Re: [PATCH] bpf: use the expected instruction for NOPs

[David Faust <david.faust@oracle.com>]

Hi Nick!

>> The BPF ISA does not have a no-op instruction.
> 
> Just an aside - wouldn't it be a good idea if the ISA did define an explicit
> encoding for no-ops ?

In my opinion, yes that would be good. But, eBPF is effectively defined
by its implementation in the kernel, so we have to support whatever the
kernel decides for the ISA.

> 
>> There are different ways
>> to achieve no-op, but in practice the Linux kernel verifier expects a
>> particular form, namely "ja 0". Update the define md_single_noop_insn
>> for BPF to reflect this.
> 
> Thanks for pointing this out.  I took the original encoding from the
> gas/testsuite/gas/all/org-1.s file, in the hopes that it would be correct
> one.
>  

I wasn't actually aware of this until yesterday. It was your patch
adding the .nop directive that prompted digging into precisely what the
kernel expects for no-op; thank you.

(The kernel eBPF verifier has an optimization built into it to remove
no-ops, and it defines no-op as 'ja 0'.)

>> gas/
>> 	* config/tc-bpf.h (md_single_noop_insn): Use 'ja 0' for no-op.
> 
> Approved and applied.
> 
> Cheers
>   Nick
> 

Thanks!

David