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

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

On 7/7/25 2:44 PM, Yonghong Song wrote:
>
>
> 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.

Sorry, I have not looked at this patch for a while.

I briefly went through the discussions, the github patch Eduard provided seems
the right choice and please ignore my above patch. Also, as discussion
with Alexei and Eduard, a little bit more work is needed in llvm to help verifier.