Kernel Korner - Network Programming in the Kernel

Here is a list of a few the FTP commands we used. Because our program provides only a basic implementation of the protocol, we discuss only the relevant commands:

Figure 2 is a diagram that shows a few FTP commands and their responses from the server.

Figure 2. The client issues FTP commands over the control connection to set up the file transfer.

Writing programs in the kernel is different from doing the same in user space.

We explain a few issues concerned with writing a network application in the kernel. Refer to Greg Kroah-Hartman's article “Things You Never Should Do in the Kernel” (see the on-line Resources). First, let's examine how a system call in user space completes its task. For example, look at the socket() system call:

sockfd = socket(AF_INET,SOCK_STREAM,0);

When a program executes a system call, it traps into the kernel via an interrupt and hands over control to the kernel. Among other things, the kernel performs various tasks, such as saving contents of registers, making changes to address space boundaries and checking for errors with system call parameters. Eventually, the sys_socket() function in the kernel is responsible for creating the socket of specified address and family type, finding an unused file descriptor and returning this number back to user space. Browsing through the kernel's code we can trace the path followed by this function (Figure 3).

Figure 3. Behind the scenes of a system call: when user space executes socket(), the kernel does necessary housekeeping and then returns a new file descriptor.

We now explain the design and implementation of a kernel FTP client. Please follow through the code available at the Linux Journal FTP site (see Resources) as you read through the article. The main functionality of this client is written in the form of a kernel module that adds a system call dynamically that user-space programs can invoke to start the FTP client process. The module allows only the root user to read a file using FTP. The user-space program that calls the system call in this module should be used with extreme caution. For example, it is easy to imagine the catastrophic results when root runs:

./a.out 10.0.0.1 10.0.0.2 foo_file /dev/hda1/*

and overwrites /dev/hda1 with a downloaded file from 10.0.0.1.

We first need to configure the Linux kernel to allow us to add new system calls via a kernel module dynamically. Starting with version 2.6, the symbol sys_call_table is no longer exported by the kernel. For our module to be able to add a system call dynamically, we need to add the following lines to arch/i386/kernel/i386_ksyms.c in the kernel source (assuming you are using a Pentium-class machine):

extern void *sys_call_table;
EXPORT_SYMBOL(sys_call_table);

After recompiling the kernel and booting the machine into it, we are all set to run the FTP client. Refer to the Kernel Rebuild HOWTO (see Resources) for details on compiling a kernel.