prog,syscalls: probe expected_attach_type field in BPF_PROG_LOAD

Discovered while running the test suite on kernel 4.14.

With the addition BPF_PROG_ATTACH tests in package link, the haveProgAttach
feature test was added, gated on 4.10. This helper uses the AttachType field,
which sets the bpf_attr expected_attach_type field, available as of 4.17.

There are more issues to address, but this patch fixes the tests on kernels
between 4.10 and 4.17.

Signed-off-by: Timo Beckers <timo@isovalent.com>

prog.go

@@ -58,7 +58,12 @@ type ProgramSpec struct {
 	Name string
 
 	// Type determines at which hook in the kernel a program will run.
-	Type       ProgramType
+	Type ProgramType
+
+	// AttachType of the program, needed to differentiate allowed context
+	// accesses in some newer program types like CGroupSockAddr.
+	//
+	// Ignored on kernels before 4.17.
 	AttachType AttachType
 
 	// Name of a kernel data structure or function to attach to. Its
@@ -265,17 +270,20 @@ func newProgramWithOptions(spec *ProgramSpec, opts ProgramOptions, handles *hand
 	}
 
 	attr := &sys.ProgLoadAttr{
-		ProgType:           sys.ProgType(spec.Type),
-		ProgFlags:          spec.Flags,
-		ExpectedAttachType: sys.AttachType(spec.AttachType),
-		License:            sys.NewStringPointer(spec.License),
-		KernVersion:        kv,
+		ProgType:    sys.ProgType(spec.Type),
+		ProgFlags:   spec.Flags,
+		License:     sys.NewStringPointer(spec.License),
+		KernVersion: kv,
 	}
 
 	if haveObjName() == nil {
 		attr.ProgName = sys.NewObjName(spec.Name)
 	}
 
+	if haveProgAttachType() == nil {
+		attr.ExpectedAttachType = sys.AttachType(spec.AttachType)
+	}
+
 	var err error
 	var targetBTF *btf.Spec
 	if opts.TargetBTF != nil {

syscalls.go

@@ -244,3 +244,32 @@ var haveProbeReadKernel = internal.FeatureTest("bpf_probe_read_kernel", "5.5", f
 	_ = fd.Close()
 	return nil
 })
+
+var haveProgAttachType = internal.FeatureTest("expected_attach_type", "4.17", func() error {
+	insns := asm.Instructions{
+		asm.Mov.Imm(asm.R0, 0),
+		asm.Return(),
+	}
+	buf := bytes.NewBuffer(make([]byte, 0, insns.Size()))
+	if err := insns.Marshal(buf, internal.NativeEndian); err != nil {
+		return err
+	}
+	bytecode := buf.Bytes()
+
+	fd, err := sys.ProgLoad(&sys.ProgLoadAttr{
+		ProgType: sys.ProgType(SocketFilter),
+		License:  sys.NewStringPointer("MIT"),
+		Insns:    sys.NewSlicePointer(bytecode),
+		InsnCnt:  uint32(len(bytecode) / asm.InstructionSize),
+		// Provide a non-zero value to trip check_uarg_tail_zero in the syscall handler.
+		ExpectedAttachType: sys.AttachType(1),
+	})
+	if errors.Is(err, unix.E2BIG) {
+		return internal.ErrNotSupported
+	}
+	if err != nil {
+		return err
+	}
+	_ = fd.Close()
+	return nil
+})

syscalls_test.go

@@ -54,3 +54,7 @@ func TestHaveInnerMaps(t *testing.T) {
 func TestHaveProbeReadKernel(t *testing.T) {
 	testutils.CheckFeatureTest(t, haveProbeReadKernel)
 }
+
+func TestHaveProgAttachType(t *testing.T) {
+	testutils.CheckFeatureTest(t, haveProgAttachType)
+}