COMP 3000 2012 Week 7 Notes

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Linux source code spelunking

see linux source cross reference

  • /documention
  • /arch --> platform specific code
  • /drives --> drivers for devices
  • /fs -> file system stuff (ext4 is here... open.c)

do\_ prefix on something is a wrapper function that preprocesses args

Looking at fs/ directory now...

  • All these help with the vfs
  • Relationship between dentry (directory entry) and inode
  • Contents are inodes, dentry is a string representingo


Filesystems

  • blocks -> actual data
  • inodes -> metadata (index node).

2 Types of inode:

    • directories and files are inodes
    • directory inode -> name to inode mapping
    • file inode -> size, last modified, pointers to data blocks, permissions, etc...
      • pointers to data wihtin inodes tell you where the data is, so that you can accessit


How to explore the kernel from userspace instead of messing about in the source code.

Look at the output of mount

  • some kernel services are mounted to various folders (ex: proc mounted to /proc)


/dev and udev

/dev is dynamically generated by udev


Now, in /dev/, do ls -l 


 drwxr-xr-x 2 root root         700 Oct  5 22:23 block
 drwxr-xr-x 2 root root         120 Oct  5 22:23 bsg
 brw-rw---- 1 root disk      7,   7 Oct  5 22:23 loop7
 crw------- 1 root root     10, 237 Oct  5 22:23 loop-control
 drwxr-xr-x 2 root root          80 Oct  5 22:23 mapper
 crw------- 1 root root     10, 227 Oct  5 22:23 mcelog
 crw-r--r-- 1 root root     10,  62 Oct  5 22:23 rfkill
 lrwxrwxrwx 1 root root           4 Oct  5 22:23 rtc -> rtc0
 crw------- 1 root root    254,   0 Oct  5 22:23 rtc0
 brw-rw---- 1 root disk      8,   0 Oct  5 22:23 sda
 brw-rw---- 1 root disk      8,   1 Oct  5 22:23 sda1
 brw-rw---- 1 root disk      8,   2 Oct  5 22:23 sda2
 crw-rw---- 1 root disk     21,   2 Oct  5 22:23 sg2
 crw------- 1 root root     21,   3 Oct  5 22:23 sg3
 lrwxrwxrwx 1 root root           8 Oct  5 22:23 shm -> /run/shm
 crw------- 1 root root     10, 231 Oct  5 22:23 snapshot
 drwxr-xr-x 3 root root         180 Oct  5 22:23 snd
 lrwxrwxrwx 1 root root          15 Oct  5 22:23 stderr -> /proc/self/fd/2
 lrwxrwxrwx 1 root root          15 Oct  5 22:23 stdout -> /proc/self/fd/1
 crw-rw-rw- 1 root tty       5,   0 Oct 17 08:47 tty
 crw--w---- 1 root tty       4,   0 Oct  5 22:23 tty0
 crw------- 1 root root    252,   5 Oct  5 22:23 usbmon5
 drwxr-xr-x 4 root root          80 Oct  5 22:23 v4l
 crw-rw---- 1 root tty       7,   0 Oct  5 22:23 vcs
 crw-rw-rw- 1 root root      1,   5 Oct  5 22:23 zero


Row 5: Major Number. Row 6: Minor Number.

Every driver may control different hardware controllers. These controllers are assigned major numbers. Each device they control is assigned a minor number. Any device can be identified with a major minor number pair. (see SCO documentation for major and minor numbers)

  • d = directory
  • b = block device --> drives, etc.
  • c = character file --> write or output sequences of characters (printers, zero, null)

random, urandom, null, zero are all device files, but they're really just special function in the kernel. They use the file api, you can read and write to most of them.


in /dev. run df . 

 Filesystem     1K-blocks  Used Available Use% Mounted on
 udev              499124     8    499116   1% /dev


The file system for this is udev, dyamically generated

udev rules are set in /etc/udev, sets who can access what devices. Sets policy.

see man udev

what is udevd vs udev?

  • udev is filesystem type and a command
  • udevd is a daemon that monitors changes

If you kill udevd, your kernel will not be informed of new changes (plugged in devices,etc..). They won't be listed in /dev/. The user cannot be informed of what is actually mounted.

LOOK UP udevd


Note: initrd (initial ram disk) has it's own /dev/ folder to allow SPECIAL devices to be mounted to before the file system and the root FS's /dev/ is loaded

/dev is defined in posix standard


/proc

Kernel state is defined as special files too! We don't need need special functions, just look at the files in /proc/

  • all numbers are process IDs
  • other devices like uptime
  • Ownership, groups, 'n stuff are all displayed in the files

Note: all of the sizes of these files are 0. Indicates that they dynamically generated

When you call ps, all it's doing is walking the proc directory.

Proc is NOT defined in the POSIX standard. It's a linux thing.

/sys/ and /proc/ are basically the same thing, but /sys is alot more regimented, proc is loose. <---- LOOOOOOK UP!


Ex: 

 strace ifconfig

You'll notice that much of what ifconfig does is reading kernel /proc/ files


NOTE: /dev/ -> devices /proc/ and /sys/ -> KERNEL STATE


LINKS

Hardlink: association between name and inode. Can have many per inode.

Symlink: pointer to a name, not to an inode. You can have pointer to a name that's a has a hardlink to an actual piece of data

ex: 

 ls -l /bin | grep gunzip 
 -rwxr-xr-x 2 root root    2251 Feb  8  2012 gunzip

Two links to gunzip's inode. Meaning, uncompress points to the same inode gunzip and uncompress binaries have the same inode

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