portlist,tstest: skip tests on kernels with /proc/net/tcp regression

Linux kernel versions 6.6.102-104 and 6.12.42-45 have a regression
in /proc/net/tcp that causes seek operations to fail with "illegal seek".
This breaks portlist tests on these kernels.

Add kernel version detection for Linux systems and a SkipOnKernelVersions
helper to tstest. Use it to skip affected portlist tests on the broken
kernel versions.

Thanks to philiptaron for the list of kernels with the issue and fix.

Updates #16966

Signed-off-by: Andrew Dunham <andrew@tailscale.com>

tstest/kernel_linux.go

@@ -0,0 +1,50 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build linux
+
+package tstest
+
+import (
+	"strconv"
+	"strings"
+
+	"golang.org/x/sys/unix"
+)
+
+// KernelVersion returns the major, minor, and patch version of the Linux kernel.
+// It returns (0, 0, 0) if the version cannot be determined.
+func KernelVersion() (major, minor, patch int) {
+	var uname unix.Utsname
+	if err := unix.Uname(&uname); err != nil {
+		return 0, 0, 0
+	}
+	release := unix.ByteSliceToString(uname.Release[:])
+
+	// Parse version string (e.g., "5.15.0-...")
+	parts := strings.Split(release, ".")
+	if len(parts) < 3 {
+		return 0, 0, 0
+	}
+
+	major, err := strconv.Atoi(parts[0])
+	if err != nil {
+		return 0, 0, 0
+	}
+
+	minor, err = strconv.Atoi(parts[1])
+	if err != nil {
+		return 0, 0, 0
+	}
+
+	// Patch version may have additional info after a hyphen (e.g., "0-76-generic")
+	// Extract just the numeric part before any hyphen
+	patchStr, _, _ := strings.Cut(parts[2], "-")
+
+	patch, err = strconv.Atoi(patchStr)
+	if err != nil {
+		return 0, 0, 0
+	}
+
+	return major, minor, patch
+}

tstest/kernel_other.go

@@ -0,0 +1,11 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !linux
+
+package tstest
+
+// KernelVersion returns (0, 0, 0) on unsupported platforms.
+func KernelVersion() (major, minor, patch int) {
+	return 0, 0, 0
+}

tstest/tstest.go

@@ -6,6 +6,7 @@ package tstest
 
 import (
 	"context"
+	"fmt"
 	"os"
 	"strconv"
 	"strings"
@@ -93,3 +94,20 @@ func Parallel(t *testing.T) {
 		t.Parallel()
 	}
 }
+
+// SkipOnKernelVersions skips the test if the current
+// kernel version is in the specified list.
+func SkipOnKernelVersions(t testing.TB, issue string, versions ...string) {
+	major, minor, patch := KernelVersion()
+	if major == 0 && minor == 0 && patch == 0 {
+		t.Logf("could not determine kernel version")
+		return
+	}
+
+	current := fmt.Sprintf("%d.%d.%d", major, minor, patch)
+	for _, v := range versions {
+		if v == current {
+			t.Skipf("skipping on kernel version %q - see issue %s", current, issue)
+		}
+	}
+}

tstest/tstest_test.go

@@ -3,7 +3,10 @@
 
 package tstest
 
-import "testing"
+import (
+	"runtime"
+	"testing"
+)
 
 func TestReplace(t *testing.T) {
 	before := "before"
@@ -22,3 +25,17 @@ func TestReplace(t *testing.T) {
 		t.Errorf("before = %q; want %q", before, "before")
 	}
 }
+
+func TestKernelVersion(t *testing.T) {
+	switch runtime.GOOS {
+	case "linux":
+	default:
+		t.Skipf("skipping test on %s", runtime.GOOS)
+	}
+
+	major, minor, patch := KernelVersion()
+	if major == 0 && minor == 0 && patch == 0 {
+		t.Fatal("KernelVersion returned (0, 0, 0); expected valid version")
+	}
+	t.Logf("Kernel version: %d.%d.%d", major, minor, patch)
+}