Understanding the linux kernel-ch4-Exception Handling

Exception handlers have a standard structure consisting of three steps:1.     Save the contents of most registers in the Kernel Mode stack (this part is coded in assembly language).2.     Handle the exception by means of a high-level C function.



3.     Exit from the handler by means of the ret_from_exception( ) function.












trap_init( )//i386/kernel/traps.c















doublefault_fn() //i386/kernel/doublefault.c
























Saving the Registers for the Exception Handler





    handler_name:







        pushl $0 /* only for some exceptions */





        pushl $do_handler_name






        jmp error_code

















error_code: 







1. Saves the registers that might be used by the high-level C function on the stack.

2. Issues a cld instruction to clear the direction flag DF of eflags , 


   thus making sure that autoincreases on the edi and esi registers will be used with string instructions 3. Copies the hardware error code saved in the stack at location esp+36 in edx. 


   Stores the value -1 in the same stack location




   this value is used to separate 0x80 exceptions from other exceptions.


4. Loads edi with the address of the high-level do_handler_name( ) C function


    saved in the stack at location esp+32; writes the contents of es in that stack location

5. Loads in the eax register the current top location of the Kernel Mode stack. 


   This address identifies the memory cell containing the last register value saved in step 1

6. Loads the user data Segment Selector into the ds and es registers.


7. Invokes the high-level C function whose address is now stored in edi












Entering and Leaving the Exception Handler





do_trap() 







store the hardware error code and the exception vector in the process descriptor of current, 

and then send a suitable signal to that process:




current->thread.error_code = error_code;





    current->thread.trap_no = vector;





    force_sig(sig_number, current);
















ret_from_exception()

















most exceptions are handled simply by sending a Unix signal to the process that caused the exception.
The action to be taken is thus deferred until the process receives the signal; as a result, the kernel is able to process the exception quickly