How can I write to TTY from a kernel module?

Question

First post to SO, so I'll try to make the question right.

I'm making a simple Linux kernel module with the goal of echoing data back to the TTY shell from where the kernel module was loaded. The problem I having is the kernel "Ooops"-ing with the following message (caught with " watch 'dmesg | tail -50' "). The kernel module's name is Seraphim:

[  184.222748] SELinux: initialized (dev proc, type proc), uses genfs_contexts
[ 1877.456607] seraphim: module verification failed: signature and/or required key  missing - tainting kernel
[ 1877.457321] ------------------
[ 1877.457324] Seraphim started.
[ 1877.457348] BUG: unable to handle kernel NULL pointer dereference at 0000000000000218
[ 1877.457411] IP: [<ffffffffa0012030>] seraphim_entry+0x30/0x1000 [seraphim]
[ 1877.457462] PGD 115a2e067 PUD 10aca8067 PMD 0
[ 1877.457498] Oops: 0000 [#1] SMP
[ 1877.457524] Modules linked in: seraphim(OF+) rfcomm bnep nf_conntrack_netbios_ns   nf_conn track_broadcast ipt_MASQUERADE ip6t_REJECT xt_conntrack ebtable_nat ebtable_broute bridge stp llce btable_filter ebtables ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_nat_ipv6 ip6table_ma etc.

The code used for writing data to the TTY terminal follows:

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/tty.h>

static void printString(char *string) {

    struct tty_struct *tty;

    tty = current->signal->tty;

    if(tty != NULL) { 

        (tty->driver->ops->write) (tty, string, strlen(string));
    }

    else
        printk("tty equals to zero");
}

What am I doing wrong?

I was following the tutorial at http://www.tldp.org/LDP/lkmpg/2.6/lkmpg.pdf but it was out of date (the kernel I am using is 3.11.10-200 on Fedora 19), so I had to rummage through 3.11.10-200 source files to find the adequate structures.

Sun · Accepted Answer · 2013-12-23T05:55:06.427

8

use tty = get_current_tty(); instead of tty = current->signal->tty;

that's it

you need to lock the tty before accessing it and get_current_tty does it internally

NOTE: get_current_tty is under EXPORT_SYMBOL_GPL, hence your module or code

edited Dec 23 '13 at 05:55

answered Dec 22 '13 at 14:22

Sun

1,505
17
25

Thanks, it worked like a charm. I've spent several hours searching for the solution to this problem. If we ever meet in real life, I owe you a beer (or a drink of your choice). – mzdv Dec 22 '13 at 18:01

How can I write to TTY from a kernel module?

1 Answers1

Linked