netdev - Re: [RFCv2 07/16] bpf: enable non-core use of the verfier

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Date:   Tue, 30 Aug 2016 18:18:06 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Jakub Kicinski <kubakici@...pl>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        netdev@...r.kernel.org, ast@...nel.org,
        dinan.gunawardena@...ronome.com, jiri@...nulli.us,
        john.fastabend@...il.com
Subject: Re: [RFCv2 07/16] bpf: enable non-core use of the verfier

On Tue, Aug 30, 2016 at 11:00:38PM +0200, Daniel Borkmann wrote:
> On 08/30/2016 10:48 PM, Alexei Starovoitov wrote:
> >On Tue, Aug 30, 2016 at 10:22:46PM +0200, Jakub Kicinski wrote:
> >>On Tue, 30 Aug 2016 21:07:50 +0200, Daniel Borkmann wrote:
> >>>>Having two modes seems more straight forward and I think we would only
> >>>>need to pay attention in the LD_IMM64 case, I don't think I've seen
> >>>>LLVM generating XORs, it's just the cBPF -> eBPF conversion.
> >>>
> >>>Okay, though, I think that the cBPF to eBPF migration wouldn't even
> >>>pass through the bpf_parse() handling, since verifier is not aware on
> >>>some of their aspects such as emitting calls directly (w/o *proto) or
> >>>arg mappings. Probably make sense to reject these (bpf_prog_was_classic())
> >>>if they cannot be handled anyway?
> >>
> >>TBH again I only use cBPF for testing.  It's a convenient way of
> >>generating certain instruction sequences.  I can probably just drop
> >>it completely but the XOR patch is just 3 lines of code so not a huge
> >>cost either...  I'll keep patch 6 in my tree for now.
> >
> >if xor matching is only need for classic, I would drop that patch
> >just to avoid unnecessary state collection. The number of lines
> >is not a concern, but extra state for state prunning is.
> >
> >>Alternatively - is there any eBPF assembler out there?  Something
> >>converting verifier output back into ELF would be quite cool.
> >
> >would certainly be nice. I don't think there is anything standalone.
> >btw llvm can be made to work as assembler only, but simple flex/bison
> >is probably better.
> 
> Never tried it out, but seems llvm backend doesn't have asm parser
> implemented?
> 
>   $ clang -target bpf -O2 -c foo.c -S -o foo.S
>   $ llvm-mc -arch bpf foo.S -filetype=obj -o foo.o
>   llvm-mc: error: this target does not support assembly parsing.
> 
> LLVM IR might work, but maybe too high level(?); alternatively, we could
> make bpf_asm from tools/net/ eBPF aware for debugging purposes. If you
> have a toolchain supporting libbfd et al, you could probably make use
> of bpf_jit_dump() (like JITs do) and then bpf_jit_disasm tool (from
> same dir as bpf_asm).

yes. llvm-based bpf asm is not complete. It's straightforward to add though.
It won't be going through IR. Only 'mc' (machine instruciton) layer.