internal: recognize register based errors from the verifier

The verifier has a bunch of places where it bails out due to register state
not being what it expects:

    λ  linux master ✓ grep -E "verbose\(env, \"R" kernel/bpf/verifier.c | wc -l
    60

These errors are difficult to understand without additional context. Include the
previous line of log output if we encounter one of these.

internal/errors.go

@@ -73,12 +73,8 @@ func (le *VerifierError) Error() string {
 	}
 
 	lines := log[n-1:]
-	if n >= 2 && (strings.HasPrefix(log[n-1], "\t") || le.Truncated) {
-		// Add one more line of context if the last line starts with a tab,
-		// or if it has been truncated.
-		// For example:
-		//     [13] STRUCT drm_rect size=16 vlen=4
-		//     \tx1 type_id=2
+	if n >= 2 && (includePreviousLine(log[n-1]) || le.Truncated) {
+		// Add one more line of context if it aids understanding the error.
 		lines = log[n-2:]
 	}
 
@@ -113,6 +109,28 @@ func (le *VerifierError) Error() string {
 	return b.String()
 }
 
+// includePreviousLine returns true if the given line likely is better
+// understood with additional context from the preceding line.
+func includePreviousLine(line string) bool {
+	// We need to find a good trade off between understandable error messages
+	// and too much complexity here. Checking the string prefix is ok, requiring
+	// regular expressions to do it is probably overkill.
+
+	if strings.HasPrefix(line, "\t") {
+		// [13] STRUCT drm_rect size=16 vlen=4
+		// \tx1 type_id=2
+		return true
+	}
+
+	if len(line) >= 2 && line[0] == 'R' && line[1] >= '0' && line[1] <= '9' {
+		// 0: (95) exit
+		// R0 !read_ok
+		return true
+	}
+
+	return false
+}
+
 // Format the error.
 //
 // Understood verbs are %s and %v, which are equivalent to calling Error(). %v

internal/errors_test.go

@@ -69,6 +69,10 @@ func TestVerifierError(t *testing.T) {
 	t.Log(truncated)
 	qt.Assert(t, truncated.Truncated, qt.IsTrue)
 	qt.Assert(t, truncated.Error(), qt.Contains, "str_off: 3166088: str_len: 228")
+
+	invalidR0 := readErrorFromFile(t, "testdata/invalid-R0.log")
+	t.Log(invalidR0)
+	qt.Assert(t, invalidR0.Error(), qt.Contains, "0: (95) exit: R0 !read_ok")
 }
 
 func ExampleVerifierError() {