* Karim Yaghmour ([email protected]) 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 <[email protected]>
*
* 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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Key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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