Re: [PATCH] Linux Kernel Markers 0.5 for Linux 2.6.17 (with probe management)

[Mathieu Desnoyers]

* Karim Yaghmour (karim@xxxxxxxxxxx) wrote:
> 
> > Hrm, your comment makes me think of an interesting idea :
> > 
> > .align
> > jump_address:
> >   near jump to end
> > setup_stack_address:
> >   setup stack
> >   call empty function
> > end:
> > 
> > So, instead of putting nops in the target area, we fill it with a useful
> > function call. Near jump being 2 bytes, it might be much easier to modify.
> > If necessary, making sure the instruction is aligned would help to change it
> > atomically. If we mark the jump address, the setup stack address and the end
> > tag address with symbols, we can easily calculate (portably) the offset of the
> > near jump to activate either the setup_stack_address or end tags.
> 
> That's another possibility. It seems more C friendly than the simple
> short-jump+3bytes.
> 
Here is the implementation :-)

Comments are welcome.


/*****************************************************************************
 * marker.h
 *
 * Code markup for dynamic and static tracing. i386 architecture optimisations.
 *
 * (C) Copyright 2006 Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
 *
 * This file is released under the GPLv2.
 * See the file COPYING for more details.
 */

#define MARK_NEAR_JUMP_PREFIX "__mark_near_jump_"
#define MARK_NEAR_JUMP_SELECT_PREFIX "__mark_near_jump_select_"

/* Note : max 256 bytes between over_label and near_jump */
#define MARK_DO_JUMP(name)      \
        do { \
                __label__ near_jump; \
                volatile static void *__mark_near_jump_##name \
                        asm (MARK_NEAR_JUMP_PREFIX#name) \
                        __attribute__((unused)) = &&near_jump; \
                volatile static void *__mark_near_jump_select_##name \
                        asm (MARK_NEAR_JUMP_SELECT_PREFIX#name) \
                        __attribute__((unused)) = &&near_jump_select; \
                asm volatile (  ".align 16;\n\t" : : ); \
                asm volatile (  ".byte 0xeb;\n\t" : : ); \
near_jump_select: \
                asm volatile (  ".byte %0-%1;\n\t" : : \
                                "m" (*&&over_label),  "m" (*&&near_jump)); \
near_jump: \
                asm volatile (  "" : : ); \
        } while(0)


To change it, we can dynamically overwrite the __mark_near_jump_select_##name
value (a byte) to (__mark_jump_call_##name - __mark_near_jump_##name).

So we have one architecture specific optimisation within the architecture
agnostic marking mechanism.

Comments are welcome.

Mathieu


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