Fix handling of function prototype type declaration inference in BTF

We currently do not handle type declaration querying for function
prototypes correctly in our BTF logic. That can lead to errors during
skeleton generation.
With this change we treat them similar to how we treat functions
themselves (which can only occur in the form of pointers).

Signed-off-by: Daniel Müller <[email protected]>

libbpf-cargo/CHANGELOG.md

@@ -1,6 +1,8 @@
 Unreleased
 ----------
 - Added `Default` impl for generated `struct` types containing pointers
+- Fixed handling of function prototypes type declaration inference in BTF and
+  skeleton generation
 - Improved error reporting in build script usage
 - Bumped minimum Rust version to `1.64`
 

libbpf-cargo/src/gen/btf.rs

@@ -134,12 +134,12 @@ impl<'s> GenBtf<'s> {
             }
             BtfKind::Struct | BtfKind::Union | BtfKind::Enum | BtfKind::Enum64 =>
                 self.get_type_name_handling_anon_types(&ty).into_owned(),
-            //    // The only way a variable references a function is through a function pointer.
-            //    // Return c_void here so the final def will look like `*mut c_void`.
-            //    //
-            //    // It's not like rust code can call a function inside a bpf prog either so we don't
-            //    // really need a full definition. `void *` is totally sufficient for sharing a pointer.
-            BtfKind::Func => "std::ffi::c_void".to_string(),
+            // The only way a variable references a function is through a function pointer.
+            // Return c_void here so the final def will look like `*mut c_void`.
+            //
+            // It's not like rust code can call a function inside a bpf prog either so we don't
+            // really need a full definition. `void *` is totally sufficient for sharing a pointer.
+            BtfKind::Func | BtfKind::FuncProto => "std::ffi::c_void".to_string(),
             BtfKind::Var(t) => self.type_declaration(t.referenced_type())?,
             _ => bail!("Invalid type: {ty:?}"),
         });

libbpf-cargo/src/test.rs

@@ -1437,6 +1437,46 @@ pub struct Bar {
     assert_definition(&btf, &struct_foo, expected_output);
 }
 
+#[test]
+fn test_btf_dump_struct_definition_func_proto() {
+    let prog_text = r#"
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+
+struct with_func_proto {
+	struct with_func_proto *next;
+	void (*func)(struct with_func_proto *);
+};
+
+struct with_func_proto w;
+"#;
+
+    let expected_output = r#"
+#[derive(Debug, Copy, Clone)]
+#[repr(C)]
+pub struct with_func_proto {
+    pub next: *mut with_func_proto,
+    pub func: *mut std::ffi::c_void,
+}
+impl Default for with_func_proto {
+    fn default() -> Self {
+        with_func_proto {
+            next: std::ptr::null_mut(),
+            func: std::ptr::null_mut(),
+        }
+    }
+}
+"#;
+
+    let mmap = build_btf_mmap(prog_text);
+    let btf = btf_from_mmap(&mmap);
+
+    // Find our struct
+    let struct_foo = find_type_in_btf!(btf, types::Struct<'_>, "with_func_proto");
+
+    assert_definition(&btf, &struct_foo, expected_output);
+}
+
 #[test]
 fn test_btf_dump_struct_definition_long_array() {
     let prog_text = r#"