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:

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On Tue, Aug 30, 2016 at 10:22:46PM +0200, Jakub Kicinski wrote:

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On Tue, 30 Aug 2016 21:07:50 +0200, Daniel Borkmann wrote:

quoted

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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.

quoted

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.