Write file from kernel land - c

Question

I would like to write into a log file from the kernel space. I can create, open & close file (/var/log/my_kern_module.log) but if I try to write something into, my module crash... So I know read/write file from kernel space is bad, but I must do it in this module. Do you have any suggestion to help me ? thx
Here you can find the code who perform write on my kernel module. It code run into a thread (kthread)
mutex_lock(&gl_mtx_writelog);
printk(KERN_EMERG "Readed : %s\n", buffer);
fd = filp_open("/var/log/my_kern_module.log", O_CREAT | O_WRONLY | O_APPEND, S_IRWXU);
if (!IS_ERR (fd)) {
fs = get_fs();
set_fs(KERNEL_DS);
do_sync_write(fd, buffer, readed, 0);
set_fs(fs);
filp_close(fd, NULL);
}
mutex_unlock(&gl_mtx_writelog);
so, i've tried to use fd->f_op->write(...), but module crash too.
BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<c10df83a>] do_sync_write+0x6a/0xe0
*pde = 00000000
Oops: 0000 [#1] SMP
last sysfs file: /sys/module/snd_mixer_oss/initstate
Modules linked in: trigger_server snd_seq_dummy snd_seq_oss snd_seq_midi_event s nd_seq snd_seq_device snd_pcm_oss snd_mixer_oss ipv6 pcmcia pcmcia_core agpgart lp fuse ppdev snd_intel8x0 snd_ac97_codec ac97_bus thermal i2c_piix4 snd_pcm the rmal_sys i2c_core snd_timer e1000 parport_pc parport snd rtc_cmos rtc_core rtc_l ib joydev psmouse soundcore snd_page_alloc evdev usbhid serio_raw hid hwmon ac b utton sg [last unloaded: pcmcia_core]
Pid: 2468, comm: kthread_cli Not tainted 2.6.33.4-smp #1 /VirtualBox
EIP: 0060:[<c10df83a>] EFLAGS: 00010287 CPU: 0
EIP is at do_sync_write+0x6a/0xe0
EAX: f6434000 EBX: 00000000 ECX: 00000200 EDX: f5c26400
ESI: f653ca80 EDI: f5cf1f14 EBP: f5cf1f98 ESP: f5cf1f0c
DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
Process kthread_cli (pid: 2468, ti=f5cf0000 task=f6434000 task.ti=f5cf0000)
Stack:
c103de14 00000441 00000000 00000442 00000000 00000001 ffffffff f653ca80
<0> 00000000 00000000 00000000 00000000 f6434000 00000000 00000000 c10384a6
<0> 0000052b 00000000 0000052b ffffffff f5c26400 00000400 f5cf1f98 00000442
Call Trace:
[<c103de14>] ? vprintk+0x184/0x3d0
[<c10384a6>] ? try_to_wake_up+0x226/0x350
[<f887b226>] ? thread_client+0x106/0x130 [trigger_server]
[<f887b120>] ? thread_client+0x0/0x130 [trigger_server]
[<c1058cf4>] ? kthread+0x74/0x80
[<c1058c80>] ? kthread+0x0/0x80
[<c10035be>] ? kernel_thread_helper+0x6/0x10
Code: 94 00 00 00 00 c7 45 98 00 00 00 00 c7 45 9c 00 00 00 00 c7 45 a0 00 00 00 00 89 45 a4 c7 45 a8 00 00 00 00 c7 45 ac 00 00 00 00 <8b> 03 8b 53 04 89 4d c8 89 45 b0 89 55 b4 eb 13 8d b6 00 00 00
EIP: [<c10df83a>] do_sync_write+0x6a/0xe0 SS:ESP 0068:f5cf1f0c
CR2: 0000000000000000
---[ end trace 32d03f08f128f335 ]---

Answer 1

One obvious problem is that the 4th parameter to do_sync_write() is a pointer to where the file offset is stored (so it can be updated). You are passing 0 which might explain the NULL pointer problem. Could try:
loff_t ppos = 0;
do_sync_write(fd, buffer, readed, &ppos);

Answer 2

Equivalent methods to do what you need are listed here.
Why writing files from the kernel is bad ?

Answer 3

This comes up all the time on LKML, and the answer is always "don't do it". There is always a better way.