[cfe-dev] BPF: question in regard to .rodata.cst32

[Y Song via cfe-dev]

On Fri, Jan 28, 2022 at 10:24 AM Vincent Li via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
>
> On Fri, Jan 28, 2022 at 10:21 AM Vincent Li <vincent.mc.li at gmail.com> wrote:
> >
> > Hi,
> >
> > Hope this is the right place to ask questions, I ran into an issue in
> > https://github.com/cilium/cilium/issues/18616 where I have defined
> > macro in header file node_config.h like below:
> >
> > grep 'VTEP' /var/run/cilium/state/globals/node_config.h
> >
> > #define ENABLE_VTEP 1
> > #define VTEP_ENDPOINT (__u32[]){ 0xeb48a90a, 0xec48a90a, 0xed48a90a,
> > 0xee48a90a, }
> > #define VTEP_MAC (__u64[]){ 0x562e984c3682, 0x552e984c3682,
> > 0x542e984c3682, 0x532e984c3682, }
> > #define VTEP_NUMS 4
> >
> > then the cilium bpf/bpf_lxc.c file include the node_config.h file and
> > code to use above macro like below:
> >
> > #ifdef ENABLE_VTEP
> >
> >         {
> >                 int i;
> >                 for (i = 0; i < VTEP_NUMS; i++) {
> >                         if (tunnel_endpoint == VTEP_ENDPOINT[i]) {
> >                                 vtep_mac = VTEP_MAC[i];
> >                                 break;
> >                        }
> >                 }
> >
> >                 if (vtep_mac && tunnel_endpoint) {
> >                         if (eth_store_daddr(ctx, (__u8 *)&vtep_mac, 0) < 0)
> >                                 return DROP_WRITE_ERROR;
> >                         return __encap_and_redirect_with_nodeid(ctx,
> > tunnel_endpoint,
> >
> >  WORLD_ID, monitor);
> >                 }
> >         }
> > #endif
> >
> > cilium will invoke clang to compile the bpf_lxc.o like below:
> >
> > clang ...CUT... -DENABLE_VTEP=1
> > -I/home/go/src/github.com/f5devcentral/cilium/bpf/include
> > -I/home/go/src/github.com/f5devcentral/cilium/bpf -D__NR_CPUS__=2 -O2
> > -g -target bpf -std=gnu89 -nostdinc -emit-llvm -Wall -Wextra -Werror
> > -Wshadow -Wno-address-of-packed-member -Wno-unknown-warning-option
> > -Wno-gnu-variable-sized-type-not-at-end -Wdeclaration-after-statement
> > -Wimplicit-int-conversion -Wenum-conversion -c bpf_lxc.c -o bpf_lxc.ll
> >
> > llc -march=bpf -mattr=dwarfris -mcpu=v2 -filetype=obj -o bpf_lxc.o bpf_lxc.ll
> >
> > the bpf_lxc.o file ended up with a .rodata.cst32 section like below
> >
> > readelf -x .rodata.cst32 /var/run/cilium/state/1378_next_fail/bpf_lxc.o
> >
> > Hex dump of section '.rodata.cst32':
> >   0x00000000 82364c98 2e560000 82364c98 2e550000 .6L..V...6L..U..
> >   0x00000010 82364c98 2e540000 82364c98 2e530000 .6L..T...6L..S..
> >
> > as you can see the .rodata.cst32 contains the VTEP_MAC data.
> >
> > but if I reduce the VTEP_ENDPOINT/VTEP_MAC member to 2 like below, no
> > any .rodata generated from the clang compiler
> >
> > grep 'VTEP' /var/run/cilium/state/globals/node_config.h
> > #define ENABLE_VTEP 1
> > #define VTEP_ENDPOINT (__u32[]){ 0xeb48a90a, 0xec48a90a, }
> > #define VTEP_MAC (__u64[]){ 0x562e984c3682, 0x552e984c3682, }
> > #define VTEP_NUMS 2
> >
> > readelf -e  /var/run/cilium/state/1378/bpf_lxc.o | grep 'rodata', nothing
> >
> > The problem I have is cilium/ebpf fail to recognize .rodata.cst32 and
> > lead cilium to fail.
> >
> > My question is why clang creates .rodata.cst32 when
> > VTEP_ENDPOINT/VTEP_MAC exceed 2 members, and why no rodata when the
> > member <= 2.
> >
> > I assume cilium/ebpf project will likely to address the issue to
> > recognize .rodata.cst32, I am just wondering if there is anything from
> > clang BPF side that needs to be addressed.
> >
> > Thank you for you patience to read this email :-)
> >
> > Regards,
> >
> > Vincent
>
> Hi Yonghong,
>
> Sorry to miss your answer to BPF mailing list before I write this
> email, I guess it works as designed then.

Right. As I explained in bpf mailing list
  https://lore.kernel.org/bpf/CAK3+h2xD5h9oKqvkCTsexKprCjL0UEaqzBJ3xR65q-k0y_Rg1A@mail.gmail.com/T/#m0e7debc59b44542c03a1adae4900701aac1302e6
This is expected based on your code pattern.

>
> Thanks
>
> Vincent
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