pkg/userns: deprecate and migrate to github.com/moby/sys/user/userns

The userns package in libcontainer was integrated into the moby/sys/user
module at commit [3778ae603c706494fd1e2c2faf83b406e38d687d][1].

This patch deprecates the containerd fork of that package, and adds it as
an alias for the moby/sys/user/userns package.

[1]: 3778ae603c

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

vendor/github.com/moby/sys/user/userns/userns.go

@@ -0,0 +1,16 @@
+// Package userns provides utilities to detect whether we are currently running
+// in a Linux user namespace.
+//
+// This code was migrated from [libcontainer/runc], which based its implementation
+// on code from [lcx/incus].
+//
+// [libcontainer/runc]: https://github.com/opencontainers/runc/blob/3778ae603c706494fd1e2c2faf83b406e38d687d/libcontainer/userns/userns_linux.go#L12-L49
+// [lcx/incus]: https://github.com/lxc/incus/blob/e45085dd42f826b3c8c3228e9733c0b6f998eafe/shared/util.go#L678-L700
+package userns
+
+// RunningInUserNS detects whether we are currently running in a Linux
+// user namespace and memoizes the result. It returns false on non-Linux
+// platforms.
+func RunningInUserNS() bool {
+	return inUserNS()
+}

vendor/github.com/moby/sys/user/userns/userns_linux.go

@@ -0,0 +1,53 @@
+package userns
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"sync"
+)
+
+var inUserNS = sync.OnceValue(runningInUserNS)
+
+// runningInUserNS detects whether we are currently running in a user namespace.
+//
+// This code was migrated from [libcontainer/runc] and based on an implementation
+// from [lcx/incus].
+//
+// [libcontainer/runc]: https://github.com/opencontainers/runc/blob/3778ae603c706494fd1e2c2faf83b406e38d687d/libcontainer/userns/userns_linux.go#L12-L49
+// [lcx/incus]: https://github.com/lxc/incus/blob/e45085dd42f826b3c8c3228e9733c0b6f998eafe/shared/util.go#L678-L700
+func runningInUserNS() bool {
+	file, err := os.Open("/proc/self/uid_map")
+	if err != nil {
+		// This kernel-provided file only exists if user namespaces are supported.
+		return false
+	}
+	defer file.Close()
+
+	buf := bufio.NewReader(file)
+	l, _, err := buf.ReadLine()
+	if err != nil {
+		return false
+	}
+
+	return uidMapInUserNS(string(l))
+}
+
+func uidMapInUserNS(uidMap string) bool {
+	if uidMap == "" {
+		// File exist but empty (the initial state when userns is created,
+		// see user_namespaces(7)).
+		return true
+	}
+
+	var a, b, c int64
+	if _, err := fmt.Sscanf(uidMap, "%d %d %d", &a, &b, &c); err != nil {
+		// Assume we are in a regular, non user namespace.
+		return false
+	}
+
+	// As per user_namespaces(7), /proc/self/uid_map of
+	// the initial user namespace shows 0 0 4294967295.
+	initNS := a == 0 && b == 0 && c == 4294967295
+	return !initNS
+}

vendor/github.com/moby/sys/user/userns/userns_linux_fuzzer.go

@@ -0,0 +1,8 @@
+//go:build linux && gofuzz
+
+package userns
+
+func FuzzUIDMap(uidmap []byte) int {
+	_ = uidMapInUserNS(string(uidmap))
+	return 1
+}

vendor/github.com/moby/sys/user/userns/userns_unsupported.go

@@ -0,0 +1,6 @@
+//go:build !linux
+
+package userns
+
+// inUserNS is a stub for non-Linux systems. Always returns false.
+func inUserNS() bool { return false }