prog: recognize ENOTSUPP in testRun()

Not all program types support PROG_TEST_RUN on all kernel versions.
Previously, when testing a program type that didn't have a test runner
implemented, the following error would be returned:

  Error when running: can't test program: can't run test: errno 524

Since this isn't overly descriptive, make it clear to the caller that
the kernel supports test runs, just not for the particular program type.

Also fixes haveProgTestRun on kernels between 4.12 (where PROG_TEST_RUN
was introduced) and 4.16 (where sk_filter test runner was added).

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

prog.go

@@ -590,6 +590,7 @@ func (p *Program) Benchmark(in []byte, repeat int, reset func()) (uint32, time.D
 
 var haveProgTestRun = internal.FeatureTest("BPF_PROG_TEST_RUN", "4.12", func() error {
 	prog, err := NewProgram(&ProgramSpec{
+		// SocketFilter does not require privileges on newer kernels.
 		Type: SocketFilter,
 		Instructions: asm.Instructions{
 			asm.LoadImm(asm.R0, 0, asm.DWord),
@@ -612,15 +613,23 @@ var haveProgTestRun = internal.FeatureTest("BPF_PROG_TEST_RUN", "4.12", func() e
 	}
 
 	err = sys.ProgRun(&attr)
-	if errors.Is(err, unix.EINVAL) {
+	switch {
+	case errors.Is(err, unix.EINVAL):
 		// Check for EINVAL specifically, rather than err != nil since we
 		// otherwise misdetect due to insufficient permissions.
 		return internal.ErrNotSupported
-	}
-	if errors.Is(err, unix.EINTR) {
+
+	case errors.Is(err, unix.EINTR):
 		// We know that PROG_TEST_RUN is supported if we get EINTR.
 		return nil
+
+	case errors.Is(err, unix.ENOTSUPP):
+		// The first PROG_TEST_RUN patches shipped in 4.12 didn't include
+		// a test runner for SocketFilter. ENOTSUPP means PROG_TEST_RUN is
+		// supported, but not for the program type used in the probe.
+		return nil
 	}
+
 	return err
 })
 
@@ -670,6 +679,10 @@ func (p *Program) testRun(in []byte, repeat int, reset func()) (uint32, []byte,
 			continue
 		}
 
+		if errors.Is(err, unix.ENOTSUPP) {
+			return 0, nil, 0, fmt.Errorf("kernel doesn't support testing program type %s: %w", p.Type(), ErrNotSupported)
+		}
+
 		return 0, nil, 0, fmt.Errorf("can't run test: %w", err)
 	}