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SubjectRe: [PATCH 2/2] printk: Add kernel parameter to control writes to /dev/kmsg
On Wed, Jun 15, 2016 at 03:40:04PM -1000, Linus Torvalds wrote:
> Possibly we could just say that if a kernel command line option has
> been given, that is absolute.
>
> And then a sysctl for when you do *not* explicitly set if on the
> kernel command line?

Ok, how about this ontop?

It is only lightly tested in a vm but basically I'm using the second
byte of devkmsg_log to set a bit in there and the sysctl handler looks
at it.

It is also visible in sysctl and we know it has been cmdline-disabled:

$ cat /proc/sys/kernel/printk_kmsg
256

---
diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
index a3683ce2a2f3..02fe4562953f 100644
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -752,6 +752,19 @@ send before ratelimiting kicks in.

==============================================================

+printk_kmsg:
+
+Control the logging to /dev/kmsg from userspace:
+
+0: default, ratelimited
+1: unlimited logging to /dev/kmsg from userspace
+2: logging to /dev/kmsg disabled
+
+The kernel command line parameter printk.kmsg= overrides this setting
+and once set, it cannot be changed by this sysctl interface anymore.
+
+==============================================================
+
randomize_va_space:

This option can be used to select the type of process address
diff --git a/include/linux/printk.h b/include/linux/printk.h
index f4da695fd615..bcf72e756122 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -171,6 +171,12 @@ extern bool printk_timed_ratelimit(unsigned long *caller_jiffies,
extern int printk_delay_msec;
extern int dmesg_restrict;
extern int kptr_restrict;
+extern unsigned int devkmsg_log;
+
+struct ctl_table;
+
+int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
+ void __user *buffer, size_t *lenp, loff_t *ppos);

extern void wake_up_klogd(void);

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 33701a166f26..9f0a885c2718 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -86,12 +86,15 @@ static struct lockdep_map console_lock_dep_map = {
};
#endif

-#define DEVKMSG_LOG_RATELIMIT 0
-#define DEVKMSG_LOG_ON 1
-#define DEVKMSG_LOG_OFF 2
+#define DEVKMSG_LOG_RATELIMIT 0
+#define DEVKMSG_LOG_ON 1
+#define DEVKMSG_LOG_OFF 2
+#define DEVKMSG_LOCK (1 << 8)
+#define DEVKMSG_LOG_MASK (DEVKMSG_LOCK - 1)
+#define DEVKMSG_LOCKED_MASK ~DEVKMSG_LOG_MASK

/* DEVKMSG_LOG_RATELIMIT by default */
-static unsigned int __read_mostly devkmsg_log;
+unsigned int __read_mostly devkmsg_log;
static int __init control_devkmsg(char *str)
{
if (!str)
@@ -101,14 +104,30 @@ static int __init control_devkmsg(char *str)
devkmsg_log = DEVKMSG_LOG_ON;
else if (!strncmp(str, "off", 3))
devkmsg_log = DEVKMSG_LOG_OFF;
+ else if (!strncmp(str, "ratelimit", 9))
+ devkmsg_log = DEVKMSG_LOG_RATELIMIT;
else
return -EINVAL;

+ /* Sysctl cannot change it anymore. */
+ devkmsg_log |= DEVKMSG_LOCK;
+
return 0;
}
__setup("printk.kmsg=", control_devkmsg);


+int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
+ void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+ if (devkmsg_log & DEVKMSG_LOCKED_MASK) {
+ if (write)
+ return -EINVAL;
+ }
+
+ return proc_dointvec_minmax(table, write, buffer, lenp, ppos);
+}
+
/*
* Number of registered extended console drivers.
*
@@ -656,11 +675,12 @@ static ssize_t devkmsg_write(struct kiocb *iocb, struct iov_iter *from)
return -EINVAL;

/* Ignore when user logging is disabled. */
- if (devkmsg_log == DEVKMSG_LOG_OFF)
+ if ((devkmsg_log & DEVKMSG_LOG_MASK) == DEVKMSG_LOG_OFF)
return len;

/* Ratelimit when not explicitly enabled or when we're not booting. */
- if ((system_state != SYSTEM_BOOTING) && (devkmsg_log != DEVKMSG_LOG_ON)) {
+ if ((system_state != SYSTEM_BOOTING) &&
+ ((devkmsg_log & DEVKMSG_LOG_MASK) != DEVKMSG_LOG_ON)) {
if (!___ratelimit(&user->rs, current->comm))
return ret;
}
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 87b2fc38398b..a29d6c4fa86c 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -814,6 +814,15 @@ static struct ctl_table kern_table[] = {
.extra2 = &ten_thousand,
},
{
+ .procname = "printk_kmsg",
+ .data = &devkmsg_log,
+ .maxlen = sizeof(unsigned int),
+ .mode = 0644,
+ .proc_handler = devkmsg_sysctl_set_loglvl,
+ .extra1 = &zero,
+ .extra2 = &two,
+ },
+ {
.procname = "dmesg_restrict",
.data = &dmesg_restrict,
.maxlen = sizeof(int),
--
Regards/Gruss,
Boris.

ECO tip #101: Trim your mails when you reply.

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