Use current kernel version as default if not specified

When a BPF program doesn't specify the target kernel version, the
most compatible option is to set the program kernel version to match
the currently running kernel.

aya/src/programs/mod.rs

@@ -88,7 +88,7 @@ pub use xdp::{Xdp, XdpError, XdpFlags};
 use crate::{
     generated::{bpf_attach_type, bpf_prog_info, bpf_prog_type},
     maps::MapError,
-    obj::{self, Function},
+    obj::{self, Function, KernelVersion},
     sys::{bpf_load_program, bpf_pin_object, bpf_prog_detach, bpf_prog_query, BpfLoadProgramAttrs},
 };
 
@@ -397,6 +397,14 @@ fn load_program(prog_type: bpf_prog_type, data: &mut ProgramData) -> Result<(),
         ..
     } = obj;
 
+    let target_kernel_version = match *kernel_version {
+        KernelVersion::Any => {
+            let (major, minor, patch) = crate::sys::kernel_version().unwrap();
+            KernelVersion::Version((major << 16) + (minor << 8) + patch)
+        }
+        _ => *kernel_version,
+    };
+
     let mut log_buf = VerifierLog::new();
     let mut retries = 0;
     let mut ret;
@@ -405,7 +413,7 @@ fn load_program(prog_type: bpf_prog_type, data: &mut ProgramData) -> Result<(),
             ty: prog_type,
             insns: instructions,
             license,
-            kernel_version: (*kernel_version).into(),
+            kernel_version: target_kernel_version.into(),
             expected_attach_type: data.expected_attach_type,
             attach_btf_obj_fd: data.attach_btf_obj_fd,
             attach_btf_id: data.attach_btf_id,