> + do_proc_dointvec_console_loglevel, NULL);
There is a new function where the last NULL parameter is not longer passed:
return proc_dointvec_conv(table, write, buffer, lenp, ppos,
do_proc_dointvec_console_loglevel);
> +}
> +
> static const struct ctl_table printk_sysctls[] = {
> {
> .procname = "printk",
Here are the above described changes made by diff:
--- a/kernel/printk/sysctl.c
+++ b/kernel/printk/sysctl.c
@@ -24,9 +24,14 @@ static int proc_dointvec_minmax_sysadmin(const struct ctl_table *table, int writ
return proc_dointvec_minmax(table, write, buffer, lenp, ppos);
}
-static int do_proc_dointvec_console_loglevel(bool *negp, unsigned long *lvalp,
- int *valp,
- int write, void *data)
+static SYSCTL_USER_TO_KERN_INT_CONV(, SYSCTL_CONV_IDENTITY)
+static SYSCTL_KERN_TO_USER_INT_CONV(, SYSCTL_CONV_IDENTITY)
+static SYSCTL_INT_CONV_CUSTOM(, sysctl_user_to_kern_int_conv,
+ sysctl_kern_to_user_int_conv, false)
+
+static int do_proc_dointvec_console_loglevel(bool *negp, unsigned long *u_ptr,
+ int *k_ptr, int dir,
+ const struct ctl_table *table)
{
int level, ret;
@@ -34,13 +39,13 @@ static int do_proc_dointvec_console_loglevel(bool *negp, unsigned long *lvalp,
* If writing, first do so via a temporary local int so we can
* bounds-check it before touching *valp.
*/
- int *intp = write ? &level : valp;
+ int *int_ptr = SYSCTL_USER_TO_KERN(dir) ? &level : k_ptr;
- ret = do_proc_dointvec_conv(negp, lvalp, intp, write, data);
+ ret = do_proc_int_conv(negp, u_ptr, int_ptr, dir, table);
if (ret)
return ret;
- if (write) {
+ if (SYSCTL_USER_TO_KERN(dir)) {
if (level != console_clamp_loglevel(level))
return -ERANGE;
@@ -54,7 +59,7 @@ static int do_proc_dointvec_console_loglevel(bool *negp, unsigned long *lvalp,
level < minimum_console_loglevel)
level = minimum_console_loglevel;
- WRITE_ONCE(*valp, level);
+ WRITE_ONCE(*k_ptr, level);
}
return 0;
@@ -64,8 +69,8 @@ static int proc_dointvec_console_loglevel(const struct ctl_table *table,
int write, void *buffer, size_t *lenp,
loff_t *ppos)
{
- return do_proc_dointvec(table, write, buffer, lenp, ppos,
- do_proc_dointvec_console_loglevel, NULL);
+ return proc_dointvec_conv(table, write, buffer, lenp, ppos,
+ do_proc_dointvec_console_loglevel);
}
static int proc_dointvec_printk_deprecated(const struct ctl_table *table, int write,
> void __init printk_sysctl_init(void)
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index cb6196e3fa99..3ed010b8f6b3 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -354,9 +354,9 @@ static void proc_put_char(void **buf, size_t *size, char c)
> }
> }
>
> -static int do_proc_dointvec_conv(bool *negp, unsigned long *lvalp,
> - int *valp,
> - int write, void *data)
> +int do_proc_dointvec_conv(bool *negp, unsigned long *lvalp,
> + int *valp,
> + int write, void *data)
> {
> if (write) {
> if (*negp) {
> @@ -380,6 +380,7 @@ static int do_proc_dointvec_conv(bool *negp, unsigned long *lvalp,
> }
> return 0;
> }
> +EXPORT_SYMBOL(do_proc_dointvec_conv);
>
> static int do_proc_douintvec_conv(unsigned long *lvalp,
> unsigned int *valp,
> @@ -471,15 +472,16 @@ static int __do_proc_dointvec(void *tbl_data, const struct ctl_table *table,
> return err;
> }
>
> -static int do_proc_dointvec(const struct ctl_table *table, int write,
> - void *buffer, size_t *lenp, loff_t *ppos,
> - int (*conv)(bool *negp, unsigned long *lvalp, int *valp,
> - int write, void *data),
> - void *data)
> +int do_proc_dointvec(const struct ctl_table *table, int write,
> + void *buffer, size_t *lenp, loff_t *ppos,
> + int (*conv)(bool *negp, unsigned long *lvalp, int *valp,
> + int write, void *data),
> + void *data)
> {
> return __do_proc_dointvec(table->data, table, write,
> buffer, lenp, ppos, conv, data);
> }
> +EXPORT_SYMBOL(do_proc_dointvec);
>
> static int do_proc_douintvec_w(unsigned int *tbl_data,
> const struct ctl_table *table,
Also all these changes in kernel/sysctl.c are not loger needed.
Best Regards,
Petr