[PATCH v4] kprobes: arm: enable OPTPROBES for ARM 32

[Masami Hiramatsu]

(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.pt at hitachi.com