Support for C-style string arguments

[jonhoo]

While all probe arguments do have to be i64, it seems like addresses to null-terminated strings are supported both by the kernel and by bpftrace. We may want to make users' lives a little easier by also accepting CStr arguments and turning them into their own address.

[cuviper]

I could see this as some kind of ProbeArg trait which casts as necessary -- CStr would just need to call as_ptr(). It might even be possible to add an associated const for the sign+size, so we don't have to force everything to i64. See also:
https://github.com/cuviper/rust-libprobe/blob/431ac2999eb88e3a8ba5ee15df13557e234d9775/src/platform/systemtap.rs#L33-L54

[Zheaoli]

So, should we pass the str or String in Rust to the probe right now?

[Zheaoli]

Seems the following code is ok

        let c_str= CString::new(path).unwrap();

        probe::probe_lazy!(opendal, write_start, c_str.as_ptr());

[cuviper]

Yeah, right now it will work with any argument that can cast as isize -- so CString and CStr won't work directly, but their as_ptr() should be fine. You could do that with str and String too, or any other slice, but you'll probably want to pass another argument for the length in that case.

[bobrik]

A native support via something like ProbeArg would indeed be nice. Currently with probe_lazy the CString created lazily are dropped and scrambled before they can be read:

probe_lazy!(proxy, h1, {
    let uri = CString::new("/lol.txt").unwrap();
    uri.as_ptr()
});

$ sudo bpftrace -e 'usdt:target/debug/examples/load_balancer:proxy:h1 { printf("uri = %s\n", str(arg0)); }'
Attaching 1 probe...
uri = �d�s�

This happens with both probe! and probe_lazy! and can be avoided by creating the CString before a call to the probe, but that defies the purpose of lazy evaluation:

let uri = CString::new("/lol.txt").unwrap();
probe_lazy!(proxy, h1, { uri.as_ptr() });

$ sudo bpftrace -e 'usdt:target/debug/examples/load_balancer:proxy:h1 { printf("uri = %s\n", str(arg0)); }'
Attaching 1 probe...
uri = /lol.txt

[cuviper]

I think you can do this and still be lazy:

let uri;
probe_lazy!(proxy, h1, {
    uri = CString::new("/lol.txt").unwrap();
    uri.as_ptr()
});

It doesn't even need mut because there's only one (conditional) initialization.

[bobrik]

Indeed that does the trick, thanks!