Re: [PATCH bpf-next v5 1/6] arm64: ftrace: Add ftrace direct call support

[Xu Kuohai <xukuohai@huawei.com>]

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
> .