Messages in this thread | | | Date | Wed, 13 Aug 2014 00:12:55 +0900 | From | Masami Hiramatsu <> | Subject | Re: [PATCH v4] kprobes: arm: enable OPTPROBES for ARM 32 |
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(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@hitachi.com
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