Opening a file from userspace from a Linux kernel module - c

Question

I've been following a tutorial for opening files from userspace from a Linux kernel module at http://www.howtoforge.com/reading-files-from-the-linux-kernel-space-module-driver-fedora-14
The code is the following:
#include <linux/module.h> // Needed by all modules
#include <linux/kernel.h> // Needed for KERN_INFO
#include <linux/fs.h> // Needed by filp
#include <asm/uaccess.h> // Needed by segment descriptors
int init_module(void)
{
// Create variables
struct file *f;
char buf[128];
mm_segment_t fs;
int i;
// Init the buffer with 0
for(i=0;i<128;i++)
buf[i] = 0;
// To see in /var/log/messages that the module is operating
printk(KERN_INFO "My module is loaded\n");
// I am using Fedora and for the test I have chosen following file
// Obviously it is much smaller than the 128 bytes, but hell with it =)
f = filp_open("/etc/fedora-release", O_RDONLY, 0);
if(f == NULL)
printk(KERN_ALERT "filp_open error!!.\n");
else{
// Get current segment descriptor
fs = get_fs();
// Set segment descriptor associated to kernel space
set_fs(get_ds());
// Read the file
f->f_op->read(f, buf, 128, &f->f_pos);
// Restore segment descriptor
set_fs(fs);
// See what we read from file
printk(KERN_INFO "buf:%s\n",buf);
}
filp_close(f,NULL);
return 0;
}
void cleanup_module(void)
{
printk(KERN_INFO "My module is unloaded\n");
}
The code is copy-pasted from the link above. On my machine, running Fedora 19 with 3.11.10-200 kernel, it seems that filp_open isn't run, providing the buf variable with null values.
What could be wrong? I am still learning the ropes of Linux kernel module development.

Answer 1

First thing you should do is to check if any errors are returned from filp_open (in fact, checking for NULL is probably an outright mistake when modern kernels are concerned). The proper sequence should be:
f = filp_open("/etc/fedora-release", O_RDONLY, 0);
if (IS_ERR(f)) {
// inspect the value of PTR_ERR(f), get the necessary clues
// negative values represent various errors
// as defined in asm-generic/errno-base.h
}
Only then you can move on to diagnosing the read.

Answer 2

977 struct file *filp_open(const char *filename, int flags, umode_t mode)
978 {
979 struct filename *name = getname_kernel(filename);
980 struct file *file = ERR_CAST(name);
981
982 if (!IS_ERR(name)) {
983 file = file_open_name(name, flags, mode);
984 putname(name);
985 }
986 return file;
987 }
Probably the error is in how you put the parameters, the flags parameter is in mode parameter position and vice versa, mode in falgs position.
source: http://lxr.free-electrons.com/source/fs/open.c#L977