Getting file descriptors and details within kernel space without open()

Question

Could anyone provide the code to overcome this problem?

Effectively how do we obtain the struct inode* from kernel level given the file /dev/driver1?

Given in user space that:

int fd; fd = open("/dev/driver1", O_RDWR | O_SYNC);

In Kernel space:

static long dev_ioctl(struct file *file, unsigned cmd, unsigned long arg) struct dev_handle *handle; handle = file->private_data;

Assuming that, we do not go by that path,

How do we obtain within the kernel itself, by eg. hard coding the file->private_data to be given to handle?

Why do you need the file descriptor number? That's not something your filesystem code should need to know. — tangrs
– tangrs, Commented Feb 25, 2014 at 9:23
I know, but some high ranking guy specified the requirements. No one can change it. — Moirisa Dikaiosýni
– Moirisa Dikaiosýni, Commented Feb 25, 2014 at 16:12
Get the high ranking guy fired. He should not be allowed anywhere near Linux kernels. His "requirements" are the exact opposite of how the Linux kernel works. The driver interfaces work on userspace pointers, and won't work with kernel ones; you'd need to construct or simulate a userspace process to be able to call these functions. The obvious intent, considering the glaring stupidity of the approach, is to avoid GPLv2 enforcement, right? Won't work, though: the kernel-userspace boundary is not a copyright law boundary, it's just a technical detail. Just ask your lawyer, okay? — Nominal Animal
– Nominal Animal, Commented Feb 27, 2014 at 8:55
The above is not to say it's impossible; just idiotic and perverse. If you really need to do this, remember you only need a userspace context (including stack, a bit of memory to hold the data, and so on). You can create a process using code similar to the usermodehelper approach, in kernel/kmod.c. — Nominal Animal
– Nominal Animal, Commented Feb 27, 2014 at 9:07

Community · Accepted Answer · 2017-05-23 12:00:35Z

You are looking for filp_open function. From file include/linux/fs.h:

struct file *filp_open(const char *filename, int flags, umode_t mode);

Here is a link to function source and documentation: http://lxr.free-electrons.com/source/fs/open.c#L937

If you really need FD, you can use sys_open (not exported in newer kernels):

long sys_open(const char __user *filename, int flags, int mode);

You can find a very good answer on a similar question: How to read/write files within a Linux kernel module?

Edit (how to get inode):

You can get cached inode from struct file:

struct file *file = ...; struct inode *inode = file->inode;

If you want it with locking: here is a background: Documentation/filesystems/path-lookup.txt

The starting point for traversing is current->fs->root. There is a number of function in kernel, that already do the work, you can find them in fs/namei.c source file.

There is a function: kern_path:

int error; struct inode *inode; struct path path; error = kern_path(pathname, LOOKUP_FOLLOW, &path); if (error) ...; inode = path.dentry->d_inode;

This would appear to yield a struct file* or an fd, and not a struct inode * which is what the OP wanted.

nos · Accepted Answer · 2014-03-03 13:21:43Z

Your code is in the dev_ioctl function ? If so, then

static long dev_ioctl(struct file *file, unsigned cmd, unsigned long arg) struct dev_handle *handle; struct inode *inode; handle = file->private_data; inode = file->f_inode;

There seems to be no sane documentation for locking requirements, so you should probably try to dig up similar code and see how it operates on the f_inode member.

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