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.
Vincent
