link: don't use regexp for ID validity checks

Replace regular expressions with custom functions checking input strings
byte by byte.

Regular expressions bring in a big library as a dependency, and they
incur some overhead during startup (to compile regexes).

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

link/kprobe.go

@@ -7,7 +7,6 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
-	"regexp"
 	"runtime"
 	"strings"
 	"sync"
@@ -27,11 +26,6 @@ var (
 		value uint64
 		err   error
 	}{}
-
-	// Allow `.` in symbol name. GCC-compiled kernel may change symbol name
-	// to have a `.isra.$n` suffix, like `udp_send_skb.isra.52`.
-	// See: https://gcc.gnu.org/gcc-10/changes.html
-	rgxKprobe = regexp.MustCompile("^[a-zA-Z_][0-9a-zA-Z_.]*$")
 )
 
 type probeType uint8
@@ -144,6 +138,33 @@ func Kretprobe(symbol string, prog *ebpf.Program, opts *KprobeOptions) (Link, er
 	return lnk, nil
 }
 
+// isValidKprobeSymbol implements the equivalent of a regex match
+// against "^[a-zA-Z_][0-9a-zA-Z_.]*$".
+func isValidKprobeSymbol(s string) bool {
+	if len(s) < 1 {
+		return false
+	}
+
+	for i, c := range []byte(s) {
+		switch {
+		case c >= 'a' && c <= 'z':
+		case c >= 'A' && c <= 'Z':
+		case c == '_':
+		case i > 0 && c >= '0' && c <= '9':
+
+		// Allow `.` in symbol name. GCC-compiled kernel may change symbol name
+		// to have a `.isra.$n` suffix, like `udp_send_skb.isra.52`.
+		// See: https://gcc.gnu.org/gcc-10/changes.html
+		case i > 0 && c == '.':
+
+		default:
+			return false
+		}
+	}
+
+	return true
+}
+
 // kprobe opens a perf event on the given symbol and attaches prog to it.
 // If ret is true, create a kretprobe.
 func kprobe(symbol string, prog *ebpf.Program, opts *KprobeOptions, ret bool) (*perfEvent, error) {
@@ -153,7 +174,7 @@ func kprobe(symbol string, prog *ebpf.Program, opts *KprobeOptions, ret bool) (*
 	if prog == nil {
 		return nil, fmt.Errorf("prog cannot be nil: %w", errInvalidInput)
 	}
-	if !rgxKprobe.MatchString(symbol) {
+	if !isValidKprobeSymbol(symbol) {
 		return nil, fmt.Errorf("symbol '%s' must be a valid symbol in /proc/kallsyms: %w", symbol, errInvalidInput)
 	}
 	if prog.Type() != ebpf.Kprobe {
@@ -471,9 +492,9 @@ func closeTraceFSProbeEvent(typ probeType, group, symbol string) error {
 // randomGroup generates a pseudorandom string for use as a tracefs group name.
 // Returns an error when the output string would exceed 63 characters (kernel
 // limitation), when rand.Read() fails or when prefix contains characters not
-// allowed by rgxTraceEvent.
+// allowed by isValidTraceID.
 func randomGroup(prefix string) (string, error) {
-	if !rgxTraceEvent.MatchString(prefix) {
+	if !isValidTraceID(prefix) {
 		return "", fmt.Errorf("prefix '%s' must be alphanumeric or underscore: %w", prefix, errInvalidInput)
 	}
 

link/perf_event.go

@@ -6,7 +6,6 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
-	"regexp"
 	"runtime"
 	"strconv"
 	"strings"
@@ -45,11 +44,6 @@ import (
 var (
 	tracefsPath = "/sys/kernel/debug/tracing"
 
-	// Trace event groups, names and kernel symbols must adhere to this set
-	// of characters. Non-empty, first character must not be a number, all
-	// characters must be alphanumeric or underscore.
-	rgxTraceEvent = regexp.MustCompile("^[a-zA-Z_][0-9a-zA-Z_]*$")
-
 	errInvalidInput = errors.New("invalid input")
 )
 
@@ -373,3 +367,28 @@ var haveBPFLinkPerfEvent = internal.FeatureTest("bpf_link_perf_event", "5.15", f
 	}
 	return err
 })
+
+// isValidTraceID implements the equivalent of a regex match
+// against "^[a-zA-Z_][0-9a-zA-Z_]*$".
+//
+// Trace event groups, names and kernel symbols must adhere to this set
+// of characters. Non-empty, first character must not be a number, all
+// characters must be alphanumeric or underscore.
+func isValidTraceID(s string) bool {
+	if len(s) < 1 {
+		return false
+	}
+	for i, c := range []byte(s) {
+		switch {
+		case c >= 'a' && c <= 'z':
+		case c >= 'A' && c <= 'Z':
+		case c == '_':
+		case i > 0 && c >= '0' && c <= '9':
+
+		default:
+			return false
+		}
+	}
+
+	return true
+}

link/perf_event_test.go

@@ -45,8 +45,8 @@ func TestTraceReadID(t *testing.T) {
 	}
 }
 
-func TestTraceEventRegex(t *testing.T) {
-	var tests = []struct {
+func TestTraceValidID(t *testing.T) {
+	tests := []struct {
 		name string
 		in   string
 		fail bool
@@ -67,8 +67,8 @@ func TestTraceEventRegex(t *testing.T) {
 				exp = "fail"
 			}
 
-			if rgxTraceEvent.MatchString(tt.in) == tt.fail {
-				t.Errorf("expected string '%s' to %s regex match", tt.in, exp)
+			if isValidTraceID(tt.in) == tt.fail {
+				t.Errorf("expected string '%s' to %s valid ID check", tt.in, exp)
 			}
 		})
 	}

link/tracepoint.go

@@ -36,7 +36,7 @@ func Tracepoint(group, name string, prog *ebpf.Program, opts *TracepointOptions)
 	if prog == nil {
 		return nil, fmt.Errorf("prog cannot be nil: %w", errInvalidInput)
 	}
-	if !rgxTraceEvent.MatchString(group) || !rgxTraceEvent.MatchString(name) {
+	if !isValidTraceID(group) || !isValidTraceID(name) {
 		return nil, fmt.Errorf("group and name '%s/%s' must be alphanumeric or underscore: %w", group, name, errInvalidInput)
 	}
 	if prog.Type() != ebpf.TracePoint {