Browsing Linux Kernel Source

Browsing Linux Kernel

Linux Day – May 6, 2007Motaz K. Saad

Department of Computer Sciencehttp://motaz.saad.googlepages.com

[email protected]

Linux Directory Structure

Structure of a Linux Based Operating System

Kernel

Get an in-depth understanding of the Linux operating system: Kernel• The Linux kernel acts as the interface between

the hardware, and the rest of the operating system.

• The Linux kernel also contains device drivers, which are specific to the hardware peripherals that you are using.

Get an in-depth understanding of the Linux operating system: Kernel• The kernel is also responsible for handling

things such as the allocation of resources (memory and CPU time),

• As well as keeping track of which applications are busy with which files,

• As well as security; and what each user is allowed to do on the operating system.

The Linux kernel version numbers

• Any even number kernel (for example 2.0.30) is a stable, released, kernel

• Any odd numbered kernel (for example 2.1.42 is a development kernel

• Kernel Source can be downloaded form kernel.org

How The Kernel Sources Are Arranged?:

• /usr/src/linux-2.6.x

How The Kernel Sources Are Arranged?

• arch: –The arch subdirectory contains all of

the architecture specific kernel code. –It has further subdirectories, one per

supported architecture, for example i386, mips, and alpha.


• include: – The include subdirectory contains most of the

include files needed to build the kernel code. – It too has further subdirectories including one for

every architecture supported. – The include/asm subdirectory is a soft link to the

real include directory needed for this architecture, for example include/asm-i386.

– To change architectures you need to edit the kernel makefile and rerun the Linux kernel configuration program.


• init:–This directory contains the

initialization code for the kernel and it is a very good place to start looking at how the kernel works.


• mm:–This directory contains all of the

memory management code. –The architecture specific memory

management code lives down in arch/*/mm/, for example arch/i386/mm/fault.c.


• drivers:–All of the system's device drivers live in

this directory. –They are further sub-divided into

classes of device driver, for example block.


• ipc: –This directory contains the kernels

inter-process communications code.

• modules: –This is simply a directory used to hold

built modules.


• fs: –All of the file system code. This is further

sub-divided into directories, one per supported file system, for example vfat, ext2, and ext3.

• kernel:– The main kernel code. Again, the

architecture specific kernel code is in arch/*/kernel.


• net: –The kernel's networking code.

• lib: –This directory contains the kernel's

library code. –The architecture specific library code

can be found in arch/*/lib/.


• Documentation: –The kernel's documentation. –Do not forget to read it.

Where to Start Looking?

• System Startup and Initialization• Memory Management• Kernel• PCI• Interprocess Communication• Interrupt Handling• Device Drivers• File Systems• Network• Modules

System Startup and Initialization

• On an Intel based system, the kernel starts when either loadlin.exe or LILO has loaded the kernel into memory and passed control to it.

• Look in arch/i386/kernel/head.S for this part. Head.S does some architecture specific setup and then jumps to the main() routine in init/main.c.

Memory Management

• This code is mostly in mm but the architecture specific code is in arch/*/mm.

• The page fault handling code is in mm/memory.c and the memory mapping and page cache code is in mm/filemap.c.

• The buffer cache is implemented in mm/buffer.c and the swap cache in mm/swap_state.c and mm/swapfile.c.

Kernel

• Most of the relevent generic code is in kernel with the architecture specific code in arch/*/kernel.

• The scheduler is in kernel/sched.c and the fork code is in kernel/fork.c.

• The bottom half handling code is in include/linux/interrupt.h.

• The task_struct data structure can be found in include/linux/sched.h.

PCI

• The PCI pseudo driver is in drivers/pci/pci.c with the system wide definitions in include/linux/pci.h.

• Each architecture has some specific PCI BIOS code, Alpha AXP's is in arch/alpha/kernel/bios32.c.

Interprocess Communication

• This is all in ipc. All System V IPC objects include an ipc_perm data structure and this can be found in include/linux/ipc.h.

• System V messages are implemented in ipc/msg.c, shared memory in ipc/shm.c and semaphores in ipc/sem.c.

• Pipes are implemented in ipc/pipe.c.

Interrupt Handling

• The kernel's interrupt handling code is almost all microprocessor (and often platform) specific.

• The Intel interrupt handling code is in arch/i386/kernel/irq.c and its definitions in include/asm-i386/irq.h.

Device Drivers

• Most of the lines of the Linux kernel's source code are in its device drivers.

• All of Linux's device driver sources are held in drivers but these are further broken out by type: – /cdrom : All of the CDROM code for Linux. It is

here that the special CDROM devices (such as Soundblaster CDROM) can be found. Note that the ide CD driver is ide-cd.c in drivers/block and that the SCSI CD driver is in scsi.c in drivers/scsi.

Device Drivers

– /pci : This are the sources for the PCI pseudo-driver. A good place to look at how the PCI subsystem is mapped and initialized. The Alpha AXP PCI fixup code is also worth looking at in arch/alpha/kernel/bios32.c.

– /scsi : This is where to find all of the SCSI code as well as all of the drivers for the scsi devices supported by Linux.

File Systems

• The sources for the EXT2 file system are all in the fs/ext2/ directory with data structure definitions in include/linux/ext2_fs.h, ext2_fs_i.h and ext2_fs_sb.h.

• The Virtual File System data structures are described in include/linux/fs.h and the code is in fs/*.

• The buffer cache is implemented in fs/buffer.c along with the update kernel daemon.

Data Structure of ext3 fs: ext3_fs.h/* * Structure of an inode on the disk */struct ext3_inode {

__le16 i_mode; /* File mode */__le16 i_uid; /* Low 16 bits of Owner Uid */__le32 i_size; /* Size in bytes */__le32 i_atime; /* Access time */__le32 i_ctime; /* Creation time */__le32 i_mtime; /* Modification time */__le32 i_dtime; /* Deletion Time */__le16 i_gid; /* Low 16 bits of Group Id */__le16 i_links_count; /* Links count */__le32 i_blocks;/* Blocks count */__le32 i_flags; /* File flags */

Here is the description of the traditional FAT entry in the currentWindows 95 filesystem: (from docs)

struct directory { // Short 8.3 names unsigned char name[8]; // file name unsigned char ext[3]; // file extension unsigned char attr; // attribute byte

unsigned char lcase; // Case for base and extensionunsigned char ctime_ms; // Creation time, millisecondsunsigned char ctime[2]; // Creation timeunsigned char cdate[2]; // Creation dateunsigned char adate[2]; // Last access dateunsigned char reserved[2]; // reserved values (ignored)

unsigned char time[2]; // time stamp unsigned char date[2]; // date stamp unsigned char start[2]; // starting cluster number unsigned char size[4]; // size of the file };

Fat fs data structure: msdos_fs.h

struct msdos_dir_entry {__u8 name[8],ext[3]; /* name and extension */__u8 attr; /* attribute bits */__u8 lcase; /* Case for base and extension */__u8 ctime_cs; /* Creation time, centiseconds (0-199) */__le16 ctime; /* Creation time */__le16 cdate; /* Creation date */__le16 adate; /* Last access date */__le16 starthi;/* High 16 bits of cluster in FAT32 */__le16 time,date,start;/* time, date and first cluster */__le32 size; /* file size (in bytes) */

};

Network

• The networking code is kept in net with most of the include files in include/net.

• The BSD socket code is in net/socket.c and the IP version 4 INET socket code is in net/ipv4/af_inet.c.

• The generic protocol support code (including the sk_buff handling routines) is in net/core with the TCP/IP networking code in net/ipv4 and net/ipv6. The network device drivers are in drivers/net.

TCP Finite State Machine (FSM )

• /net/ipv4/tcp.c– TCP_SYN_SENT– TCP_SYN_RECV– TCP_ESTABLISHED– TCP_FIN_WAIT1– TCP_FIN_WAIT2– TCP_CLOSING– TCP_TIME_WAIT– TCP_CLOSE_WAIT– TCP_LAST_ACK– TCP_CLOSE

Big-Endian & Little-Endian

Big-Endian Little-Endian

Endianness in networking

• Networks generally use big-endian order.• In fact, the Internet Protocol (IP) defines a standard

big-endian network byte order. • This byte order is used for all numeric values in the

packet headers and by many higher level protocols and file formats that are designed for use over IP.

• The Berkeley sockets API defines a set of functions to convert 16- and 32-bit integers to and from network byte order:– The htonl (host-to-network-long) and htons (host-to-

network-short) functions convert 32-bit and 16-bit values respectively from machine (host) to network order

– Whereas the ntohl and ntohs functions convert from network to host order.

Determining the byte order

#define BIG_ENDIAN 0

#define LITTLE_ENDIAN 1

int TestByteOrder() { short int word = 0x0001;

char *byte = (char *) &word;

return(byte[0] ? LITTLE_ENDIAN : BIG_ENDIAN);

}

Endianess in kernel: linux/include/ip.hstruct iphdr {#if defined(__LITTLE_ENDIAN_BITFIELD)

__u8 ihl:4,version:4;

#elif defined (__BIG_ENDIAN_BITFIELD)__u8 version:4,

ihl:4;#else#error "Please fix <asm/byteorder.h>"#endif

__u8 tos;__be16 tot_len;__be16 id;__be16 frag_off;__u8 ttl;__u8 protocol;__u16 check;__be32 saddr;__be32 daddr;/*The options start here. */

};

Endianness in TCP Heaher include/linux/tcp.h

struct tcphdr {__u16 source;__u16 dest;__u32 seq;__u32 ack_seq;

#if defined(__LITTLE_ENDIAN_BITFIELD)__u16 res1:4,

doff:4,fin:1,syn:1,rst:1,psh:1,ack:1,urg:1,ece:1,cwr:1;

Endianness in TCP Heaher include/linux/tcp.h (Cont.)

#elif defined(__BIG_ENDIAN_BITFIELD)__u16 doff:4,

res1:4,cwr:1,ece:1,urg:1,ack:1,psh:1,rst:1,syn:1,fin:1;

Modules

• The kernel module code is partially in the kernel and partially in the modules package. The kernel code is all in kernel/modules.c with the data structures and kernel demon kerneld messages in include/linux/module.h and include/linux/kerneld.h respectively.

• You may want to look at the structure of an ELF object file in include/linux/elf.h.

Thank you!

Browsing Linux Kernel Source

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