Messages in this thread | | | Subject | Re: [PATCH v15 04/10] arm64: Kprobes with single stepping support | From | David Long <> | Date | Fri, 22 Jul 2016 11:51:32 -0400 |
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On 07/22/2016 06:16 AM, Catalin Marinas wrote: > On Thu, Jul 21, 2016 at 02:33:52PM -0400, David Long wrote: >> On 07/21/2016 01:23 PM, Marc Zyngier wrote: >>> On 21/07/16 17:33, David Long wrote: >>>> On 07/20/2016 12:09 PM, Marc Zyngier wrote: >>>>> On 08/07/16 17:35, David Long wrote: >>>>>> +#define MAX_INSN_SIZE 1 >>>>>> +#define MAX_STACK_SIZE 128 >>>>> >>>>> Where is that value coming from? Because even on my 6502, I have a 256 >>>>> byte stack. >>>>> >>>> >>>> Although I don't claim to know the original author's thoughts I would >>>> guess it is based on the seven other existing implementations for >>>> kprobes on various architectures, all of which appear to use either 64 >>>> or 128 for MAX_STACK_SIZE. The code is not trying to duplicate the >>>> whole stack. > [...] >>> My main worry is that whatever value you pick, it is always going to be >>> wrong. This is used to preserve arguments that are passed on the stack, >>> as opposed to passed by registers). We have no idea of what is getting >>> passed there so saving nothing, 128 bytes or 2kB is about the same. It >>> is always wrong. >>> >>> A much better solution would be to check the frame pointer, and copy the >>> delta between FP and SP, assuming it fits inside the allocated buffer. >>> If it doesn't, or if FP is invalid, we just skip the hook, because we >>> can't reliably execute it. >> >> Well, this is the way it works literally everywhere else. It is a documented >> limitation (Documentation/kprobes.txt). Said documentation may need to be >> changed along with the suggested fix. > > The document states: "Up to MAX_STACK_SIZE bytes are copied". That means > the arch code could always copy less but never more than MAX_STACK_SIZE. > What we are proposing is that we should try to guess how much to copy > based on the FP value (caller's frame) and, if larger than > MAX_STACK_SIZE, skip the probe hook entirely. I don't think this goes > against the kprobes.txt document but at least it (a) may improve the > performance slightly by avoiding unnecessary copy and (b) it avoids > undefined behaviour if we ever encounter a jprobe with arguments passed > on the stack beyond MAX_STACK_SIZE. >
OK, it sounds like an improvement. I do worry a little about unexpected side effects. I'm just asking if we can accept the existing code as now complete enough (in that I believe it matches the other implementations) and make this enhancement something for the next release cycle, allowing the existing code to be exercised by a wider audience and providing ample time to test the new modification? I'd hate to get stuck in a mode where this patch gets repeatedly delayed for changes that go above and beyond the original design.
Thanks, -dl
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