Port most syscalls to VFS2.

pipe and pipe2 aren't ported, pending a slight rework of pipe FDs for VFS2.
mount and umount2 aren't ported out of temporary laziness. access and faccessat
need additional FSImpl methods to implement properly, but are stubbed to
prevent googletest from CHECK-failing. Other syscalls require additional
plumbing.

Updates #1623

PiperOrigin-RevId: 297188448

2 files changed, 68 insertions, 0 deletions

diff --git a/pkg/gohacks/BUILD b/pkg/gohacks/BUILD
new file mode 100644
index 000000000..798a65eca
--- /dev/null
+++ b/pkg/gohacks/BUILD
@@ -0,0 +1,11 @@
+load("//tools:defs.bzl", "go_library")
+
+package(licenses = ["notice"])
+
+go_library(
+    name = "gohacks",
+    srcs = [
+        "gohacks_unsafe.go",
+    ],
+    visibility = ["//:sandbox"],
+)
diff --git a/pkg/gohacks/gohacks_unsafe.go b/pkg/gohacks/gohacks_unsafe.go
new file mode 100644
index 000000000..aad675172
--- /dev/null
+++ b/pkg/gohacks/gohacks_unsafe.go
@@ -0,0 +1,57 @@
+// Copyright 2020 The gVisor 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 gohacks contains utilities for subverting the Go compiler.
+package gohacks
+
+import (
+	"reflect"
+	"unsafe"
+)
+
+// Noescape hides a pointer from escape analysis. Noescape is the identity
+// function but escape analysis doesn't think the output depends on the input.
+// Noescape is inlined and currently compiles down to zero instructions.
+// USE CAREFULLY!
+//
+// (Noescape is copy/pasted from Go's runtime/stubs.go:noescape().)
+//
+//go:nosplit
+func Noescape(p unsafe.Pointer) unsafe.Pointer {
+	x := uintptr(p)
+	return unsafe.Pointer(x ^ 0)
+}
+
+// ImmutableBytesFromString is equivalent to []byte(s), except that it uses the
+// same memory backing s instead of making a heap-allocated copy. This is only
+// valid if the returned slice is never mutated.
+func ImmutableBytesFromString(s string) []byte {
+	shdr := (*reflect.StringHeader)(unsafe.Pointer(&s))
+	var bs []byte
+	bshdr := (*reflect.SliceHeader)(unsafe.Pointer(&bs))
+	bshdr.Data = shdr.Data
+	bshdr.Len = shdr.Len
+	bshdr.Cap = shdr.Len
+	return bs
+}
+
+// StringFromImmutableBytes is equivalent to string(bs), except that it uses
+// the same memory backing bs instead of making a heap-allocated copy. This is
+// only valid if bs is never mutated after StringFromImmutableBytes returns.
+func StringFromImmutableBytes(bs []byte) string {
+	// This is cheaper than messing with reflect.StringHeader and
+	// reflect.SliceHeader, which as of this writing produces many dead stores
+	// of zeroes. Compare strings.Builder.String().
+	return *(*string)(unsafe.Pointer(&bs))
+}