Linux Kernel File Operations

With the support of VFS, user-State processes can use read and write to read and write file systems of any type, but how can we operate files without such a system calling in the Linux kernel? We know that when read and write are in the kernel state, sys_read and sys_write are actually executed. However, when we look at the kernel source code, we find that the functions of these operation files are not exported (exported using export_symbol ), that is to say, it cannot be used in the kernel module. How can this problem be solved?

 

Through viewing the source code of sys_open, we found that it mainly uses the do_filp_open () function, which is in FS/nameI. c, and in the modified file, the filp_open function also calls the do_filp_open function, and the interface is very similar to the sys_open function. The call parameters are the same as those of sys_open, and export_symbol is used to export the function, so we guess this function can open a file, which is the same as open. Using the same search method, we found a group of functions that operate files in the kernel, as shown below:

 

Function	Function prototype
Open a file	StructFile * filp_open (Const Char* Filename,IntFlags,IntMode)
Read files	Ssize_tVfs_read (StructFile * file,Char_ User * Buf,Size_tCount, Loff_t * POS)
Write files	Ssize_tVfs_write (StructFile * file,Const Char_ User * Buf,Size_tCount, loff_t * POS)
Close file	IntFilp_close (StructFile * filp, fl_owner_t ID)

 

We noticed that in the vfs_read and vfs_write functions, the Buf parameter points to the memory address of the user space. If we use the pointer of the kernel space directly,-efalut is returned. So we need to use
Set_fs () and get_fs () macros are used to change the kernel's processing method for memory address check. Therefore, the file read/write process in the kernel space is as follows:

1. Mm_segment_t FS = get_fs ();
2. Set_fs (kernel_fs );
3. // Vfs_write ();
4. Vfs_read ();
5. Set_fs (FS );

The following is an example of file operations in the kernel:

# Include # includestatic char Buf [] = "hello"; static char buf1 [10]; int _ init hello_init (void) {struct file * FP; mm_segment_t FS; loff_t Pos; printk ("Hello enter \ n"); FP = filp_open ("/home/Niutao/kernel_file", o_rdwr | o_creat, 0644 ); if (is_err (FP) {printk ("Create File error \ n"); Return-1;} FS = get_fs (); set_fs (kernel_ds); Pos = 0; vfs_write (FP, Buf, sizeof (BUF), & Pos); Pos = 0; vfs_read (FP, buf1, sizeof (BUF), & Pos); printk ("Read: % s \ n ", buf1); filp_close (FP, null); set_fs (FS); Return 0;} void _ exit hello_exit (void) {printk ("Hello exit \ n");} module_init (hello_init); module_exit (hello_exit); module_license ("GPL ");

The above content from: http://www.ej38.com/showinfo/linux-192439.html

Another question is, what method should I use to obtain the file attributes (directory files, common files, uid, and GID) in the kernel state?

The user-state method is called by the stat system, but the kernel-state system call is not exported, so it cannot be used.

I don't know if sys_stat and vfs_stat can be implemented. No, I'm trying ........

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