program: add get_fd_by_pin() api

After a eBPF program is loaded to the kernel and the loader process exited,
a user still may want to attach this program to PROG_ARRAY of another one.
Add an API to recover the file descriptor from a pinned path.

Signed-off-by: Ivan Koveshnikov <[email protected]>

libbpf-rs/src/program.rs

@@ -624,6 +624,30 @@ impl<'obj> Program<'obj> {
         Ok(prog_info.id)
     }
 
+    /// Returns program fd by pin
+    pub fn get_fd_by_pin<P: AsRef<Path>>(path: P) -> Result<OwnedFd> {
+        let path_c = util::path_to_cstring(path)?;
+        let path_ptr = path_c.as_ptr();
+        let fd = util::parse_ret_i32(unsafe { libbpf_sys::bpf_obj_get(path_ptr) })?;
+
+        let fd = unsafe { OwnedFd::from_raw_fd(fd) };
+
+        // A pinned path may represent any kind of an object, verify that
+        // it represents a program before returning descriptor to the caller.
+        let mut info: libbpf_sys::bpf_prog_info = unsafe { mem::zeroed() };
+        let mut size = mem::size_of_val(&info) as u32;
+        // SAFETY: All pointers are derived from references and hence valid.
+        util::parse_ret(unsafe {
+            libbpf_sys::bpf_prog_get_info_by_fd(
+                fd.as_raw_fd(),
+                &mut info as *mut libbpf_sys::bpf_prog_info,
+                &mut size as *mut u32,
+            )
+        })?;
+
+        Ok(fd)
+    }
+
     /// Returns flags that have been set for the program.
     pub fn flags(&self) -> u32 {
         unsafe { libbpf_sys::bpf_program__flags(self.ptr.as_ptr()) }

libbpf-rs/tests/test.rs

@@ -748,6 +748,28 @@ fn test_object_loading_pinned_map_from_path() {
     );
 }
 
+#[tag(root)]
+#[test]
+fn test_object_loading_from_pin() {
+    bump_rlimit_mlock();
+
+    let path = "/sys/fs/bpf/myprog_test_pin_to_load_from_path";
+    let prog_name = "handle__sched_switch";
+
+    let mut obj = get_test_object("runqslower.bpf.o");
+    let mut prog = get_prog_mut(&mut obj, prog_name);
+    prog.pin(path).expect("pinning prog failed");
+    let prog_id = Program::get_id_by_fd(prog.as_fd()).expect("failed to determine prog id");
+
+    let pinned_prog_fd = Program::get_fd_by_pin(path).expect("failed to get fd of pinned prog");
+    let pinned_prog_id =
+        Program::get_id_by_fd(pinned_prog_fd.as_fd()).expect("failed to determine pinned prog id");
+
+    assert_eq!(prog_id, pinned_prog_id);
+
+    prog.unpin(path).expect("unpinning program failed");
+}
+
 #[tag(root)]
 #[test]
 fn test_object_loading_loaded_map_from_id() {