Re: [PATCH v2] arm: eBPF JIT compiler

[Daniel Borkmann]

On 06/13/2017 08:56 AM, Shubham Bansal wrote:
> Hi Daniel, Kees, David, Russel,
>
> > > Any plans to implement above especially BPF_JMP | BPF_CALL in near future?
> > > Reason why I'm asking is that i) currently the arm32 cBPF JIT implements
> > > all of the cBPF extensions (except SKF_AD_RANDOM and SKF_AD_VLAN_TPID).
> > > Some of the programs that were JITed before e.g. using SKF_AD_CPU would now
> > > fall back to the eBPF interpreter due to lack of translation in JIT, but
> > > also ii) that probably most (if not all) of eBPF programs use BPF helper
> > > calls heavily, which will still redirect them to the interpreter right now
> > > due to lack of BPF_JMP | BPF_CALL support, so it's really quite essential
> > > to have it implemented.
> >
> > I can try for BPF_JMP | BPF_CALL. I didn't do it last time because I
> > thought, it would make the code look messy and become pain to get it
> > through the review.
> > For this, I have to map eBPF arguments with arm ABI arguments and move
> > ebpf arguments to corresponding arm ABI arguments, as eBPF arguments
> > doesn't match with arm ABI arguments.
> > Let me try that if its possible.
>
> Okay. I looked at it, tried few different solutions also. There is a
> problem with implementing BPF_JMP | BPF_CALL.
> Problem is transition between 4 byte and 8 byte arguments. Lets take a
> look a the following example to get a more clear look at the problem.
>
> Lets consider this function :
> CASE 1:                            foo(int a, int b, long long c, int d)
> For calling this function in arm 32 arch, I have to pass the arguments
> as following:
>                                           a -> r0
>                                           b -> r1
>                                           c -> r2, r3
>                                           d -> stack_top
>
> Now consider an another example function :
> CASE 2:                           bar(int a, int b, int c, int d)
> For calling this function in arm32 arch, I have to pass the arguments
> as following:
>                                         a -> r0
>                                         b -> r1
>                                         c -> r2
>                                         d -> r3
>
> So, you can clearly see the problem with it. There is no way of
> knowing which of the above way to pass the arguments. There are
> solutions possible:

Right.

> 1. One thing I can do is look at the address of the function to call
> and pass the argument accordingly but thats not really a robust
> solution as we have to change the arm32 JIT each time we add any new
> BPF helper function.

Yeah, that would be rather ugly.

> 2. Another solution is, if any of you guys can assure/confirm me that
> there will be only 4 byte argument passed to BPF helper functions in
> arm32 as of now and in future including the pointer as well, then I
> can just assume that each argument is passed as 4 byte value and my
> trimming the 8byte arguments to 4 bytes arguments wouldn't be a
> problem. In that case, arguments for CASE 1 and CASE 2 will be passed
> in the same way, i.e.
>                                         a -> r0
>                                         b -> r1
>                                         c -> r2
>                                         d -> r3
>
> Let me know what you think. I don't think I can find the solution to
> this problem other than those mentioned above. Would love to here any
> ideas from you guys.

Not all of the helpers have 4 or less byte arguments only, there are a
few with 8 byte arguments, so making that general assumption wouldn't
work. I guess what could be done is that helpers have a flag in struct
bpf_func_proto which indicates for JITs that all args are 4 byte on 32bit
so you could probably use convention similar to case2 for them. Presumably
for that information to process, the JIT might need to be reworked to
extract that via bpf_analyzer() that does a verifier run to re-analyze
the program like in nfp JIT case.

The other option could perhaps be to check the interpreter disasm of
___bpf_prog_run() with regards to how it handles BPF_JMP | BPF_CALL
helper call and do something similarly generic in the JIT as well.

> > > Did you also manage to get the BPF selftest suite running in the meantime
> > > (tools/testing/selftests/bpf/)? There are a couple of programs that clang
> > > will compile (test_pkt_access.o, test_xdp.o, test_l4lb.o, test_tcp_estats.o)
> > > and then test run.
> I will run these tests tonight. Hopefully I will be able to run them.

Ok.

> Any comments are welcome. Would love to here what you think about my
> solutions above.