BPF tail call not called - c

Question

In the following code BPF program tail_prog is not getting tail called from main_prog:
#include <linux/bpf.h>
#include <bpf/bpf_helpers.h>
struct bpf_map_def SEC("maps") jump_table = {
.type = BPF_MAP_TYPE_PROG_ARRAY,
.key_size = sizeof(__u32),
.value_size = sizeof(__u32),
.max_entries = 8,
};
SEC("xdp")
int main_prog(struct xdp_md *ctx) {
bpf_printk("Making tail call");
bpf_tail_call(ctx, &jump_table, 0);
return XDP_PASS;
}
SEC("xdp_1")
int tail_prog(struct xdp_md *ctx) {
bpf_printk("Inside tail call");
return XDP_PASS;
}
char _license[] SEC("license") = "GPL";
I observe only the print in main_prog is printed.
I'm loading the BPF programs using Cilium's eBPF Go package. Here's the code for loading the programs and maps:
type BpfObjects struct {
MainProg *ebpf.Program `ebpf:"main_prog"`
TailProg *ebpf.Program `ebpf:"tail_prog"`
JumpTable *ebpf.Map `ebpf:"jump_table"`
}
var objects BpfObjects
spec, err := ebpf.LoadCollectionSpec("prog.o")
if err != nil {
log.Fatalln("ebpf.LoadCollectionSpec", err)
}
if err := spec.LoadAndAssign(&objects, nil); err != nil {
log.Fatalln("ebpf.LoadAndAssign", err)
}
objects.JumpTable.Update(0, objects.TailProg.FD(), ebpf.UpdateAny)
According to this, the jump table has be initialized from user space, which is what I think the last line above is supposed to do. However, I don't see any difference whether that line is there or not.

Answer 1

I wasn't looking at the error returned from the Update function: Update can't marshal key: encoding int: binary.Write: invalid type int. Therefore, the program array map was not updated. I changed to the following:
err = objects.JumpTable.Update(uint32(0), uint32(objects.CopyHttpHostnameProg.FD()), ebpf.UpdateAny)
if err != nil {
println("Update", err.Error())
}
If you pass 0 as the key, the size of the key is 8 bytes, which is why you have to do uint32(0), which matches the map's definition. Now the tail calls succeed.