How do I rewrite global variables before loading a BPF object?

[Ephemeraler]

Before that, I use libbpf and C to write bpf controller. In *.bpf.c, I declare a global variable (filter_pid). and I set value (skel->bss->filter_pid = 1234) in *.c. Now, what should I do in cilium/ebpf. when I executed go generate, I might not find this variable In bpf_**.go. 3Q.

[ti-mo]

@Ephemeraler ebpf-go implements this using CollectionSpec.RewriteConstants().

Given the following global variable in C:

volatile const uint32_t arg;

The following Go code takes care of rewriting that constant to hold the value 1 at runtime:

err := spec.RewriteConstants(map[string]interface{}{
	"arg":  uint32(1),
})

[ti-mo]

@hossein-bakhtiari-revolut To be safe, always make section names part of the variable declaration:

static volatile const uint32_t some_variable __section(".rodata.foo");

Mind, .rodata is considered a prefix, you can use any name you'd like. Variables declared const should never end up in .bss, but the same advice applies for non-const globals. If you declare static volatiles, explicitly put them into .data to avoid them ending up in .bss if you leave them uninitialized.

[g0dA]

@thediveo sry, i just saw this email reminder

the code example

// in *.c
volatile const uint32_t arg;


// in *.go
err := spec.RewriteConstants(map[string]interface{}{
    "arg":  uint32(1),
})

but this variable will in .rodata section, this need BPF_F_RDONLY, was patched from bpf: add program side {rd, wr}only support for maps

so, this feature also need your kernel version support

[thediveo]

can you please elaborate? my example works for me.

[g0dA]

useful is good, i just say that kernel version have effect, if your kernel version is low, you may get an error like:

map create: read- and write-only maps not supported (requires >= v5.2)

[thediveo]

thanks for clarifying!