prog: test for linking unimplemented forward function declarations

This test makes sure the linker allows calls into missing symbols
so that they can be provided by the caller after the CollectionSpec
has been emitted.

Makefile

@@ -34,6 +34,7 @@ TARGETS := \
 	testdata/iproute2_map_compat \
 	testdata/map_spin_lock \
 	testdata/subprog_reloc \
+	testdata/fwd_decl \
 	internal/btf/testdata/relocs
 
 .PHONY: all clean container-all container-shell generate

linker_test.go

@@ -1,9 +1,11 @@
 package ebpf
 
 import (
+	"errors"
 	"testing"
 
 	"github.com/cilium/ebpf/asm"
+	"github.com/cilium/ebpf/internal"
 	"github.com/cilium/ebpf/internal/testutils"
 )
 
@@ -54,3 +56,47 @@ func TestFindReferences(t *testing.T) {
 		t.Errorf("Expected return code 1337, got %d", ret)
 	}
 }
+
+func TestForwardFunctionDeclaration(t *testing.T) {
+	testutils.Files(t, testutils.Glob(t, "testdata/fwd_decl-*.elf"), func(t *testing.T, file string) {
+		coll, err := LoadCollectionSpec(file)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if coll.ByteOrder != internal.NativeEndian {
+			t.Skip()
+		}
+
+		spec := coll.Programs["call_fwd"]
+
+		// This program calls an unimplemented forward function declaration.
+		_, err = NewProgram(spec)
+		if !errors.Is(err, errUnsatisfiedReference) {
+			t.Fatal("Expected an error wrapping errUnsatisfiedReference, got:", err)
+		}
+
+		// Append the implementation of fwd().
+		spec.Instructions = append(spec.Instructions,
+			asm.Mov.Imm32(asm.R0, 23).Sym("fwd"),
+			asm.Return(),
+		)
+
+		// The body of the subprog we appended does not come with BTF func_infos,
+		// so the verifier will reject it. Load without BTF.
+		spec.BTF = nil
+
+		prog, err := NewProgram(spec)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		ret, _, err := prog.Test(make([]byte, 14))
+		if err != nil {
+			t.Fatal("Running program:", err)
+		}
+		if ret != 23 {
+			t.Fatalf("Expected 23, got %d", ret)
+		}
+	})
+}

testdata/fwd_decl.c

@@ -0,0 +1,10 @@
+/* This file excercises the ELF loader. It is not a valid BPF program. */
+
+#include "common.h"
+
+// Forward function declaration, never implemented.
+int fwd();
+
+__section("socket") int call_fwd() {
+	return fwd();
+}