Re: Encoding of V4 32-bit JA

[Eduard Zingerman <eddyz87@gmail.com>]

On Sun, 2023-07-23 at 21:14 +0200, Jose E. Marchesi wrote:
> > On Fri, 2023-07-21 at 18:19 +0200, Jose E. Marchesi wrote:
> > > Hi Yonghong.
> > > 
> > > This is from the v4 instructions proposal:
> > > 
> > >     ========  =====  =========================  ============
> > >     code      value  description                notes
> > >     ========  =====  =========================  ============
> > >     BPF_JA    0x00   PC += imm                  BPF_JMP32 only
> > > 
> > > Is this instruction using source 1 instead of 0?  Otherwise, it would
> > > have exactly the same encoding than the V3< JA instruction.  Is that
> > > what is intended?
> > > 
> > > TIA.
> > > 
> > 
> > Hi Jose,
> > 
> > I think that assumption is that `BPF_JMP32 | BPF_JA` is currently free:
> > - documentation [1] implies that only `BPF_JMP` should be used for `BPF_JA`
> >   (see "notes" column for the first line)
> > - BPF verifier rejects `BPF_JMP32 | BPF_JA`
> > - clang always generates `BPF_JMP | BPF_JA`
> 
> Makes sense, thanks for the info.
> 
> Do you know the precise pseudo-c assembly syntax to use for this
> instruction?

In [1] Yonghong uses the following form:

  gotol +0xcd9b

But it seems to be not specified in the documentation for the patch-set v3.

[1] https://reviews.llvm.org/D144829

> 
> > Thanks,
> > Eduard
> > 
> > [1] https://www.kernel.org/doc/html/latest/bpf/instruction-set.html#jump-instructions