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

[Yonghong Song <yonghong.song@linux.dev>]

On 7/7/25 12:07 PM, Eduard Zingerman wrote:
> On Thu, 2025-07-03 at 11:21 -0700, Eduard Zingerman wrote:
>
> [...]
>
>>>>>    .jumptables
>>>>>      <subprog-rel-off-0>
>>>>>      <subprog-rel-off-1> | <--- jump table #1 symbol:
>>>>>      <subprog-rel-off-2> |        .size = 2   // number of entries in the jump table
>>>>>      ...                          .value = 1  // offset within .jumptables
>>>>>      <subprog-rel-off-N>                          ^
>>>>>                                                   |
>>>>>    .text                                          |
>>>>>      ...                                          |
>>>>>      <insn-N>     <------ relocation referencing -'
>>>>>      ...                  jump table #1 symbol
> [...]
>
> I think I got it working in:
> https://github.com/eddyz87/llvm-project/tree/separate-jumptables-section
>
> Changes on top of Yonghong's work.
> An example is in the attachment the gist is:
>
> -------------------------------
>
> $ clang --target=bpf -c -o jump-table-test.o jump-table-test.c
> There are 8 section headers, starting at offset 0xaa0:
>
> Section Headers:
>    [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
>    ...
>    [ 4] .jumptables       PROGBITS        0000000000000000 000220 000260 00      0   0  1
>    ...
>
> Symbol table '.symtab' contains 8 entries:
>     Num:    Value          Size Type    Bind   Vis       Ndx Name
>       ...
>       3: 0000000000000000   256 NOTYPE  LOCAL  DEFAULT     4 .BPF.JT.0.0
>       4: 0000000000000100   352 NOTYPE  LOCAL  DEFAULT     4 .BPF.JT.0.1
>       ...
>
> $ llvm-objdump --no-show-raw-insn -Sdr jump-table-test.o
> jump-table-test.o:      file format elf64-bpf
>
> Disassembly of section .text:
>
> 0000000000000000 <foo>:
>         ...
>         6:       r2 <<= 0x3
>         7:       r1 = 0x0 ll
>                  0000000000000038:  R_BPF_64_64  .jumptables
>         9:       r1 += r2
>        10:       r1 = *(u64 *)(r1 + 0x0)
>        11:       gotox r1
>        ...
>        34:       r2 <<= 0x3
>        35:       r1 = 0x100 ll
>                  0000000000000118:  R_BPF_64_64  .jumptables
>        37:       r1 += r2
>        38:       r1 = *(u64 *)(r1 + 0x0)
>        39:       gotox r1
>        ...
>
> -------------------------------
>
> The changes only touch BPF backend. Can be simplified a bit if I move
> MachineFunction::getJTISymbol to TargetLowering in the shared LLVM
> parts.

Thanks, Eduard. I actually also explored a little bit and came up with
the below patch:
   https://github.com/yonghong-song/llvm-project/tree/br-jt-v6-seperate-jmptable
the top commit is the addition on top of https://github.com/llvm/llvm-project/pull/133856.
I tried to leverage existing llvm infrastructure and it will support ELF/XCOFF/COFF
and all backends.

Anton, besides Eduard's patch, please also take a look at the above patch.