Doc: update Documentation/exception.txt

     Kernel level exception handling in Linux 2.1.8

     Kernel level exception handling in Linux

  Commentary by Joerg Pommnitz <joerg@raleigh.ibm.com>

When a process runs in kernel mode, it often has to access user

void do_page_fault(struct pt_regs *regs, unsigned long error_code)

in arch/i386/mm/fault.c. The parameters on the stack are set up by 

the low level assembly glue in arch/i386/kernel/entry.S. The parameter

in arch/x86/mm/fault.c. The parameters on the stack are set up by

the low level assembly glue in arch/x86/kernel/entry_32.S. The parameter

regs is a pointer to the saved registers on the stack, error_code

contains a reason code for the exception.

Since we jump to the contents of fixup, fixup obviously points

to executable code. This code is hidden inside the user access macros.

I have picked the get_user macro defined in include/asm/uaccess.h as an

example. The definition is somewhat hard to follow, so let's peek at 

I have picked the get_user macro defined in arch/x86/include/asm/uaccess.h

as an example. The definition is somewhat hard to follow, so let's peek at

the code generated by the preprocessor and the compiler. I selected

the get_user call in drivers/char/console.c for a detailed examination.

the get_user call in drivers/char/sysrq.c for a detailed examination.

The original code in console.c line 1405:

The original code in sysrq.c line 587:

        get_user(c, buf);

The preprocessor output (edited to become somewhat readable):