Linux-kernel: printk from "open" syscall don't work

6
votes

I have a doubt.

I opened the kernel and I changed the directory linux-3.1.1/fs/open.c

I changed the follow code in the open.c.

SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, int, mode)
{
    long ret;
    printk(KERN_EMERG "Testing\n");
    ... 
}

I put this line only: printk(KERN_EMERG "Testing");

And I include the libraries:<linux/kernel.h> and <linux/printk.h>

So I compiled and rebooted my linux(Ubuntu). During the rebooting appeared a lot of "Testing" on the screen. So up to now its Ok.

But now I have a problem. I created this program in c.

int main()
{
    size_t filedesc = open("testefile2.txt",O_CREAT | O_WRONLY,0640);
    printf("%d",filedesc);
}

I compiled this program and executed and works good. But I don´t understand why the "Testing" didn't appeared on the shell. I mean , if when I reboot the pc appeared a lot of the word "Testing" , why this word doens´t appear when I execute the program above. Just to add I include this libraries in this code above:

unistd.h , fcntl.h , stdio.h , stdlib.h

Thank you guys.

2
clinuxlinux-kernelsystem-callsprintk
I tried fixing the formatting on this, but it seems I made it worse. Anyone see where it's screwed up? I don't see anything wrong with the way the code blocks are formatted... :( – Tim Coker
I think its better now.Sorry,I didnt know how to use this format correctly.Thank you. – UserJ
Maybe your printk() isn't appearing where you expect it to appear. When you open another virtual console (or terminal) and type sudo tail -f /var/log/dmesg, do you see your Testing messages? – mpontillo
Mike is probalby correct. Your shell is run under a different process than the console that the kernel is connected to. Ubuntu almost certainly redirects the output to the system log. – Falmarri
When I type this sudo tail -f /var/log/dmesg doesnt appear "Testing" Basically I`m open a shell and typing this:gcc -o t test.c And then : ./t But its prints only 3 – UserJ

2 Answers

6
votes

printk calls appear in the kernel message buffer, not in your process' stdout/stderr

4
votes

But I don´t understand why the "Testing" didn't appeared on the shell.

I think, this is effect of printk's messages suppression. (more exactly:rate limiting)

Check the messages log or console for 

printk: ### messages suppressed.

string.

This feature will stop printing a message, if there were a lot of messages in recent time.

Actual code is as 3.1 kernel: http://lxr.linux.no/#linux+v3.1.1/kernel/printk.c#L1621

1621 * printk rate limiting, lifted from the networking subsystem.
1622 *
1623 * This enforces a rate limit: not more than 10 kernel messages
1624 * every 5s to make a denial-of-service attack impossible.
1625 */
1626 DEFINE_RATELIMIT_STATE(printk_ratelimit_state, 5 * HZ, 10);
1627
1628 int __printk_ratelimit(const char *func)

So, As the open syscall is very-very popular (just do an strace -e open /bin/ls - I'll get 15 open syscalls for just starting an simplest ls), the rate limiting will be in effect. It will limit your message to be printed only one time in 5 seconds; not more than 10 messages in single "burst".

I can only suggest to create a special user with known UID and add an UID checking before printk in your additional printk-in-open code.