Move sys to pkg/sys

Signed-off-by: Derek McGowan <derek@mcg.dev>

pkg/sys/userns_unsafe_linux.go

@@ -0,0 +1,94 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package sys
+
+import (
+	"runtime"
+	"syscall"
+	"unsafe"
+)
+
+// ForkUserns is to fork child process with user namespace. It returns child
+// process's pid and pidfd reference to the child process.
+//
+// Precondition: The runtime OS thread must be locked, which is GO runtime
+// requirement.
+//
+// Beside this, the child process sets PR_SET_PDEATHSIG with SIGKILL so that
+// the parent process's OS thread must be locked. Otherwise, the exit event of
+// parent process's OS thread will send kill signal to the child process,
+// even if parent process is still running.
+//
+//go:norace
+//go:noinline
+func ForkUserns() (_pid uintptr, _pidfd uintptr, _ syscall.Errno) {
+	var (
+		pidfd     uintptr
+		pid, ppid uintptr
+		err       syscall.Errno
+	)
+
+	ppid, _, err = syscall.RawSyscall(uintptr(syscall.SYS_GETPID), 0, 0, 0)
+	if err != 0 {
+		return 0, 0, err
+	}
+
+	beforeFork()
+	if runtime.GOARCH == "s390x" {
+		// NOTE:
+		//
+		// On the s390 architectures, the order of the first two
+		// arguments is reversed.
+		//
+		// REF: https://man7.org/linux/man-pages/man2/clone.2.html
+		pid, _, err = syscall.RawSyscall(syscall.SYS_CLONE,
+			0,
+			uintptr(syscall.CLONE_NEWUSER|syscall.SIGCHLD|syscall.CLONE_PIDFD),
+			uintptr(unsafe.Pointer(&pidfd)),
+		)
+	} else {
+		pid, _, err = syscall.RawSyscall(syscall.SYS_CLONE,
+			uintptr(syscall.CLONE_NEWUSER|syscall.SIGCHLD|syscall.CLONE_PIDFD),
+			0,
+			uintptr(unsafe.Pointer(&pidfd)),
+		)
+	}
+	if err != 0 || pid != 0 {
+		afterFork()
+		return pid, pidfd, err
+	}
+	afterForkInChild()
+
+	if _, _, err = syscall.RawSyscall(syscall.SYS_PRCTL, syscall.PR_SET_PDEATHSIG, uintptr(syscall.SIGKILL), 0); err != 0 {
+		goto err
+	}
+
+	pid, _, err = syscall.RawSyscall(syscall.SYS_GETPPID, 0, 0, 0)
+	if err != 0 {
+		goto err
+	}
+
+	// exit if re-parent
+	if pid != ppid {
+		goto err
+	}
+
+	_, _, err = syscall.RawSyscall(syscall.SYS_PPOLL, 0, 0, 0)
+err:
+	syscall.RawSyscall(syscall.SYS_EXIT, uintptr(err), 0, 0)
+	panic("unreachable")
+}