What's wrong with vfs_stat() call?

Question

I'm trying to do a stat on files,

    struct kstat stat;
    int error = vfs_stat ("/bin/ls", &stat); // /bin/ls exists

    if (error)
    {
            printk (KERN_INFO "error code %d\n", error);
    }
    else
    {
            printk (KERN_INFO "mode of ls: %o\n", stat.mode);
            printk (KERN_INFO "owner of ls: %o\n", stat.uid);
    }

    return error;

But error was always set to 14 (Bad Address), what's wrong with the code?

I'm running 3.9 kernel.

Answer 1

vfs_stat() is defined as:

int vfs_stat(const char __user *name, struct kstat *stat);

and __user is defined as:

# define __user __attribute__((noderef, address_space(1)))

In other words, vfs_stat() only supports using filename that is pointer into user space, and should not be dereferenced inside kernel space. Note that "/bin/ls" does NOT point into user space, but into kernel space, and thus cannot be used here.

Actually, error message 14 (bad address) tells this issue right into your face :)

Answer 2

Use the following code:

#include <linux/uaccess.h>

int error;
mm_segment_t old_fs = get_fs();

set_fs(KERNEL_DS);
error = vfs_stat ("/bin/ls", &stat);
set_fs(old_fs);

...