readCon speedup

When looking at runc strace output, I noticed that selinux attr files
are read byte by byte:

	read(8, "u", 1) = 1
	read(8, "n", 1) = 1
	....

Apparently, this happens because fmt.Fscanf("%s") works that way.

Let's read the whole file via ioutil.ReadAll instead.

The added benchmark shows a drastic improvement:

name            old time/op    new time/op    delta
CurrentLabel-4    26.4µs ± 1%     4.3µs ± 1%   -83.73%  (p=0.004 n=6+5)

name            old alloc/op   new alloc/op   delta
CurrentLabel-4      352B ± 0%      776B ± 0%  +120.45%  (p=0.002 n=6+6)

name            old allocs/op  new allocs/op  delta
CurrentLabel-4      9.00 ± 0%      7.00 ± 0%   -22.22%  (p=0.002 n=6+6)

Note that increased allocation size is caused by the fact that ReadAll
preallocates a 512 bytes buffer, which is very reasonable.

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

go-selinux/selinux_linux.go

@@ -274,12 +274,15 @@ func readCon(fpath string) (string, error) {
 	if err := isProcHandle(in); err != nil {
 		return "", err
 	}
+	return readConFd(in)
+}
 
-	var retval string
-	if _, err := fmt.Fscanf(in, "%s", &retval); err != nil {
+func readConFd(in *os.File) (string, error) {
+	data, err := ioutil.ReadAll(in)
+	if err != nil {
 		return "", err
 	}
-	return strings.Trim(retval, "\x00"), nil
+	return string(bytes.TrimSuffix(data, []byte{0})), nil
 }
 
 // classIndex returns the int index for an object class in the loaded policy,
@@ -664,11 +667,7 @@ func readWriteCon(fpath string, val string) (string, error) {
 		return "", err
 	}
 
-	var retval string
-	if _, err := fmt.Fscanf(f, "%s", &retval); err != nil {
-		return "", err
-	}
-	return strings.Trim(retval, "\x00"), nil
+	return readConFd(f)
 }
 
 // setExecLabel sets the SELinux label that the kernel will use for any programs

go-selinux/selinux_linux_test.go

@@ -509,3 +509,17 @@ func BenchmarkChcon(b *testing.B) {
 		}
 	}
 }
+
+func BenchmarkCurrentLabel(b *testing.B) {
+	var (
+		l   string
+		err error
+	)
+	for n := 0; n < b.N; n++ {
+		l, err = CurrentLabel()
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+	b.Log(l)
+}