[patch] printk from userspace

Andrew Morton
2002-08-14T03:04:10Z

The patch allows userspace to issue printk's, via sys_syslog():

#include <sys/klog.h>

int do_syslog(char *msg)
{
return klogctl(10, msg, strlen(msg));
}

main()
{
char big[2000];

do_syslog("<1>one\n");
do_syslog("<2>two");
do_syslog("<3>three\n");
memset(big, 'a', sizeof(big));
big[1999] = 0;
do_syslog(big);
}

The main use of this is within hpa's klibc - initial userspace needs a
way of logging information and this API allows that information to be
captured into the printk ringbuffer. It ends up in /var/log/messages.

Messages are truncated at 1024 characters by printk's vsprintf().

Requires CAP_SYS_ADMIN.

printk.c | 16 ++++++++++++++++
1 files changed, 16 insertions(+)

--- 2.5.31/kernel/printk.c~hpa-printk Tue Aug 13 20:15:32 2002
+++ 2.5.31-akpm/kernel/printk.c Tue Aug 13 20:15:32 2002
@@ -25,6 +25,7 @@
#include <linux/module.h>
#include <linux/interrupt.h> /* For in_interrupt() */
#include <linux/config.h>
+#include <linux/slab.h>
#include <linux/delay.h>

#include <asm/uaccess.h>
@@ -163,12 +164,15 @@ __setup("console=", console_setup);
* 7 -- Enable printk's to console
* 8 -- Set level of messages printed to console
* 9 -- Return number of unread characters in the log buffer
+ * 10 -- Printk from userspace. Includes loglevel. Returns number of
+ * chars printed.
*/
int do_syslog(int type, char * buf, int len)
{
unsigned long i, j, limit, count;
int do_clear = 0;
char c;
+ char *lbuf = NULL;
int error = 0;

switch (type) {
@@ -283,11 +287,23 @@ int do_syslog(int type, char * buf, int
error = log_end - log_start;
spin_unlock_irq(&logbuf_lock);
break;
+ case 10:
+ lbuf = kmalloc(len + 1, GFP_KERNEL);
+ error = -ENOMEM;
+ if (lbuf == NULL)
+ break;
+ error = -EFAULT;
+ if (copy_from_user(lbuf, buf, len))
+ break;
+ lbuf[len] = '\0';
+ error = printk("%s", lbuf);
+ break;
default:
error = -EINVAL;
break;
}
out:
+ kfree(lbuf);
return error;
}

.