On 2/2/22 08:17, Yonghong Song wrote:
>
>
> On 2/1/22 10:07 AM, Vincent Li wrote:
>> On Fri, Jan 28, 2022 at 10:27 AM Vincent Li <vincent.mc.li@xxxxxxxxx> wrote:
>>>
>>> On Thu, Jan 27, 2022 at 5:50 PM Yonghong Song <yhs@xxxxxx> wrote:
>>>>
>>>>
>>>>
>>>> On 1/25/22 12:32 PM, Vincent Li wrote:
>>>>> On Tue, Jan 25, 2022 at 9:52 AM Vincent Li <vincent.mc.li@xxxxxxxxx> wrote:
>>>>>>
>>>>>> this is macro I suspected in my implementation that could cause issue with BTF
>>>>>>
>>>>>> #define ENABLE_VTEP 1
>>>>>> #define VTEP_ENDPOINT (__u32[]){0xec48a90a, 0xee48a90a, 0x1f48a90a,
>>>>>> 0x2048a90a, }
>>>>>> #define VTEP_MAC (__u64[]){0x562e984c3682, 0x582e984c3682,
>>>>>> 0x5eaaed93fdf2, 0x5faaed93fdf2, }
>>>>>> #define VTEP_NUMS 4
>>>>>>
>>>>>> On Tue, Jan 25, 2022 at 9:38 AM Vincent Li <vincent.mc.li@xxxxxxxxx> wrote:
>>>>>>>
>>>>>>> Hi
>>>>>>>
>>>>>>> While developing Cilium VTEP integration feature
>>>>>>> https://github.com/cilium/cilium/pull/17370, I found a strange issue
>>>>>>> that seems related to BTF and probably caused by my specific
>>>>>>> implementation, the issue is described in
>>>>>>> https://github.com/cilium/cilium/issues/18616, I don't know much about
>>>>>>> BTF and not sure if my implementation is seriously flawed or just some
>>>>>>> implementation bug or maybe not compatible with BTF. Strangely, the
>>>>>>> issue appears related to number of VTEPs I use, no problem with 1 or 2
>>>>>>> VTEP, 3, 4 VTEPs will have problem with BTF, any guidance from BTF
>>>>>>> experts are appreciated :-).
>>>>>>>
>>>>>>> Thanks
>>>>>>>
>>>>>>> Vincent
>>>>>
>>>>> Sorry for previous top post
>>>>>
>>>>> it looks the compiler compiles the cilium bpf_lxc.c to bpf_lxc.o
>>>>> differently and added " [21] .rodata.cst32 PROGBITS
>>>>> 0000000000000000 00011e68" when following macro exceeded 2 members
>>>>>
>>>>> #define VTEP_ENDPOINT (__u32[]){0xec48a90a, 0xee48a90a, 0x1f48a90a,
>>>>> 0x2048a90a, }
>>>>>
>>>>> no ".rodata.cst32" compiled in bpf_lxc.o when above VTEP_ENDPOINT
>>>>> member <=2. any reason why compiler would do that?
>>>>
>>>> Regarding to why compiler generates .rodata.cst32, the reason is
>>>> you have some 32-byte constants which needs to be saved somewhere.
>>>> For example,
>>>>
>>>> $ cat t.c
>>>> struct t {
>>>> long c[2];
>>>> int d[4];
>>>> };
>>>> struct t g;
>>>> int test()
>>>> {
>>>> struct t tmp = {.c = {1, 2}, .d = {3, 4}};
>>>> g = tmp;
>>>> return 0;
>>>> }
>>>>
>>>> $ clang -target bpf -O2 -c t.c
>>>> $ llvm-readelf -S t.o
>>>> ...
>>>> [ 4] .rodata.cst32 PROGBITS 0000000000000000 0000a8 000020
>>>> 20 AM 0 0 8
>>>> ...
>>>>
>>>> In the above code, if you change the struct size, say from 32 bytes to
>>>> 40 bytes, the rodata.cst32 will go away.
>>>
>>> Thanks Yonghong! I guess it is cilium/ebpf needs to recognize rodata.cst32 then
>>
>> Hi Yonghong,
>>
>> Here is a follow-up question, it looks cilium/ebpf parse vmlinux and
>> stores BTF type info in btf.Spec.namedTypes, but the elf object file
>> provided by user may have section like rodata.cst32 generated by
>> compiler that does not have accompanying BTF type info stored in
>> btf.Spec.NamedTypes for the rodata.cst32, how vmlinux can be
>> guaranteed to have every BTF type info from application/user provided
>> elf object file ? I guess there is no guarantee.
>
> vmlinux holds kernel types. rodata.cst32 holds data. If the type of
> rodata.cst32 needs to be emitted, the type will be encoded in bpf
> program BTF.
>
> Did you actually find an issue with .rodata.cst32 section? Such a
> section is typically generated by the compiler for initial data
> inside the function and llvm bpf backend tries to inline the
> values through a bunch of load instructions. So even you see
> .rodata.cst32, typically you can safely ignore it.
>
>>
>> Vincent
>
Hi Yonghong,
Thanks for the reproducer. Couldn't figure out what to do with .rodata.cst32,
since there are no symbols and no BTF info for that section.
The values found in .rodata.cst32 are indeed inlined in the bytecode as you
mentioned, so it seems like we can ignore it.
Why does the compiler emit these sections? cilium/ebpf assumed up until now
that all sections starting with '.rodata' are datasecs and must be loaded into
the kernel, which of course needs accompanying BTF.
What other .rodata.* should we expect?
Thanks,
Timo
