I know all the discussions about why one should not read/write files from kernel, instead how to use /proc or netlink to do that. I want to read/write anyway. I have also read Driving Me Nuts - Things You Never Should Do in the Kernel.
However, the problem is that 2.6.30 does not export sys_read()
. Rather it's wrapped in SYSCALL_DEFINE3
. So if I use it in my module, I get the following warnings:
WARNING: "sys_read" [xxx.ko] undefined!
WARNING: "sys_open" [xxx.ko] undefined!
Obviously insmod
cannot load the module because linking does not happen correctly.
Questions:
sys_read()
/sys_open()
are not exported)?SYSCALL_DEFINEn()
from within the kernel?You should be aware that you should avoid file I/O from within Linux kernel when possible. The main idea is to go "one level deeper" and call VFS level functions instead of the syscall handler directly:
Includes:
#include <linux/fs.h>
#include <asm/segment.h>
#include <asm/uaccess.h>
#include <linux/buffer_head.h>
Opening a file (similar to open):
struct file *file_open(const char *path, int flags, int rights)
{
struct file *filp = NULL;
mm_segment_t oldfs;
int err = 0;
oldfs = get_fs();
set_fs(get_ds());
filp = filp_open(path, flags, rights);
set_fs(oldfs);
if (IS_ERR(filp)) {
err = PTR_ERR(filp);
return NULL;
}
return filp;
}
Close a file (similar to close):
void file_close(struct file *file)
{
filp_close(file, NULL);
}
Reading data from a file (similar to pread):
int file_read(struct file *file, unsigned long long offset, unsigned char *data, unsigned int size)
{
mm_segment_t oldfs;
int ret;
oldfs = get_fs();
set_fs(get_ds());
ret = vfs_read(file, data, size, &offset);
set_fs(oldfs);
return ret;
}
Writing data to a file (similar to pwrite):
int file_write(struct file *file, unsigned long long offset, unsigned char *data, unsigned int size)
{
mm_segment_t oldfs;
int ret;
oldfs = get_fs();
set_fs(get_ds());
ret = vfs_write(file, data, size, &offset);
set_fs(oldfs);
return ret;
}
Syncing changes a file (similar to fsync):
int file_sync(struct file *file)
{
vfs_fsync(file, 0);
return 0;
}
[Edit] Originally, I proposed using file_fsync, which is gone in newer kernel versions. Thanks to the poor guy suggesting the change, but whose change was rejected. The edit was rejected before I could review it.