(2014/08/12 22:03), Wang Nan wrote:
> Hi Masami and everyone,
>
> When checking my code I found a problem: if we replace a stack operatinon
> instruction,
> it is possible that the emulate execution of such instruction destroy the
> stack used
> by kprobeopt:
>
>> +
>> +asm (
>> + ".global optprobe_template_entry\n"
>> + "optprobe_template_entry:\n"
>> + " sub sp, sp, #80\n"
>> + " stmia sp, {r0 - r14} \n"
>
> Here, trampoline code sub sp with 80 (0x50, I choose this number without much
> thinking), and then
> use stmia to push r0 - r14 (registers except pc) onto the stack. Assume the
> original sp is
> 0xd0000050, the stack becomes:
>
> 0xd0000000: r0
> 0xd0000004: r1
> 0xd0000008: r2
> ...
> 0xd0000038: r14
> 0xd000003c: r15 (place holder)
> 0xd0000040: cpsr (place holder)
> 0xd0000044: ?
> 0xd0000048: ?
> 0xd000004c: ?
> 0xd0000050: original stack
>
> If the replaced code operates stack, for example, push {r0 - r10}, it will
> overwrite our register.
> For that reason, sub sp, #80 is not enough, we need at least 64 bytes stack
> space, so the first instruction
> here should be sub sp, #128.
>
> However, it increase stack requirement. Moreover, although rare, there may be
> sp relative addressing,
> such as: str r1, [sp, #-132].
Hmm, I see the increasing stack is clearly hard to emulate, but
why is it hard to emulate sp relative instruction? It should
access the memory under the stack pointer.
> To make every situations safe, do you think we need to alloc a pre-cpu
> optprobe private stack?
Of course, that is one possible idea, but the simplest way is just not
optimizing such instructions. Why not can_optimize() check that? ;)
Thank you,
--
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu...@hitachi.com
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
