Re: [RFC bpf-next 8/9] libbpf: support llvm-generated indirect jumps

[Anton Protopopov <a.s.protopopov@gmail.com>]

On 25/07/09 10:11PM, Eduard Zingerman wrote:
> On Wed, 2025-07-09 at 01:38 -0700, Eduard Zingerman wrote:
> > On Tue, 2025-07-08 at 22:58 -0700, Eduard Zingerman wrote:
> > 
> > [...]
> > 
> > > This seems to work:
> > > https://github.com/eddyz87/llvm-project/tree/separate-jumptables-section.1
> 
> [...]
> 
> > I think this is a correct form, further changes should be LLVM
> > internal.
> 
> Pushed yet another update. Jump table entries computation was off by 1.
> Here is a comment from the commit:
> 
> --- 8< --------------------------------
> 
> Emit JX instruction anchor label:
> 
>        .reloc 0, FK_SecRel_8, BPF.JT.0.0
>        gotox r1
>   .LBPF.JX.0.0:                          <--- this
> 
> This label is used to compute jump table entries:
> 
>                  .--- basic block label
>                  v
>   .L0_0_set_7 = LBB0_7 - .LBPF.JX.0.0    <---- JX anchor label
>   ...
>   BPF.JT.0.0:                            <---- JT definition
>        .long   .L0_0_set_7
> 
> The anchor needs to be placed after gotox to follow BPF
> jump offset rules: dest_pc == jump_pc + off + 1.
> For example:
> 
>   1: gotox r1 // suppose r1 value corresponds to to LBB0_7
>      ...
>   5: <insn>   // LBB0_7 physical address
> 
> In order to jump to 5 from 1 offset read from jump table has to be 3,
> hence anchor should be placed at 2.
> 
> -------------------------------- >8 ---
> 
> Please let me know if this works end-to-end.

Thanks! I will be testing this today with my patchset.