What should I have to fix a bug?

1. Bug. Well known particular bug.
2. A buggy kernel.
3. Bit of luck.

Note:

• Having a repeatable bug is more then 50% of success.
• All examples are from 2.6.17.13 (i386)

Function printk().

printk is a very useful function similar to printf(). This function works everywhere and at any time (apart from early stage of booting the kernel when video isn't initialized). It uses log levels to tell the console the importance of each message.

Full list of levels:

1. KERN_EMERG <-- the most important

2. KERN_ALERT
3. KERN_CRIT
4. KERN_ERR
5. KERN_WARNING
6. KERN_NOTICE
7. KERN_INFO

8. KERN_DEBUG <-- the least important

The console will print messages only with a level higher than console_loglevel. By default printk uses DEFAULT_MESSAGE_LOGLEVEL == KERN_WARNING (but this may be changed in the future).

printk() uses a cyclic buffer to manage the messages. Next klogd reads the messages (using /proc/kmsg) from the buffer and gives them to syslogd which writes them to /var/log/messages. (You can configure syslogd by editing /etc/syslog.conf).

Examples:

printk(loglevel "mesages");

'

From: Linux/arch/mips/mm/tlbex.c

185         if (arg & ~RS_MASK)
186                 printk(KERN_WARNING "TLB synthesizer field overflow\n");

37         if (!(errst0 & PI_ERR_ST0_VALID_MASK)) {
38                 printk();
39                 return;
40         }     

732         if (omap_dma_in_1510_mode()) {
733                 printk(KERN_ERR "DMA linking is not supported in 1510 mode\n");
734                 BUG();
735                 return;
736         } 

1221         BUG_ON(lcd_dma.active); 

40                 if (options & DMA_PANIC_ON_ERROR)
41                         panic("request_dma error!"); 

398         if (!sht->detect) {
399                 printk(KERN_WARNING "scsi_register() called on new-style "
400                                     "template for driver %s\n", sht->name);
401                 dump_stack();
402         } 

1. SysRq+b Restart computer

2. SysRq+e Send SIGTERM to all tasks (with out init !!!)

3. SysRq+h Help

4. SysRq+i Send SIGKILL to all tasks (with out init !!!)

5. SysRq+k Kill all tasks ran from this console

6. SysRq+l Send SIGTKILL to all tasks (with init !!!)

7. SysRq+m Dump core and show it on console

8. SysRq+o Halt system and shutdown it

9. SysRq+p Print CPU registers on console

10. SysRq+r Change keyboard from RAW to XLATE

11. SysRq+s Save dirty buffers on HDD

12. SysRq+t Show current task info on console

13. SysRq+u Unmount all filesystems, and remount read only

Note that every user can use SysRq keys, and it can work improperly on an unstable system.

How to use non-exist debugger?

Note: use Additional compiling options to make gdb more functional.

1. gdb (it is not a patch it is GNU debugger)

+ simple to use

• gdb vmlinux /proc/kcore call gdb with vmlinux(not compressed image of kernel, in the top level kernel source directory) and optional /proc/kcore - memory of kernel(you must be root)
• p varaible - show what variable contain
• disassemble name_of_function - disassemble function
• etc.
• - you can not change variables or other data

• kgdb (connect 2 computers, one with kdb and second with gdb)

• +you can change data +all functions of basic gdb are implemented

-you must configure connection

1. kdb

+only one computer needed +all functions of gdb implemented +simple to use

• type "break" in console

• starts on oops

When everything fail.