unix: add Mremap for linux

For golang/go#60409

Change-Id: I75a9732ee996f0aeb91599d80803f96ada468c27
GitHub-Last-Rev: c348b6194d9fbe92f4f912c66709b0af75810e99
GitHub-Pull-Request: golang/sys#164
Reviewed-on: https://go-review.googlesource.com/c/sys/+/502715
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>

unix/mremap_test.go

@@ -0,0 +1,47 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build linux
+// +build linux
+
+package unix_test
+
+import (
+	"testing"
+
+	"golang.org/x/sys/unix"
+)
+
+func TestMremap(t *testing.T) {
+	b, err := unix.Mmap(-1, 0, unix.Getpagesize(), unix.PROT_NONE, unix.MAP_ANON|unix.MAP_PRIVATE)
+	if err != nil {
+		t.Fatalf("Mmap: %v", err)
+	}
+	if err := unix.Mprotect(b, unix.PROT_READ|unix.PROT_WRITE); err != nil {
+		t.Fatalf("Mprotect: %v", err)
+	}
+
+	b[0] = 42
+
+	bNew, err := unix.Mremap(b, unix.Getpagesize()*2, unix.MREMAP_MAYMOVE)
+	if err != nil {
+		t.Fatalf("Mremap2: %v", err)
+	}
+	bNew[unix.Getpagesize()+1] = 84 // checks
+
+	if bNew[0] != 42 {
+		t.Fatal("first element value was changed")
+	}
+	if len(bNew) != unix.Getpagesize()*2 {
+		t.Fatal("new memory len not equal to specified len")
+	}
+	if cap(bNew) != unix.Getpagesize()*2 {
+		t.Fatal("new memory cap not equal to specified len")
+	}
+
+	_, err = unix.Mremap(b, unix.Getpagesize(), unix.MREMAP_FIXED)
+	if err != unix.EINVAL {
+		t.Fatalf("unix.MREMAP_FIXED should be forbidden")
+	}
+}