Add get key_size and value_size for OpenMapImpl

[fkcrazy001]

Currently there is no way to get a map's key_size and value_size from OpenMapImpl. This makes it difficult to use libbpf-rs as a “parser-only” library.

There is a valid use case where people only want to use libbpf-rs to open eBPF object files and extract metadata (programs, maps, etc.), without actually loading/attaching them to the kernel. For example, a custom eBPF map implementation + uBPF could be used to run eBPF programs entirely in userspace. In that scenario, we need access to map metadata such as key_size and value_size in order to recreate the maps on the userspace side.

Right now, some of the APIs needed for this are not exposed. I have a local vendored version where I implemented this functionality myself, but I would like to propose adding an official API for it.

My suggestion is:

• Expose map metadata (at least key_size and value_size, and possibly other fields like flags) from OpenMapImpl or from a related public type.
• Ensure that these fields can be queried before any actual kernel loading/attachment happens, so libbpf-rs can be used purely as a parser/inspector for eBPF object files.

Would this be something you’d consider supporting? I think this just requires adding some wrapper of libbpf c apis in Rust

[d-e-s-o]

Not opposed to adding it, but it depends on what the implementation looks like, given that this is a rather niche use case. Did you just extend the OpenMapImpl impl for your use case?

[d-e-s-o]

And actually seeing that we already expose name and map_type getters on OpenMapImpl, which are already duplicated with MapCore methods, I suppose adding key_size and value_size would be acceptable.

[fkcrazy001]

Not opposed to adding it, but it depends on what the implementation looks like, given that this is a rather niche use case. Did you just extend the OpenMapImpl impl for your use case?

Yes，actually I add some apis like max_entries for type OpenMap. Below is my implementation, quite straighforward. And if you're willing to, I'd like to pull request

impl<'obj> OpenMap<'obj> {
    // ...other methods
    /// Retrieve the map flags.
    pub fn map_flags(&self) -> u32 {
        unsafe { libbpf_sys::bpf_map__map_flags(self.ptr.as_ptr()) }
    }

    /// Retrieve the map numa node.
    pub fn numa_node(&self) -> u32 {
        unsafe { libbpf_sys::bpf_map__numa_node(self.ptr.as_ptr()) }
    }

    /// Retrieve the key size of the map in bytes.
    pub fn key_size(&self) -> u32 {
        unsafe { libbpf_sys::bpf_map__key_size(self.ptr.as_ptr()) }
    }

    /// Retrieve the value size of the map in bytes.
    pub fn value_size(&self) -> u32 {
        unsafe { libbpf_sys::bpf_map__value_size(self.ptr.as_ptr()) }
    }
}

[d-e-s-o]

Sure, feel free to open a PR.

[fkcrazy001]

PR Merged: #1385