Messages in this thread | | | Date | Wed, 10 Aug 2022 16:10:23 +0800 | Subject | Re: [PATCH bpf-next v5 1/6] arm64: ftrace: Add ftrace direct call support | From | Xu Kuohai <> |
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On 8/10/2022 1:03 AM, Florent Revest wrote: > On Thu, Jun 9, 2022 at 6:27 AM Xu Kuohai <xukuohai@huawei.com> wrote: >> On 6/7/2022 12:35 AM, Mark Rutland wrote: >>> On Thu, May 26, 2022 at 10:48:05PM +0800, Xu Kuohai wrote: >>>> On 5/26/2022 6:06 PM, Mark Rutland wrote: >>>>> On Thu, May 26, 2022 at 05:45:03PM +0800, Xu Kuohai wrote: >>>>>> On 5/25/2022 9:38 PM, Mark Rutland wrote: >>>>>>> On Wed, May 18, 2022 at 09:16:33AM -0400, Xu Kuohai wrote: >>>>>>>> As noted in that thread, I have a few concerns which equally apply here: >>>>>>> >>>>>>> * Due to the limited range of BL instructions, it's not always possible to >>>>>>> patch an ftrace call-site to branch to an arbitrary trampoline. The way this >>>>>>> works for ftrace today relies upon knowingthe set of trampolines at >>>>>>> compile-time, and allocating module PLTs for those, and that approach cannot >>>>>>> work reliably for dynanically allocated trampolines. >>>>>> >>>>>> Currently patch 5 returns -ENOTSUPP when long jump is detected, so no >>>>>> bpf trampoline is constructed for out of range patch-site: >>>>>> >>>>>> if (is_long_jump(orig_call, image)) >>>>>> return -ENOTSUPP; >>>>> >>>>> Sure, my point is that in practice that means that (from the user's PoV) this >>>>> may randomly fail to work, and I'd like something that we can ensure works >>>>> consistently. >>>>> >>>> >>>> OK, should I suspend this work until you finish refactoring ftrace? >>> >>> Yes; I'd appreciate if we could hold on this for a bit. >>> >>> I think with some ground work we can avoid most of the painful edge cases and >>> might be able to avoid the need for custom trampolines. >>> >> >> I'v read your WIP code, but unfortunately I didn't find any mechanism to >> replace bpf trampoline in your code, sorry. >> >> It looks like bpf trampoline and ftrace works can be done at the same >> time. I think for now we can just attach bpf trampoline to bpf prog. >> Once your ftrace work is done, we can add support for attaching bpf >> trampoline to regular kernel function. Is this OK? > > Hey Mark and Xu! :) > > I'm interested in this feature too and would be happy to help. > > I've been trying to understand what you both have in mind to figure out a way > forward, please correct me if I got anything wrong! :) > > > It looks like, currently, there are three places where an indirection to BPF is > technically possible. Chronologically these are: > > - the function's patchsite (currently there are 2 nops, this could become 4 > nops with Mark's series on per call-site ops) > > - the ftrace ops (currently called by iterating over a global list but could be > called more directly with Mark's series on per-call-site ops or by > dynamically generated branches with Wang's series on dynamic trampolines) > > - a ftrace trampoline tail call (currently, this is after restoring a full > pt_regs but this could become an args only restoration with Mark's series on > DYNAMIC_FTRACE_WITH_ARGS) > > > If we first consider the situation when only a BPF program is attached to a > kernel function: > - Using the patchsite for indirection (proposed by Xu, same as on x86) > Pros: > - We have BPF trampolines anyway because they are required for orthogonal > features such as calling BPF programs as functions, so jumping into that > existing JITed code is straightforward > - This has the minimum overhead (eg: these trampolines only save the actual > number of args used by the function in ctx and avoid indirect calls) > Cons: > - If the BPF trampoline is JITed outside BL's limits, attachment can > randomly fail > > - Using a ftrace op for indirection (proposed by Mark) > Pros: > - BPF doesn't need to care about BL's range, ftrace_caller will be in range > Cons: > - The ftrace trampoline would first save all args in an ftrace_regs only for > the BPF op to then re-save them in a BPF ctx array (as per BPF calling > convention) so we'd effectively have to do the work of saving args twice > - BPF currently uses DYNAMIC_FTRACE_WITH_DIRECT_CALLS APIs. Either arm64 > should implement DIRECT_CALLS with... an indirect call :) (that is, the > arch_ftrace_set_direct_caller op would turn back its ftrace_regs into > arguments for the BPF trampoline) or BPF would need to use a different > ftrace API just on arm64 (to define new ops, which, unless if they would be > dynamically JITed, wouldn't be as performant as the existing BPF > trampolines) > > - Using a ftrace trampoline tail call for indirection (not discussed yet iiuc) > Pros: > - BPF also doesn't need to care about BL's range > - This also leverages the existing BPF trampolines > Cons: > - This also does the work of saving/restoring arguments twice > - DYNAMIC_FTRACE_WITH_DIRECT_CALLS depends on DYNAMIC_FTRACE_WITH_REGS now > although in practice the registers kept by DYNAMIC_FTRACE_WITH_ARGS > should be enough to call BPF trampolines > > If we consider the situation when both ftrace ops and BPF programs are attached > to a kernel function: > - Using the patchsite for indirection can't solve this > > - Using a ftrace op for indirection (proposed by Mark) or using a ftrace > trampoline tail call as an indirection (proposed by Xu, same as on x86) have > the same pros & cons as in the BPF only situation except that this time we > pay the cost of registers saving twice for good reasons (we need args in both > ftrace_regs and the BPF ctx array formats anyway) > > > Unless I'm missing something, it sounds like the following approach would work: > - Always patch patchsites with calls to ftrace trampolines (within BL ranges) > - Always go through ops and have arch_ftrace_set_direct_caller set > ftrace_regs->direct_call (instead of pt_regs->orig_x0 in this patch) > - If ftrace_regs->direct_call != 0 at the end of the ftrace trampoline, tail > call it > > Once Mark's series on DYNAMIC_FTRACE_WITH_ARGS is merged, we would need to have > DYNAMIC_FTRACE_WITH_DIRECT_CALLS > depend on DYNAMIC_FTRACE_WITH_REGS || DYNAMIC_FTRACE_WITH_ARGS > BPF trampolines (the only users of this API now) only care about args to the > attachment point anyway so I think this would work transparently ? > > Once Mark's series on per-callsite ops is merged, the second step (going > through ops) would be significantly faster in the situation where only one > program is used, therefore one arch_ftrace_set_direct_caller op. > > Once Wang's series on dynamic trampolines is merged, the second step (going > through ops) would also be significantly faster in the case when multiple ops > are attached. > > > What are your thoughts? If this sounds somewhat sane, I'm happy to help out > with the implementation as well :) >
Hi Florent,
I'm struggling with how to attach bpf trampoline to regular kernel functions. I think your suggestion is fine. Thanks for the help!
> Thanks! > Florent > .
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