feat: Improved bpf_loop probe

The previously used ASM probe didn't work reliably on new kernels, which
were rejecting the program, because of lack of valid function pointer
being used as a callback.

Introduce a small C probe which has a minimal, but valid `bpf_loop`
call.

Fixes #300

crates/bpf-feature-autodetect/build.rs

@@ -1,3 +1,5 @@
 fn main() -> Result<(), Box<dyn std::error::Error>> {
-    bpf_builder::build("test_lsm", "src/test_lsm.bpf.c")
+    bpf_builder::build("test_bpf_loop", "src/test_bpf_loop.bpf.c")?;
+    bpf_builder::build("test_lsm", "src/test_lsm.bpf.c")?;
+    Ok(())
 }

crates/bpf-feature-autodetect/src/bpf_loop.rs

@@ -0,0 +1,29 @@
+use aya::{include_bytes_aligned, programs::TracePoint, Bpf, BpfError};
+use log::warn;
+
+fn load_probe() -> Result<(), BpfError> {
+    let mut bpf = Bpf::load(include_bytes_aligned!(concat!(
+        env!("OUT_DIR"),
+        "/test_bpf_loop.none.bpf.o"
+    )))?;
+    let program: &mut TracePoint = bpf.program_mut("probe_bpf_loop").unwrap().try_into()?;
+    program.load()?;
+    Ok(())
+}
+
+pub fn bpf_loop_supported() -> bool {
+    match load_probe() {
+        Ok(_) => true,
+        Err(e) => {
+            let err_msg = "`bpf_loop` helper is not supported by the kernel";
+
+            if let Ok(true) = std::env::var("RUST_BACKTRACE").map(|s| s == "1") {
+                warn!("{err_msg}: {e}");
+            } else {
+                warn!("{err_msg}");
+            }
+
+            false
+        }
+    }
+}

crates/bpf-feature-autodetect/src/lib.rs

@@ -10,12 +10,16 @@ use libc::SYS_bpf;
 use thiserror::Error;
 
 pub mod atomic;
+pub mod bpf_loop;
 pub mod func;
 pub mod insn;
 pub mod kernel_version;
 pub mod lsm;
 
-use crate::{atomic::atomics_supported, func::func_id_supported, lsm::lsm_supported};
+use crate::{
+    atomic::atomics_supported, bpf_loop::bpf_loop_supported, func::func_id_supported,
+    lsm::lsm_supported,
+};
 
 /// Size of the eBPF verifier log.
 const LOG_SIZE: usize = 4096;
@@ -34,12 +38,7 @@ pub fn autodetect_features() -> BpfFeatures {
             bpf_func_id::BPF_FUNC_get_current_task_btf,
             BpfProgType::BPF_PROG_TYPE_CGROUP_SKB,
         ),
-        bpf_loop: func_id_supported(
-            "bpf_loop",
-            bpf_func_id::BPF_FUNC_loop,
-            // Program type doesn't matter here.
-            BpfProgType::BPF_PROG_TYPE_KPROBE,
-        ),
+        bpf_loop: bpf_loop_supported(),
         lsm: lsm_supported(),
     }
 }

crates/bpf-feature-autodetect/src/test_bpf_loop.bpf.c

@@ -0,0 +1,12 @@
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+
+static int callback(void *ctx, u32 index) {
+  return 0;
+}
+
+SEC("tracepoint")
+int probe_bpf_loop(void *ctx) {
+  bpf_loop(5, callback, NULL, 0);
+  return 0;
+}